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Multiple lightning strikes on a city at night
Lightning is one of the most dramatic effects of electricity.

Electricity (from the New Latin ēlectricus, "amber-like"[a]) is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning and static electricity, but in addition, less familiar concepts, such as the electromagnetic field and electromagnetic induction.

In general usage, the word "electricity" is adequate to refer to a number of physical effects. In scientific usage, however, the term is vague, and these related, but distinct, concepts are better identified by more precise terms:

Electrical phenomena have been studied since antiquity, though advances in the science were not made until the seventeenth and eighteenth centuries. Practical applications for electricity however remained few, and it would not be until the late nineteenth century that engineers were able to put it to industrial and residential use. The rapid expansion in electrical technology at this time transformed industry and society. Electricity's extraordinary versatility as a source of energy means it can be put to an almost limitless set of applications which include transport, heating, lighting, communications, and computation. The backbone of modern industrial society is, and for the foreseeable future can be expected to remain, the use of electrical power.[1]

Contents

History

A bust of a bearded man with dishevelled hair
Thales, the earliest researcher into electricity

Long before any knowledge of electricity existed people were aware of shocks from electric fish. Ancient Egyptian texts dating from 2750 BC referred to these fish as the "Thunderer of the Nile", and described them as the "protectors" of all other fish. They were again reported millennia later by ancient Greek, Roman and Arabic naturalists and physicians.[2] Several ancient writers, such as Pliny the Elder and Scribonius Largus, attested to the numbing effect of electric shocks delivered by catfish and torpedo rays, and knew that such shocks could travel along conducting objects.[3] Patients suffering from ailments such as gout or headache were directed to touch electric fish in the hope that the powerful jolt might cure them.[4] Possibly the earliest and nearest approach to the discovery of the identity of lightning, and electricity from any other source, is to be attributed to the Arabs, who before the 15th century had the Arabic word for lightning (raad) applied to the electric ray.[5]

That certain objects such as rods of amber could be rubbed with cat's fur and attract light objects like feathers was known to ancient cultures around the Mediterranean. Thales of Miletos made a series of observations on static electricity around 600 BC, from which he believed that friction rendered amber magnetic, in contrast to minerals such as magnetite, which needed no rubbing.[6][7] Thales was incorrect in believing the attraction was due to a magnetic effect, but later science would prove a link between magnetism and electricity. According to a controversial theory, the Parthians may have had knowledge of electroplating, based on the 1936 discovery of the Baghdad Battery, which resembles a galvanic cell, though it is uncertain whether the artifact was electrical in nature.[8]

A half-length portrait of a bald, somewhat portly man in a three-piece suit.
Benjamin Franklin conducted extensive research on electricity in the 18th century

Electricity would remain little more than an intellectual curiosity for millennia until 1600, when the English physician William Gilbert made a careful study of electricity and magnetism, distinguishing the lodestone effect from static electricity produced by rubbing amber.[6] He coined the New Latin word electricus ("of amber" or "like amber", from ήλεκτρον [elektron], the Greek word for "amber") to refer to the property of attracting small objects after being rubbed.[9] This association gave rise to the English words "electric" and "electricity", which made their first appearance in print in Thomas Browne's Pseudodoxia Epidemica of 1646.[10]

Further work was conducted by Otto von Guericke, Robert Boyle, Stephen Gray and C. F. du Fay. In the 18th century, Benjamin Franklin conducted extensive research in electricity, selling his possessions to fund his work. In June 1752 he is reputed to have attached a metal key to the bottom of a dampened kite string and flown the kite in a storm-threatened sky.[11] A succession of sparks jumping from the key to the back of the hand showed that lightning was indeed electrical in nature.[12]

Half-length portrait oil painting of a man in a dark suit
Michael Faraday formed the foundation of electric motor technology

In 1791, Luigi Galvani published his discovery of bioelectricity, demonstrating that electricity was the medium by which nerve cells passed signals to the muscles.[13] Alessandro Volta's battery, or voltaic pile, of 1800, made from alternating layers of zinc and copper, provided scientists with a more reliable source of electrical energy than the electrostatic machines previously used.[13] The recognition of electromagnetism, the unity of electric and magnetic phenomena, is due to Hans Christian Ørsted and André-Marie Ampère in 1819-1820; Michael Faraday invented the electric motor in 1821, and Georg Ohm mathematically analysed the electrical circuit in 1827.[13]

While it had been the early 19th century that had seen rapid progress in electrical science, the late 19th century would see the greatest progress in electrical engineering. Through such people as Nikola Tesla, Thomas Edison, Ottó Bláthy, Sir Charles Parsons, George Westinghouse, Ernst Werner von Siemens, Alexander Graham Bell and Lord Kelvin, electricity was turned from a scientific curiosity into an essential tool for modern life, becoming a driving force for the Second Industrial Revolution.[14]

Concepts

Electric charge

Electric charge is a property of certain subatomic particles, which gives rise to and interacts with, the electromagnetic force, one of the four fundamental forces of nature. Charge originates in the atom, in which its most familiar carriers are the electron and proton. It is a conserved quantity, that is, the net charge within an isolated system will always remain constant regardless of any changes taking place within that system.[15] Within the system, charge may be transferred between bodies, either by direct contact, or by passing along a conducting material, such as a wire.[16] The informal term static electricity refers to the net presence (or 'imbalance') of charge on a body, usually caused when dissimilar materials are rubbed together, transferring charge from one to the other.

A clear glass dome has an external electrode which connects through the glass to a pair of gold leaves. A charged rod touches the external electrode and makes the leaves repel.
Charge on a gold-leaf electroscope causes the leaves to visibly repel each other

The presence of charge gives rise to the electromagnetic force: charges exert a force on each other, an effect that was known, though not understood, in antiquity.[17] A lightweight ball suspended from a string can be charged by touching it with a glass rod that has itself been charged by rubbing with a cloth. If a similar ball is charged by the same glass rod, it is found to repel the first: the charge acts to force the two balls apart. Two balls that are charged with a rubbed amber rod also repel each other. However, if one ball is charged by the glass rod, and the other by an amber rod, the two balls are found to attract each other. These phenomena were investigated in the late eighteenth century by Charles-Augustin de Coulomb, who deduced that charge manifests itself in two opposing forms. This discovery led to the well-known axiom: like-charged objects repel and opposite-charged objects attract.[17]

The force acts on the charged particles themselves, hence charge has a tendency to spread itself as evenly as possible over a conducting surface. The magnitude of the electromagnetic force, whether attractive or repulsive, is given by Coulomb's law, which relates the force to the product of the charges and has an inverse-square relation to the distance between them.[18][19] The electromagnetic force is very strong, second only in strength to the strong interaction,[20] but unlike that force it operates over all distances.[21] In comparison with the much weaker gravitational force, the electromagnetic force pushing two electrons apart is 1042 times that of the gravitational attraction pulling them together.[22]

The charge on electrons and protons is opposite in sign, hence an amount of charge may be expressed as being either negative or positive. By convention, the charge carried by electrons is deemed negative, and that by protons positive, a custom that originated with the work of Benjamin Franklin.[23] The amount of charge is usually given the symbol Q and expressed in coulombs;[24] each electron carries the same charge of approximately −1.6022×10−19 coulomb. The proton has a charge that is equal and opposite, and thus +1.6022×10−19  coulomb. Charge is possessed not just by matter, but also by antimatter, each antiparticle bearing an equal and opposite charge to its corresponding particle.[25]

Charge can be measured by a number of means, an early instrument being the gold-leaf electroscope, which although still in use for classroom demonstrations, has been superseded by the electronic electrometer.[16]

Electric current

The movement of electric charge is known as an electric current, the intensity of which is usually measured in amperes. Current can consist of any moving charged particles; most commonly these are electrons, but any charge in motion constitutes a current.

By historical convention, a positive current is defined as having the same direction of flow as any positive charge it contains, or to flow from the most positive part of a circuit to the most negative part. Current defined in this manner is called conventional current. The motion of negatively-charged electrons around an electric circuit, one of the most familiar forms of current, is thus deemed positive in the opposite direction to that of the electrons.[26] However, depending on the conditions, an electric current can consist of a flow of charged particles in either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation.

Two metal wires form an inverted V shape. A blindingly bright orange-white electric arc flows between their tips.
An electric arc provides an energetic demonstration of electric current

The process by which electric current passes through a material is termed electrical conduction, and its nature varies with that of the charged particles and the material through which they are travelling. Examples of electric currents include metallic conduction, where electrons flow through a conductor such as metal, and electrolysis, where ions (charged atoms) flow through liquids. While the particles themselves can move quite slowly, sometimes with an average drift velocity only fractions of a millimetre per second,[16] the electric field that drives them itself propagates at close to the speed of light, enabling electrical signals to pass rapidly along wires.[27]

Current causes several observable effects, which historically were the means of recognising its presence. That water could be decomposed by the current from a voltaic pile was discovered by Nicholson and Carlisle in 1800, a process now known as electrolysis. Their work was greatly expanded upon by Michael Faraday in 1833.[28] Current through a resistance causes localised heating, an effect James Prescott Joule studied mathematically in 1840.[28] One of the most important discoveries relating to current was made accidentally by Hans Christian Ørsted in 1820, when, while preparing a lecture, he witnessed the current in a wire disturbing the needle of a magnetic compass.[29] He had discovered electromagnetism, a fundamental interaction between electricity and magnetics.

In engineering or household applications, current is often described as being either direct current (DC) or alternating current (AC). These terms refer to how the current varies in time. Direct current, as produced by example from a battery and required by most electronic devices, is a unidirectional flow from the positive part of a circuit to the negative.[30] If, as is most common, this flow is carried by electrons, they will be travelling in the opposite direction. Alternating current is any current that reverses direction repeatedly; almost always this takes the form of a sinusoidal wave.[31] Alternating current thus pulses back and forth within a conductor without the charge moving any net distance over time. The time-averaged value of an alternating current is zero, but it delivers energy in first one direction, and then the reverse. Alternating current is affected by electrical properties that are not observed under steady state direct current, such as inductance and capacitance.[32] These properties however can become important when circuitry is subjected to transients, such as when first energised.

Electric field

The concept of the electric field was introduced by Michael Faraday. An electric field is created by a charged body in the space that surrounds it, and results in a force exerted on any other charges placed within the field. The electric field acts between two charges in a similar manner to the way that the gravitational field acts between two masses, and like it, extends towards infinity and shows an inverse square relationship with distance.[21] However, there is an important difference. Gravity always acts in attraction, drawing two masses together, while the electric field can result in either attraction or repulsion. Since large bodies such as planets generally carry no net charge, the electric field at a distance is usually zero. Thus gravity is the dominant force at distance in the universe, despite being much weaker.[22]

Field lines emanating from a positive charge above a plane conductor

An electric field generally varies in space,[33] and its strength at any one point is defined as the force (per unit charge) that would be felt by a stationary, negligible charge if placed at that point.[34] The conceptual charge, termed a 'test charge', must be vanishingly small to prevent its own electric field disturbing the main field and must also be stationary to prevent the effect of magnetic fields. As the electric field is defined in terms of force, and force is a vector, so it follows that an electric field is also a vector, having both magnitude and direction. Specifically, it is a vector field.[34]

The study of electric fields created by stationary charges is called electrostatics. The field may be visualised by a set of imaginary lines whose direction at any point is the same as that of the field. This concept was introduced by Faraday,[35] whose term 'lines of force' still sometimes sees use. The field lines are the paths that a point positive charge would seek to make as it was forced to move within the field; they are however an imaginary concept with no physical existence, and the field permeates all the intervening space between the lines.[35] Field lines emanating from stationary charges have several key properties: first, that they originate at positive charges and terminate at negative charges; second, that they must enter any good conductor at right angles, and third, that they may never cross nor close in on themselves.[36]

A hollow conducting body carries all its charge on its outer surface. The field is therefore zero at all places inside the body.[37] This is the operating principal of the Faraday cage, a conducting metal shell which isolates its interior from outside electrical effects.

The principles of electrostatics are important when designing items of high-voltage equipment. There is a finite limit to the electric field strength that may be withstood by any medium. Beyond this point, electrical breakdown occurs and an electric arc causes flashover between the charged parts. Air, for example, tends to arc across small gaps at electric field strengths which exceed 30 kV per centimetre. Over larger gaps, its breakdown strength is weaker, perhaps 1 kV per centimetre.[38] The most visible natural occurrence of this is lightning, caused when charge becomes separated in the clouds by rising columns of air, and raises the electric field in the air to greater than it can withstand. The voltage of a large lightning cloud may be as high as 100 MV and have discharge energies as great as 250 kWh.[39]

The field strength is greatly affected by nearby conducting objects, and it is particularly intense when it is forced to curve around sharply pointed objects. This principle is exploited in the lightning conductor, the sharp spike of which acts to encourage the lightning stroke to develop there, rather than to the building it serves to protect.[40]

Electric potential

Two AA batteries each have a plus sign marked at one end.
A pair of AA cells. The + sign indicates the polarity of the potential difference between the battery terminals.

The concept of electric potential is closely linked to that of the electric field. A small charge placed within an electric field experiences a force, and to have brought that charge to that point against the force requires work. The electric potential at any point is defined as the energy required to bring a unit test charge from an infinite distance slowly to that point. It is usually measured in volts, and one volt is the potential for which one joule of work must be expended to bring a charge of one coulomb from infinity.[41] This definition of potential, while formal, has little practical application, and a more useful concept is that of electric potential difference, and is the energy required to move a unit charge between two specified points. An electric field has the special property that it is conservative, which means that the path taken by the test charge is irrelevant: all paths between two specified points expend the same energy, and thus a unique value for potential difference may be stated.[41] The volt is so strongly identified as the unit of choice for measurement and description of electric potential difference that the term voltage sees greater everyday usage.

For practical purposes, it is useful to define a common reference point to which potentials may be expressed and compared. While this could be at infinity, a much more useful reference is the Earth itself, which is assumed to be at the same potential everywhere. This reference point naturally takes the name earth or ground. Earth is assumed to be an infinite source of equal amounts of positive and negative charge, and is therefore electrically uncharged – and unchargeable.[42]

Electric potential is a scalar quantity, that is, it has only magnitude and not direction. It may be viewed as analogous to height: just as a released object will fall through a difference in heights caused by a gravitational field, so a charge will 'fall' across the voltage caused by an electric field.[43] As relief maps show contour lines marking points of equal height, a set of lines marking points of equal potential (known as equipotentials) may be drawn around an electrostatically charged object. The equipotentials cross all lines of force at right angles. They must also lie parallel to a conductor's surface, otherwise this would produce a force that will move the charge carriers to even the potential of the surface.

The electric field was formally defined as the force exerted per unit charge, but the concept of potential allows for a more useful and equivalent definition: the electric field is the local gradient of the electric potential. Usually expressed in volts per metre, the vector direction of the field is the line of greatest slope of potential, and where the equipotentials lie closest together.[16]

Electromagnetism

A wire carries a current towards the reader. Concentric circles representing the magnetic field circle anticlockwise around the wire, as viewed by the reader.
Magnetic field circles around a current

Ørsted's discovery in 1821 that a magnetic field existed around all sides of a wire carrying an electric current indicated that there was a direct relationship between electricity and magnetism. Moreover, the interaction seemed different from gravitational and electrostatic forces, the two forces of nature then known. The force on the compass needle did not direct it to or away from the current-carrying wire, but acted at right angles to it.[29] Ørsted's slightly obscure words were that "the electric conflict acts in a revolving manner." The force also depended on the direction of the current, for if the flow was reversed, then the force did too.[44]

Ørsted did not fully understand his discovery, but he observed the effect was reciprocal: a current exerts a force on a magnet, and a magnetic field exerts a force on a current. The phenomenon was further investigated by Ampère, who discovered that two parallel current-carrying wires exerted a force upon each other: two wires conducting currents in the same direction are attracted to each other, while wires containing currents in opposite directions are forced apart.[45] The interaction is mediated by the magnetic field each current produces and forms the basis for the international definition of the ampere.[45]

A cut-away diagram of a small electric motor
The electric motor exploits an important effect of electromagnetism: a current through a magnetic field experiences a force at right angles to both the field and current

This relationship between magnetic fields and currents is extremely important, for it led to Michael Faraday's invention of the electric motor in 1821. Faraday's homopolar motor consisted of a permanent magnet sitting in a pool of mercury. A current was allowed through a wire suspended from a pivot above the magnet and dipped into the mercury. The magnet exerted a tangential force on the wire, making it circle around the magnet for as long as the current was maintained.[46]

Experimentation by Faraday in 1831 revealed that a wire moving perpendicular to a magnetic field developed a potential difference between its ends. Further analysis of this process, known as electromagnetic induction, enabled him to state the principle, now known as Faraday's law of induction, that the potential difference induced in a closed circuit is proportional to the rate of change of magnetic flux through the loop. Exploitation of this discovery enabled him to invent the first electrical generator in 1831, in which he converted the mechanical energy of a rotating copper disc to electrical energy.[46] Faraday's disc was inefficient and of no use as a practical generator, but it showed the possibility of generating electric power using magnetism, a possibility that would be taken up by those that followed on from his work.

Faraday's and Ampère's work showed that a time-varying magnetic field acted as a source of an electric field, and a time-varying electric field was a source of a magnetic field. Thus, when either field is changing in time, then a field of the other is necessarily induced.[47] Such a phenomenon has the properties of a wave, and is naturally referred to as an electromagnetic wave. Electromagnetic waves were analysed theoretically by James Clerk Maxwell in 1864. Maxwell developed a set of equations that could unambiguously describe the interrelationship between electric field, magnetic field, electric charge, and electric current. He could moreover prove that such a wave would necessarily travel at the speed of light, and thus light itself was a form of electromagnetic radiation. Maxwell's Laws, which unify light, fields, and charge are one of the great milestones of theoretical physics.[47]

Electric circuits

A basic electric circuit. The voltage source V on the left drives a current I around the circuit, delivering electrical energy into the resistor R. From the resistor, the current returns to the source, completing the circuit.

An electric circuit is an interconnection of electric components such that electric charge is made to flow along a closed path (a circuit), usually to perform some useful task.

The components in an electric circuit can take many forms, which can include elements such as resistors, capacitors, switches, transformers and electronics. Electronic circuits contain active components, usually semiconductors, and typically exhibit non-linear behaviour, requiring complex analysis. The simplest electric components are those that are termed passive and linear: while they may temporarily store energy, they contain no sources of it, and exhibit linear responses to stimuli.[48]

The resistor is perhaps the simplest of passive circuit elements: as its name suggests, it resists the current through it, dissipating its energy as heat. The resistance is a consequence of the motion of charge through a conductor: in metals, for example, resistance is primarily due to collisions between electrons and ions. Ohm's law is a basic law of circuit theory, stating that the current passing through a resistance is directly proportional to the potential difference across it. The resistance of most materials is relatively constant over a range of temperatures and currents; materials under these conditions are known as 'ohmic'. The ohm, the unit of resistance, was named in honour of Georg Ohm, and is symbolised by the Greek letter Ω. 1 Ω is the resistance that will produce a potential difference of one volt in response to a current of one amp.[48]

The capacitor is a device capable of storing charge, and thereby storing electrical energy in the resulting field. Conceptually, it consists of two conducting plates separated by a thin insulating layer; in practice, thin metal foils are coiled together, increasing the surface area per unit volume and therefore the capacitance. The unit of capacitance is the farad, named after Michael Faraday, and given the symbol F: one farad is the capacitance that develops a potential difference of one volt when it stores a charge of one coulomb. A capacitor connected to a voltage supply initially causes a current as it accumulates charge; this current will however decay in time as the capacitor fills, eventually falling to zero. A capacitor will therefore not permit a steady state current, but instead blocks it.[48]

The inductor is a conductor, usually a coil of wire, that stores energy in a magnetic field in response to the current through it. When the current changes, the magnetic field does too, inducing a voltage between the ends of the conductor. The induced voltage is proportional to the time rate of change of the current. The constant of proportionality is termed the inductance. The unit of inductance is the henry, named after Joseph Henry, a contemporary of Faraday. One henry is the inductance that will induce a potential difference of one volt if the current through it changes at a rate of one ampere per second.[48] The inductor's behaviour is in some regards converse to that of the capacitor: it will freely allow an unchanging current, but opposes a rapidly changing one.

Production and uses

Generation and transmission

A wind farm of about a dozen three-bladed white wind turbines.
Wind power is of increasing importance in many countries

Thales' experiments with amber rods were the first studies into the production of electrical energy. While this method, now known as the triboelectric effect, is capable of lifting light objects and even generating sparks, it is extremely inefficient.[49] It was not until the invention of the voltaic pile in the eighteenth century that a viable source of electricity became available. The voltaic pile, and its modern descendant, the electrical battery, store energy chemically and make it available on demand in the form of electrical energy.[49] The battery is a versatile and very common power source which is ideally suited to many applications, but its energy storage is finite, and once discharged it must be disposed of or recharged. For large electrical demands electrical energy must be generated and transmitted continuously over conductive transmission lines.

Electrical power is usually generated by electro-mechanical generators driven by steam produced from fossil fuel combustion, or the heat released from nuclear reactions; or from other sources such as kinetic energy extracted from wind or flowing water. The modern steam turbine invented by Sir Charles Parsons in 1884 today generates about 80 percent of the electric power in the world using a variety of heat sources. Such generators bear no resemblance to Faraday's homopolar disc generator of 1831, but they still rely on his electromagnetic principle that a conductor linking a changing magnetic field induces a potential difference across its ends.[50] The invention in the late nineteenth century of the transformer meant that electrical power could be transmitted more efficiently at a higher voltage but lower current. Efficient electrical transmission meant in turn that electricity could be generated at centralised power stations, where it benefited from economies of scale, and then be despatched relatively long distances to where it was needed.[51][52]

Since electrical energy cannot easily be stored in quantities large enough to meet demands on a national scale, at all times exactly as much must be produced as is required.[51] This requires electricity utilities to make careful predictions of their electrical loads, and maintain constant co-ordination with their power stations. A certain amount of generation must always be held in reserve to cushion an electrical grid against inevitable disturbances and losses.

Demand for electricity grows with great rapidity as a nation modernises and its economy develops. The United States showed a 12% increase in demand during each year of the first three decades of the twentieth century,[53] a rate of growth that is now being experienced by emerging economies such as those of India or China.[54][55] Historically, the growth rate for electricity demand has outstripped that for other forms of energy.[56]

Environmental concerns with electricity generation have led to an increased focus on generation from renewable sources, in particular from wind and hydropower. While debate can be expected to continue over the environmental impact of different means of electricity production, its final form is relatively clean.[57]

Uses

The light bulb, an early application of electricity, operates by Joule heating: the passage of current through resistance generating heat

Electricity is an extremely flexible form of energy, and has been adapted to a huge, and growing, number of uses.[58] The invention of a practical incandescent light bulb in the 1870s led to lighting becoming one of the first publicly available applications of electrical power. Although electrification brought with it its own dangers, replacing the naked flames of gas lighting greatly reduced fire hazards within homes and factories.[59] Public utilities were set up in many cities targeting the burgeoning market for electrical lighting.

The Joule heating effect employed in the light bulb also sees more direct use in electric heating. While this is versatile and controllable, it can be seen as wasteful, since most electrical generation has already required the production of heat at a power station.[60] A number of countries, such as Denmark, have issued legislation restricting or banning the use of electric heating in new buildings.[61] Electricity is however a highly practical energy source for refrigeration,[62] with air conditioning representing a growing sector for electricity demand, the effects of which electricity utilities are increasingly obliged to accommodate.[63]

Electricity is used within telecommunications, and indeed the electrical telegraph, demonstrated commercially in 1837 by Cooke and Wheatstone, was one of its earliest applications. With the construction of first intercontinental, and then transatlantic, telegraph systems in the 1860s, electricity had enabled communications in minutes across the globe. Optical fibre and satellite communication technology have taken a share of the market for communications systems, but electricity can be expected to remain an essential part of the process.

The effects of electromagnetism are most visibly employed in the electric motor, which provides a clean and efficient means of motive power. A stationary motor such as a winch is easily provided with a supply of power, but a motor that moves with its application, such as an electric vehicle, is obliged to either carry along a power source such as a battery, or to collect current from a sliding contact such as a pantograph, placing restrictions on its range or performance.

Electronic devices make use of the transistor, perhaps one of the most important inventions of the twentieth century,[64] and a fundamental building block of all modern circuitry. A modern integrated circuit may contain several billion miniaturised transistors in a region only a few centimetres square.[65]

Electricity and the natural world

Physiological effects

A voltage applied to a human body causes an electric current through the tissues, and although the relationship is non-linear, the greater the voltage, the greater the current.[66] The threshold for perception varies with the supply frequency and with the path of the current, but is about 0.1 mA to 1 mA for mains-frequency electricity, though a current as low as a microamp can be detected as an electrovibration effect under certain conditions.[67] If the current is sufficiently high, it will cause muscle contraction, fibrillation of the heart, and tissue burns.[66] The lack of any visible sign that a conductor is electrified makes electricity a particular hazard. The pain caused by an electric shock can be intense, leading electricity at times to be employed as a method of torture. Death caused by an electric shock is referred to as electrocution. Electrocution is still the means of judicial execution in some jurisdictions, though its use has become rarer in recent times.[68]

Electrical phenomena in nature

The electric eel, Electrophorus electricus

Electricity is not a human invention, and may be observed in several forms in nature, a prominent manifestation of which is lightning. Many interactions familiar at the macroscopic level, such as touch, friction or chemical bonding, are due to interactions between electric fields on the atomic scale. The Earth's magnetic field is thought to arise from a natural dynamo of circulating currents in the planet's core.[69] Certain crystals, such as quartz, or even sugar, generate a potential difference across their faces when subjected to external pressure.[70] This phenomenon is known as piezoelectricity, from the Greek piezein (πιέζειν), meaning to press, and was discovered in 1880 by Pierre and Jacques Curie. The effect is reciprocal, and when a piezoelectric material is subjected to an electric field, a small change in physical dimensions take place.[70]

Some organisms, such as sharks, are able to detect and respond to changes in electric fields, an ability known as electroreception,[71] while others, termed electrogenic, are able to generate voltages themselves to serve as a predatory or defensive weapon.[3] The order Gymnotiformes, of which the best known example is the electric eel, detect or stun their prey via high voltages generated from modified muscle cells called electrocytes.[3][4] All animals transmit information along their cell membranes with voltage pulses called action potentials, whose functions include communication by the nervous system between neurons and muscles.[72] An electric shock stimulates this system, and causes muscles to contract.[73] Action potentials are also responsible for coordinating activities in certain plants and mammals.[72]

Cultural perception

In the 19th and early 20th century, electricity was not part of the everyday life of many people, even in the industrialised Western world. The popular culture of the time accordingly often depicts it as a mysterious, quasi-magical force that can slay the living, revive the dead or otherwise bend the laws of nature.[74] This attitude began with the 1771 experiments of Luigi Galvani in which the legs of dead frogs were shown to twitch on application of animal electricity. "Revitalization" or resusciation of apparently dead or drowned persons was reported in the medical literature shortly after Galvani's work. These results were known to Mary Shelley when she authored Frankenstein (1819), although she does not name the method of revitalization of the monster. The revitalization of monsters with electricity later became a stock theme in horror films.

As the public familiarity with electricity as the lifeblood of the Second Industrial Revolution grew, its wielders were more often cast in a positive light,[75] such as the workers who "finger death at their gloves' end as they piece and repiece the living wires" in Rudyard Kipling's 1907 poem The Sons of Martha.[75] Electrically powered vehicles of every sort featured large in adventure stories such as those of Jules Verne or the Tom Swift books.[75] The masters of electricity, whether fictional or real—including scientists such as Thomas Edison, Charles Steinmetz or Nikola Tesla—were popularly conceived of as having wizard-like powers.[75]

With electricity ceasing to be a novelty and becoming a necessity of everyday life in the later half of the 20th century, it required particular attention by popular culture only when it stops flowing,[75] an event that usually signals disaster.[75] The people who keep it flowing, such as the nameless hero of Jimmy Webb’s song "Wichita Lineman" (1968),[75] are still often cast as heroic, wizard-like figures.[75]

See also

Notes

a. ^  the New Latin ēlectricus, "amber-like", came from the classical Latin electrum, itself coming from the Greek ἤλεκτρον, (elektron), meaning amber

References

  1. ^ Jones, D.A., "Electrical engineering: the backbone of society", Proceedings of the IEE: Science, Measurement and Technology 138 (1): 1–10  
  2. ^ Moller, Peter (December 1991), "Review: Electric Fish", BioScience 41 (11): 794–6 [794], doi:10.2307/1311732  
  3. ^ a b c Bullock, Theodore H. (2005), Electroreception, Springer, pp. 5–7, ISBN 0387231927  
  4. ^ a b Morris, Simon C. (2003), Life's Solution: Inevitable Humans in a Lonely Universe, Cambridge University Press, pp. 182–185, ISBN 0521827043  
  5. ^ The Encyclopedia Americana; a library of universal knowledge (1918), New York: Encyclopedia Americana Corp
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Bibliography

  • Bird, John (2007), Electrical and Electronic Principles and Technology, 3rd edition, Newnes, ISBN 0-978-8556-6  
  • Duffin, W.J. (1980), Electricity and Magnetism, 3rd edition, McGraw-Hill, ISBN 007084111X  
  • Edminister, Joseph (1965), Electric Circuits, 2nd Edition, McGraw-Hill, ISBN 07084397X  
  • Hammond, Percy (1981), Electromagnetism for Engineers, Pergamon, ISBN 0-08-022104-1  
  • Morely, A.; Hughes, E (1994), Principles of Electricity, Fifth edition, Longman, ISBN 0-582-22874-3  
  • Naidu, M.S.; Kamataru, V. (1982), High Voltage Engineering, Tata McGraw-Hill, ISBN 0-07-451786-4  
  • Nilsson, James; Riedel, Susan (2007), Electric Circuits, Prentice Hall, ISBN 978-0131989252  
  • Patterson, Walter C. (1999), Transforming Electricity: The Coming Generation of Change, Earthscan, ISBN 185383341X  
  • Sears, et al., Francis (1982), University Physics, Sixth Edition, Addison Wesley, ISBN 0-2010-7199-1  
  • Benjamin, P. (1898). A history of electricity (The intellectual rise in electricity) from antiquity to the days of Benjamin Franklin. New York: J. Wiley & Sons.

External links


Quotes

Up to date as of January 14, 2010

From Wikiquote

Quotes about electricity

Unsourced

  • "So astounding are the facts in this connection, that it would seem as though the Creator, himself had electrically designed this planet ..." ~ Nikola Tesla
  • "Without electricty, there can be no art." ~ Nam June Paik
  • "We will make electricity so cheap that only the rich will burn candles." ~ Thomas Edison
  • "Go hug your girlfriend! Or if you don't have any, go find one! You won't find the meaning of life with electricity!" ~ Pex Tufvesson (Kaktus & Mahoney)

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Travel guide

Up to date as of January 14, 2010
(Redirected to Electrical systems article)

From Wikitravel

Universal Plug Adaptor
Universal Plug Adaptor

This article is a travel topic.

Electrical systems differ around the world. Some use 110-120 volts and others 220-240 volts. Some use 50 hertz and others 60 hertz. The plugs are also different and often incompatible. However, travelers with electrical appliances can take a few steps to ensure that they can be used at their destination.

Map of the world coloured by voltage and frequency
Map of the world coloured by voltage and frequency

Start by taking a look at the back of the device you want to use. If it says "100-240V, 50/60 Hz" or a greater range, it will work anywhere in the world, and you can skip right to the next section. If it doesn't, keep reading.

Dealing with electricity differences can be daunting, but it actually isn't too hard. There are only two main type of electric systems used around the world, with varying physical connections:

  • 110-120 volt, at 60 hertz frequency (in general: North and Central Americas, Western Japan)
  • 220-240 volt, at 50 hertz frequency (in general: the rest of the world, with some exceptions)

Occasionally, you will find 110-120 volts @ 50 Hz such as in Tokyo, Japan; and conversely, 220-240 volts @ 60 Hz such as in the Philippines and some parts of Brazil. Such systems are not common worldwide, though.

If the voltage and frequency for your device is the same as where you are travelling, then you need only worry about the physical plug. (The small difference between 110V and 120V is within the tolerances of most electrical devices. Likewise for 220V and 240V.)

If the voltage for your device is not the same, then you will need a transformer or converter to convert the voltage.

Giving a device a lower voltage than it was designed for is generally not dangerous; the device may not work correctly, but no dramatic failure is likely. Giving any device a voltage higher than it was designed for is dangerous, and will very likely damage the device. If you put 230 volts into a device designed for 110 it may melt, catch fire, or even explode.

Plug types
Plug types
Map of the world coloured by type of plug used
Map of the world coloured by type of plug used

A device that lets you insert a plug into a different socket is an adapter: these are small, cheap and safe. For example, between England and Germany, you need only an adapter. You stick your British plug in the adapter, which connects the rectangular phase and neutral prongs to the round German ones and puts the ground where the German outlet expects it, and you're good to go.

Unfortunately, there are many different plugs in the world. The three most widespread standards are:

  • The "American" (Type A) plug, with two vertical pins
  • The "European" (Type C) plug, with two round pins
  • The "British" (Type G) plug, with three rectangular pins

If your device has one of these plugs and you can adapt it to the others, you've got 90% of the world covered. (The main exceptions are South Africa, Australia, New Zealand, Argentina and parts of China, which use a Type I plug with two slanted pins.) Adapters between Type A and Type C and from C to G are tiny and cheap; converting Type A into G or Type G into anything else, on the other hand, needs a bulkier model.

For hobbyists: if you can't find an adapter, and you're staying for a longer time, just buy a separate plug at your destination, remove the existing plug and attach the new one. Unlike adapters, plugs are always available, and they're generally cheaper too. Caution: only try this if you know what you're doing! (Fire and/or electrocution are possible if inexperienced.)

As a last resort, a Type C plug can be forced into a Type G socket without any converter at all if you ignore what your mother told you and stick a pen or similar pointy object into the center (ground) hole, which fools the socket into thinking a ground pin has been inserted and opens up the other holes. Disable power to the socket and try to use something non-conductive to do this! Forcing a type C plug into a type G socket will damage the socket and will probably get you into trouble with the owner of the socket.

There's one more complication to consider: any two-pin socket is ungrounded, while all three-pin plugs are grounded. Trying to get grounding to work makes life more difficult, as any of sockets C, D, E, F, H, J, K or L will happily accept the ungrounded plug C, but will not work with any grounded variant other than their own. It's thus very tempting to use an adapter to turn a three-pin into a two-pin, but this will disable grounding, potentially leaving you vulnerable to electrocution and other electrical nastiness.

A last word of warning: many developing countries use multi-plug sockets that accept (say) both Type A and Type C. Don't assume the voltage is correct just because the plug fits, since a Thai Type A+C socket still carries 220V and may destroy American (110V) Type A devices.

Technical details

The difference between a transformer and converter is the way they deal with the wave-form of the electricity. Converters simply chop the wave in half. This is relatively simple and can be done in a small amount of space, so converters are comparatively light-weight and inexpensive. Transformers alter the length of the wave. This is more complicated and takes up more space: transformers are basically chunks of iron specially-wrapped in wires. So they are larger, heavier and more expensive. Electric appliances can function with either a full or half-sine wave, whereas electronic devices must have a full sine wave.

If you are using a 220V-240V appliance at 110V you will need a transformer.

If you are using a 110V appliance at 220V-240V you can also use a transformer, but may be able to get away with a (cheaper) converter.

If your device is an electric appliance with a heating element or mechanical motor such as a an iron or hair-dryer, then you can probably just use a converter. If your device is electronic, using electronic chips or circuits, such as a computer, printer, TV, microwave, VCR or even a battery charger, you will need a transformer.

Transformers

  • They have two different types: "step-up" and "step-down". Step-up transformers allow you to plug a higher-voltage device into a lower-voltage power socket (e.g., using a UK device in the US.). Step-down transformers allow you to plug a lower-voltage device into a higher-voltage socket (e.g., using a US device in the UK.). Some transformers offer both. Take care to use the right type: if you plugged a 110-to-220 V step-up transformer into a 220 V socket you would get 440 V and a fried device.
  • You must also make sure that the power rating ("wattage") of your transformer is at least 10% greater than that of the device - otherwise the transformer can overheat and even catch fire. Before buying a transformer, look for the "input" figure - usually on the device's plug or in the manual. Some don't display wattage, but you can work it out simply by multiplying the voltage (V) and the current (amps (A); if mA, divide by 1,000). The resulting figure is the same as the wattage.
  • Transformers can be used with both electronic devices (e.g., those with chips and circuits) or electrical appliances (e.g., those with heating elements and motors). They can usually operate for a much longer time than converters.

Converters

These lighter-weight, less expensive devices can handle large wattage loads of up to 1600 watts, but they only step-down voltage, not raise it. They are suitable for those in 110-120V countries traveling to where the voltage is 220-240V. Converters are designed to operate for only an hour or two at a time, not continuously. As stated above, they cannot be used with electronic devices: devices that use chips or circuits, such as a computers, printers, VCRs or even battery chargers. A converter is normally given to a appliance that converts AC to DC.

Frequency (Hz)

Frequency is generally not a problem - most travel items will work on either 50 or 60 Hz. If all the electrical appliance does is produce heat or light, then the frequency is unlikely to matter.

Frequency is most likely to affect clocks and devices with motors. They may run faster or slower than they should and may be damaged in the long run as a result. Again, though, some motorised devices may function correctly on either 50 or 60 Hz - especially if they also operate on batteries. Just look on the label or plug.

However, you still may need to be careful if you have a sensitive or expensive device that converts AC (power from the wall) into DC (battery-like current) - especially if you also need to convert the voltage. A device will convert AC to DC to either : 1) save battery power by allowing you to plug into the mains or 2) to charge a battery in the device. The design of power supplies where AC is converted into DC does take frequency into account. Even though 60 Hz converts a little more easily to DC than 50 Hz does, there's enough tolerance in most small appliances and electronic gadgets that you can ignore frequency. However, if you also need to change the voltage (because the voltage of your device is different from the mains power voltage), you cannot use a switching-type converter. You must use the heavier iron-core transformer. If in doubt, consult a reputable electrical goods dealer.

If your device won't operate with a different frequency (powerful motors and non-quartz clocks), there is really nothing you can do to change it. Unlike voltage, frequency cannot easily be converted. Foreign embassies may have to use huge generators to provide current compatible with equipment from home.

If you desperately need to have power at your home country's frequency, you might try using a 12 volt DC to AC converter intended for vehicle use. However, most of these (especially those commonly found in stores) output a "sawtooth" wave instead of a sine wave. (Check the manufacturer's website if you need a sine wave output. It may be special order.) Make sure the wattage of the converter is sufficient for whatever device you need to operate, and the 12V battery has enough amps for the job. For example, 12V times 15 amps gives 180 watts (or less after losses are included).

Japan is a special case. East Japan (eg Tokyo) uses 50 Hz and west Japan (eg Osaka) uses 60 Hz. Equipment made for the Japanese market may have a switch to select 50 Hz or 60 Hz.

Unstable supply

In many developing countries, electrical supply is highly erratic and you need to take precautions to protect your equipment.

The main danger is power spikes, where the amount of power supplied temporarily surges to dangerous levels, with potentially catastrophic consequences. In developed countries, the main source of spikes is lightning strikes, but in developing countries they're most often associated with power outages since when the power comes back on, it rarely does so smoothly. The cheapest method of protection is thus simply to disconnect electronic devices as soon as the power goes out, and wait a few minutes after the power comes back on until plugging them back in.

Surge protectors are devices designed specially to protect against spikes and surges, and some are available in portable travel-sized versions. Some surge protectors can also be fitted to a telephone line to protect your phone or laptop modem. The most common variety use a metal oxide varistor (MOV), which shorts to ground if a given voltage is exceeded. These are easily destroyed by larger spikes, and better models will have a light indicating when the MOV has broken down, but you still need to keep an eye on them as the device will still continue to give power even if the protection is gone. There are also surge protectors with fuses, which are fail-safe (a blown fuse will stop power) and replaceable, but there still is a risk of a short, sharp spike which can pass through and damage your device before the fuse blows.

In some (mostly poor) regions, you may experience electricity voltage drops. Instead of 240V for example, you may only get 200V or even less (50% of the nominal supply voltage is not unknown). This happens especially if you're at "the end of the line" (far from the source or transformer) and is caused by the resistance of the electric lines themselves. Some appliances, such as light bulbs and heating equipment just keep working under a lower voltage, although a 20% voltage drop will cause a 36% power drop. Most electronic devices also keep working, but voltage drops are critical for fluorescent lamps and refrigerators, which may stop working altogether (usually without being damaged, when the voltage returns to normal, they will start working again).

Voltage drops can be solved with a special device called a voltage stabiliser. A stabiliser will raise the voltage again to its normal level. The principle is the same as for switching converters, except that stabilisers will produce a stable output, even with an unstable input. Stabilisers come in different power ranges, but they're all large, bulky and not practical to carry around. Be aware that some appliances, such as refrigerators, briefly consume twice or 3 times more power at start-up; the stabiliser should be able to provide this power. Voltage stabilisers can introduce surges if there is a power outage. The cheaper relay type can also damage electronic equipment.

Appliances

If you are buying new appliances, get in the habit of checking the voltage. A dual-voltage hair straightener will cost you no more than a single voltage one, and save considerable hassle when travelling.

Laptop computers

Virtually all laptop computers (including those with internal power supplies) will handle a range of 100 to 240 volts and a frequency of 50 to 60Hz fine. In other words, you might not need a converter/transformer; most power supplies have supported ranges printed directly on them (like on this image), so have a look. However, you will definitely need to make sure that you have the plug that matches the outlet for the country you are going to.

If you are taking a laptop, you can use it to charge other items using a USB port on the laptop, even if they are normally not connected to it - this can save you a bundle of transformers in your luggage.

Radios

Radios also tend to be interchangeable from country to country. The exact FM range being used can vary from country to country though, so you may not be able to access all stations. In the US, only odd channels (88.1,88.3, 100.1 etc) are used. A radio intended for the US market will not work correctly in most other countries. Japan, in particular, has an FM band from 76 MHz to 90 MHz rather than the more common 87.5 MHz to 108 MHz. The countries of the former Soviet Union also use a similar band. For the medium wave band, channel spacings (the difference between each valid frequency) can be 9kHz or 10kHz (for USA). Some digital radios will have a switch or setting to choose which channel spacing is used. Without this, they will not work correctly outside their intended market. Old-fashioned analog-dial tuners don't have this limitation.

Mobile phones and digital cameras

Chargers for these may work with both 110V and 240V systems, though you may still need an adaptor plug or have to use the shaver socket. You may be able to get a second charger for the other voltage system, or even a dual voltage charger designed for both systems. However, your mobile phone handset may not be compatible with the country's network, or you may be limited to certain cellular providers. (See Telephone service for travel#Cell phones.)

Equipment using standard batteries

Battery sizes and voltages tend to be standard from place to place, and equipment that uses off-the-shelf batteries tends to be interchangeable. It may be difficult to get good quality batteries in some countries, especially alkaline batteries which are needed my most electronic equipment. If a cheaper battery is used, make sure to remove it as soon as it is exhausted or if the equipment will not be used for a while (risk of leakage).

Be cautious with:

Generators

In many countries without fully developed electrical power distribution systems, the use of generators is common. Generator supplies can be very good, however, in many places they are not, and can cause damage to sensitive equipment if it is connected. The voltage, frequency, and waveform shape (it should be a smooth sine-wave) can vary. In some places, people modify generators to run faster. This gives more voltage and power but increases the frequency too. The part of a generator that keeps it running at a constant speed is called the governor. If this is tampered with, the output voltage could rise sufficiently to cause damage. The best advice is not to connect valuable equipment to the supply, or at the very least disconnect it as soon as it is finished with.

If you are unsure about the quality of generator in use, there are a few simple rules. If it runs from petrol/gasoline it is bad - anyone serious about using generator power uses a diesel oil powered system. A good quality generator will have a low engine speed. 1500RPM for 50Hz or 1800RPM for 60Hz. If the engine speed is 3000RPM+, it is not a good machine.

Lamps

Lamps and their light bulbs are very sensitive to voltage. If you shift between voltage systems, you will need to change the light bulbs to match the voltage, unless the lamp is designed to operate on both systems, say through a low voltage adaptor. If you buy a lamp abroad, you may need to have an electrician completely rewire a lamp when you get home to comply with your country's electrical safety standards. This may not be a problem for a one-off special item, but if you are going into the importing business it could be a showstopper.

Also watch out for the light bulb connection. In 110-120V systems this is often a screw connector while in 220-240V systems it is often a bayonet connector. These connectors also come in at least two different sizes. Be sure you can obtain light bulbs of the right voltage, size, and connector shape in the country you intend to use the lamp, and at a reasonable price, otherwise the lamp may become little more than junk when the bulb fails.

Electric motors

The electric motors in things like refrigerators, vacuum cleaners, washing machines and other whiteware are often sensitive to frequency. Older hairdryers and electric shavers might be also. Even if you use a step-up or step-down transformer, the different supply frequencies mean motors run at the wrong speed and burn out. The larger and more powerful the motor is, the more this is true. Don't, for example, bring a vacuum cleaner from the USA to Europe (or vice versa). It's almost guaranteed to fail -- even if you have a voltage converter.

Electric shavers

Hotels often provide a special electrical outlet specifically for electric shavers. They allow any voltage shaver to be plugged into them and be used safely in front of the bathroom mirror. They may also accept your cellphone adaptor or similar low power battery charging unit. Many – but not all – electric shavers sold today are dual voltage 50/60Hz and some will even recharge the battery at 12V DC (such as in an automobile). Check the label and instructions for compatibility.

Hairdryers

Hairdryers are a particular risk; if you accidentally plug your 100-120 Volt hairdryer into a 240 Volt outlet you may find it catching fire in your hands! Similarly a 220-240 Volt hairdryer in a 120 Volt outlet may run slowly and not heat up enough. Most good hotels and motels will be able to supply a hair drier, it may even be a room fitting. However it may be worthwhile buying or borrowing a hairdryer suited for the electrical system of countries you may be travelling in.

Many new hairdryers sold in 100-120V countries are dual voltage with settings for 100-120V and 220-240V. Even though it's motorized, it will work on either 50 or 60 Hz. Be sure to "lock out" the high setting when it's plugged into 220-240 volts, or it will quickly burn out, or worse! (The low setting will be as powerful as the high setting was at 120V, with low speed unavailable.)

Clocks

An electric clock of any sort is sensitive to voltage. If the voltage if doubled or halved it will not function and may burn out. Furthermore, the electric frequency (50 or 60 Hz) is used in cheap clocks - such as many clock radio style clocks - to keep the time. If the clock has a quartz crystal it uses this for the timekeeping and operates independently of the line frequency. Thus, if a clock made for North America were used in Europe – even with a voltage adapter – it would lose 10 minutes per hour! Obviously, not a great idea if you have a train to catch.

Video equipment

Televisions, many radios, video and DVD players, as well as videotapes, are often specific to the broadcast system used in the country that they are sold in, usually associated with the frequency of the country's electric current. For example, North America is 60 Hz and its television is 30 frames per second, while Europe is 50 Hz and its television is 25 frames per second. The main three television broadcast systems are PAL, the closest to a worldwide standard, NTSC, used mostly in the Americas and some East Asian countries (notably Japan, South Korea, and Taiwan) and SECAM, originally from France and adopted by much of Eastern Europe and the Middle East, but there are various incompatibilities even within these supposed standards. There is no difference between PAL and SECAM for unconverted DIGITAL video including DVDs. However, any analog output to a television set would be in the native format of the country of location. Brazil, Philippines uses a hybrid PAL/NTSC standard called "PAL-M". In Brazil, Philippines, DVDs and video tape are the same as NTSC (without region coding -- see below), but all players and TV sets are useless outside the country unless they have a separate NTSC setting.

Before purchasing any video equipment, read the manual and warranty carefully. For TVs and VCRs don't forget about cable television frequencies; they may not be the same, even if everything else is. Television sets often won't work correctly in another country from where they were sold, even if the voltage and video standard are the same. For example, a television set made for the USA will skip a few channels in Japan. Furthermore, many countries have or are in the process of switching to digital over-the-air broadcasting, ( dates by country). Unless you have an internationally compatible device you may find your expensive looking system is little more than worthless junk in another country because it won't work with your country's broadcast system. Your warranty is probably only valid in the country of purchase, and you may need to return the goods to the place you purchased them from.

The final problem with transporting TVs is that many European countries, notoriously the UK, require a license to watch any live TV (over-the-air, cable, satellite, & even live-streams on the internet). Fees can be hefty (in addition to being charged for the license).

DVD and Blu-Ray, infuriatingly, have completely artificial limitations introduced in the form of region coding, which attempts to limits the region where the discs can be used, as a technique to keep the various regions as separate markets. For example, a Region 1 player in North America will not play a Region 3 DVD from Hong Kong. The workaround are to obtain either a regionless DVD player which ignores the code, purchase multi-region discs (regions 1 & 3 in this case), or better yet, region 0 discs that can be played on any device.

Technically, there is no such thing as a NTSC or PAL DVD disc, as all color information is the same for both. When discs are labeled as such, what they're refering to is the picture size and frame rate (i.e. number of frames per second) that are used in most (but not all!) countries that have TV broadcasts on this same system. Many NTSC players cannot play PAL DVDs, unless that's a specific feature included (many Philips and JVC models include this). PAL DVD players are generally much better at playing NTSC, but it's not a certainty. If all else fails, a computer DVD-ROM can play any DVD movie, though there's a limit on how many times you can change the region code. Unlike analogue television sets, computer monitors can automatically handle both 25 (PAL & SECAM) and 30 (NTSC) frames per second, as well as various picture sizes. This also applies to LCD and plasma "flat panel" television sets, but don't expect their tuner to be compatible outside the country in which they were sold.

Videocameras can usually be charged with both electrical systems so you can record during travels and view it back home. Digital cameras and videocameras can usually output to both PAL, NTSC, and SECAM, so you can view your recording while travelling. Bring an RCA (yellow plug) to SCART adaptor if you plan to view video from a camcorder on a European television set.

VHS and other tape formats, while becoming out of date, still exist. There is no compatibility between NTSC and PAL. Professional conversions will probably cost most than the original. Some manufacturers, such as Philips and Samsung, make or did make several VHS VCR models that will play and record any foreign video format, though these machines are hard to find (try online and mail order) and relatively expensive. Also, you won't be able to make copies of tapes in the format that's not of your country if both the master and the needed copy(s) are of that foreign format. (Unless, of course, you're wealthy enough to afford two of these machines.) You could do a double conversion, but the quality will suffer greatly. One workaround would be to make a DVD in this foreign format, then play it back again to the multi-format VCR. You'll need a computer video capture card capable of both formats (many aren't), DVD production software reset to this foreign format, and a DVD player also capable of playing with an output (not just internal conversion!) of both formats (most aren't, except for the Philips brand) to play it back to the multi-format VCR. If you just want DVD copies, skip the last step of the DVD player playing it back to the VCR, and burn as many copies as you need.

Converting DVDs from one format to another if required, can be done on a computer, or you can get it done professionally. Regular blank discs and software work fine for making copies of a foreign format, as it's all just a bunch of one's and zero's and no different than copying anything else. However, as noted previously, converting it and playing it on a television set is another matter.

Stay safe

The first time you use electrical equipment on a voltage system you haven't used before, watch for excessive heat, strange smells, and smoke. This especially true for those residing in countries with 120V (USA, Canada, Japan, etc.) visiting places with the higher voltage. Smoke is a sure sign your equipment cannot cope with the voltage system.

If your electrical equipment gets very hot, smells of burning (there is a distinct smell of electrically fried circuit boards) or starts to smoke, turn it off at the wall or the main switch immediately, then carefully unplug the equipment. Do not disconnect or unplug by just grabbing the smoking device, its plug or cord, and then unplugging it, as these parts are probably very hot, and the insulation could be melted or unsafe, which could result in electrocution.

You may find your expensive equipment has been fried and needs to be replaced because the wrong voltage was used. However, if the equipment only got hot and did not smoke or produce strange burning smells you may be lucky. Some older devices have fuses that you may be able to replace. New devices, such as gaming consoles, will trip a circuit breaker. Disconnect them from all power and leave them for 60 minutes or so, and the circuit breaker will normally reset.

Do not rely on fuses to protect your equipment. If a fuse does blow, you should have things checked by an electrician before using the suspect equipment again.

In third-world countries with frequent blackouts, it's not at all uncommon for a visitor to plug something in, and have the power go out coincidentially. Always check the neighborhood first, before blaming the appliance or looking at the fuse/circuit breaker.

This is a guide article. It has good, detailed information covering the entire topic. Plunge forward and help us make it a star!

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

ELECTRICITY. This article is devoted to a general sketch of the history of the development of electrical knowledge on both the theoretical and the practical sides. The two great branches of electrical theory which concern the phenomena of electricity at rest, or " frictional " or " static " electricity, and of electricity in motion, or electric currents, are treated in two separate articles, Electrostatics and Electrokinetics. The phenomena attendant on the passage of electricity through solids, through liquids and through gases, are described in the article Electric conduction, and also Electrolysis, and the propagation of electrical vibrations in Electric Waves. The interconnexion of magnetism (which has an article to itself) and FIG. 8. - Action of the Wimshurst Machine.

electricity is discussed in Electromagnetism, and these manifestations in nature in Atmospheric Electricity; Aurora Polaris and Terrestrial magnetism. The general principles of electrical engineering will be found in Electricity Supply, and further details respecting the generation and use of electrical power are given in such articles as Dynamo; Motors, Electric; Transformers; Accumulator; Power Transmission: Electric; Traction; Lighting: Electric; Electrochemistry and Electrometallurgy. The principles of telegraphy (land, submarine and wireless) and of telephony are discussed in the articles Telegraph and Telephone, and various electrical instruments are treated in separate articles such as Amperemeter; Electrometer; Galvanometer; Voltmeter; Wheatstone'S Bridge; Potentiometer; Meter, Electric; Electrophorus; Leyden Jar; &C.

The term " electricity " is applied to denote the physical agency which exhibits itself by effects of attraction and repulsion when particular substances are rubbed or heated, also in certain chemical and physiological actions and in connexion with moving magnets and metallic circuits. The name is derived from the word electrica, first used by William Gilbert (1544-1603) in his epoch-making treatise De magnete, magneticisque corporibus, et de magno magnete tellure, published in 1600, 1 to denote substances which possess a similar property to amber (= electrum, from iiXecrpov) of attracting light objects when rubbed. Hence the phenomena came to be collectively called electrical, a term first used by William Barlowe, archdeacon of Salisbury, in 1618, and the study of them, electrical science.

Historical Sketch. Gilbert was the first to conduct systematic scientific experiments on electrical phenomena. Prior to his date the scanty knowledge possessed by the ancients and enjoyed in the middle ages began and ended with facts said to have been familiar to Thales of Miletus (600 B.C.) and mentioned by Theophrastus (321 B.C.) and Pliny (A.D. 70), namely, that amber, jet and one or two other substances possessed the power, when rubbed, of attracting fragments of straw, leaves or feathers. Starting with careful and accurate observations on facts concerning the mysterious properties of amber and the lodestone, Gilbert laid the foundations of modern electric and magnetic science on the true experimental and inductive basis. The subsequent history of electricity may be divided into four well-marked periods. The first extends from the date of publication of Gilbert's great treatise in 1600 to the invention by Volta of the voltaic pile and the first production of the electric current in 1799. The second dates from Volta's discovery to the discovery by Faraday in 1831 of the induction of electric currents and the creation of currents by the motion of conductors in magnetic fields, which initiated the era of modern electrotechnics. The third covers the period between 1831 and Clerk Maxwell's enunciation of the electromagnetic theory of light in 1865 and the invention of the self-exciting dynamo, which marks another great epoch in the development of the subject; and the fourth comprises the modern development of electric theory and of absolute quantitative measurements, and above all, of the applications of this knowledge in electrical engineering. We shall sketch briefly the historical progress during these various stages, and also the growth of electrical theories of electricity during that time.

First Period. - Gilbert was probably led to study the phenomena of the attraction of iron by the lodestone in consequence of his conversion to the Copernican theory of the earth's motion, and thence proceeded to study the attractions produced by amber. An account of his electrical discoveries is given in the De magnete, lib. ii. cap. 2.2 He invented the versorium or 1 Gilbert's work, On the Magnet, Magnetic Bodies and the Great Magnet, the Earth, has been translated from the rare folio Latin edition of 1600, but otherwise reproduced in its original form by the chief members of the Gilbert Club of England, with a series of valuable notes by Prof. S. P. Thompson (London, 1900). See also The Electrician, February 21, 1902.

2 See The Intellectual Rise in Electricity, ch. x., by Park Benjamin (London, 1895).

electrical needle and proved that innumerable bodies he called electrica, when rubbed, can attract the needle of the versorium (see Electroscope). Robert Boyle added many new facts and gave an account of them in his book, The Origin of Electricity. He showed that the attraction between the rubbed body and the test object is mutual. Otto von Guericke (1602-1686) constructed the first electrical machine with a revolving ball of sulphur (see Electrical Machine), and noticed that light objects were repelled after being attracted by excited electrics. Sir Isaac Newton substituted a ball of glass for sulphur in the electrical machine and made other not unimportant additions to electrical knowledge. Francis Hawksbee (d. 1713) published in his book Physico-Mechanical Experiments (1709), and in several Memoirs in the Phil. Trans. about 1707, the results of his electrical inquiries. He showed that light was produced when mercury was shaken up in a glass tube exhausted of its air. Dr Wall observed the spark and crackling sound when warm amber was rubbed, and compared them with thunder and lightning (Phil. Trans., 1708, 26, p. 69). Stephen Gray (1696-1736) noticed in 1720 that electricity could be excited by the friction of hair, silk, wool, paper and other bodies. In 1729 Gray made the important discovery that some bodies were conductors and others nonconductors of electricity. In conjunction with his friend Granville Wheeler (d. 1770), he conveyed the electricity from rubbed glass, a distance of 886 ft., along a string supported on silk threads (Phil. Trans., 1 7351 73 6, 39, pp. 16, 166 and 400). Jean Theophile Desaguliers (1683-1744) announced soon after that electrics were non-conductors, and conductors were nonelectrics. C. F. de C. du Fay (1699-1739) made the great discovery that electricity is of two kinds, vitreous and resinous (Phil. Trans., 1733, 38, p. 263), the first being produced when glass, crystal, &c. are rubbed with silk, and the second when resin, amber, silk or paper, &c. are excited by friction with flannel. He also discovered that a body charged with positive or negative electricity repels a body free to move when the latter is charged with electricity of like sign, but attracts it if it is charged with electricity of opposite sign, i.e. positive repels positive and negative repels negative, but positive attracts negative. It is to du Fay also that we owe the abolition of the distinction between electrics and non-electrics. He showed that all substances could be electrified by friction, but that to electrify conductors they must be insulated or supported on non-conductors. Various improvements were made in the electrical machine, and thereby experimentalists were provided with the means of generating strong electrification; C. F. Ludolff (1707-1763) of Berlin in 1744 succeeded in igniting ether with the electric spark (Phil. Trans., 1 744, 43, p. 167).

For a very full list of the papers and works of these early electrical philosophers, the reader is referred to the bibliography on Electricity in Dr Thomas Young's Natural Philosophy, vol. ii. p. 415.

In 1745 the important invention of the Leyden jar or condenser was made by E. G. von Kleist of Kammin, and almost simultaneously by Cunaeus and Pieter van Musschenbroek (1692-1761) of Leiden (see Leyden Jar). Sir William Watson (1715-1787) in England first observed the flash of light when a Leyden jar is discharged, and he and Dr John Bevis (1695-1771) suggested coating the jar inside and outside with tinfoil. Watson carried out elaborate experiments to discover how far the electric discharge of the jar could be conveyed along metallic wires and was able to accomplish it for a distance of 2 m., making the important observation that the electricity appeared to be transmitted instantaneously.

Table of contents

Franklin's Researches

Benjamin Franklin (1706-1790) was one of the great pioneers of electrical science, and made the evermemorable experimental identification of lightning and electric spark. He argued that electricity is not created by friction, but merely collected from its state of diffusion through other matter by which it is attracted. He asserted that the glass globe, when rubbed, attracted the electrical fire, and took it from the rubber, the same globe being disposed, when the friction ceases, to give out its electricity to any body which has less. In the case of the charged Leyden jar, he asserted that the inner coating of tinfoil had received more than its ordinary quantity of electricity, and was therefore electrified positively, or plus, while the outer coating of tinfoil having had its ordinary quantity of electricity diminished, was electrified negatively, or minus. Hence the cause of the shock and spark when the jar is discharged, or when the superabundant or plus electricity of the inside is transferred by a conducting body to the defective or minus electricity of the outside. This theory of the Leyden phial Franklin supported very ingeniously by showing that the outside and the inside coating possessed electricities of opposite sign, and that, in charging it, exactly as much electricity is added on one side as is subtracted from the other. The abundant discharge of electricity by points was observed by Franklin is his earliest experiments, and also the power of points to conduct it copiously from an electrified body. Hence he was furnished with a simple method of collecting electricity from other bodies, and he was enabled to perform those remarkable experiments which are chiefly connected with his name. Hawksbee, Wall and J. A. Nollet (1700-1770) had successively suggested the identity of lightning and the electric spark, and of thunder and the snap of the spark. Previously to the year 1750, Franklin drew up a statement, in which he showed that all the general phenomena and effects which were produced by electricity had their counterparts in lightning. After waiting some time for the erection of a spire at Philadelphia, by means of which he hoped to bring down the electricity of a thunderstorm, he conceived the idea of sending up a kite among thunder-clouds. With this view he made a small cross of two small light strips of cedar, the arms being sufficiently long to reach to the four corners of a large thin silk handkerchief when extended. The corners of the handkerchief were tied to the extremities of the cross, and when the body of the kite was thus formed, a tail, loop and string were added to it. The body was made of silk to enable it to bear the violence and wet of a thunderstorm. A very sharp pointed wire was fixed at the top of the upright stick of the cross, so as to rise a foot or more above the wood. A silk ribbon was tied to the end of the twine next the hand, and a key suspended at the junction of the twine and silk. In company with his son, Franklin raised the kite like a common one, in the first thunderstorm, which happened in the month of June 1752. To keep the silk ribbon dry, he stood within a door, taking care that the twine did not touch the frame of the door; and when the thunder-clouds came over the kite he watched the state of the string. A cloud passed without any electrical indications, and he began to despair of success. At last, however, he saw the loose filaments of the twine standing out every way, and he found them to be attracted by the approach of his finger. The suspended key gave a spark on the application of his knuckle, and when the string had become wet with the rain the electricity became abundant. A Leyden jar was charged at the key, and by the electric fire thus obtained spirits were inflamed, and many other experiments performed which had been formerly made by excited electrics. In subsequent trials with another apparatus, he found that the clouds were sometimes positively and sometimes negatively electrified, and so demonstrated the perfect identity of lightning and electricity. Having thus succeeded in drawing the electric fire from the clouds, Franklin conceived the idea of protecting buildings from lightning by erecting on their highest parts pointed iron wires or conductors communicating with the ground. The electricity of a hovering or a passing cloud would thus be carried off slowly and silently; and if the cloud was highly charged, the lightning would strike in preference the elevated conductors.' The most important of Franklin's electrical writings are his Experiments and Observations on Electricity made at Philadelphia, 1 75 1 - 1 754; his Letters on Electricity; and various memoirs and letters in the Phil. Trans. from 1756 to 1760.

About the same time that Franklin was making his kite 1 See Sir Oliver Lodge, " Lightning, Lightning Conductors and Lightning Protectors," Journ. Inst. Elec. Eng. (1889), 18, p. 386, and the discussion on the subject in the same volume; also the book by the same author on Lightning Conductors and Lightning Guards (London, 1892).

experiment in America, T. F. Dalibard (1703-1779) and others in France had erected a long iron rod at Mar11, and obtained results agreeing with those of Franklin. Similar investigations were pursued by many others, among whom Father G. B. Beccaria (1716-1781) deserves especial mention. John Canton (1718-1772) made the important contribution to knowledge that electricity of either sign could be produced on nearly any body by friction with appropriate substances, and that a rod of glass roughened on one half was excited negatively in the rough part and positively in the smooth part by friction with the same rubber. Canton first suggested the use of an amalgam of mercury and tin for use with glass cylinder electrical machines to improve their action. His most important discovery, however, was that of electrostatic induction, the fact that one electrified body can produce charges of electricity upon another insulated body, and that when this last is touched it is left electrified with a charge of opposite sign to that of the inducing charge (Phil. Trans., 1 7531 754). We shall make mention lower down of Canton's contributions to electrical theory. Robert Symmer (d. 1763) showed that quite small differences determined the sign of the electrification that was generated by the friction of two bodies one against the other. Thus wearing a black and a white silk stocking one over the other, he found they were electrified oppositely when rubbed and drawn off, and that such a rubbed silk stocking when deposited in a Leyden jar gave up its electrification to the jar (Phil. Trans., 1 759). Ebenezer Kinnersley (1711-1778) of Philadelphia made useful observations on the elongation and fusion of iron wires by electrical discharges (Phil. Trans., 1763). A contemporary of Canton and co-discoverer with him of the facts of electrostatic induction was the Swede, Johann Karl Wilcke (1732-1796), then resident in Germany, who in 1762 published an account of experiments in which a metal plate held above the upper surface of a glass table was subjected to the action of a charge on an electrified metal plate held below the glass (Kon. Schwedische Akad. Abhandl., 1762, 24, p. 213).

Pyro-electricity

The subject of pyro-electricity, or the power possessed by some minerals of becoming electrified when merely heated, and of exhibiting positive and negative electricity, now began to attract notice. It is possible that the lyncurium of the ancients, which according to Theophrastus attracted light bodies, was tourmaline, a mineral found in Ceylon, which had been christened by the Dutch with the name of aschentrikker, or the attractor of ashes. In 1717 Louis Lemery exhibited to the Paris Academy of Sciences a stone from Ceylon which attracted light bodies; and Linnaeus in mentioning his experiments gives the stone the name of lapis electricus. Giovanni Caraffa, duca di Noja (1715-1768), was led in 1758 to purchase some of the stones called tourmaline in Holland, and, assisted by L. J. M. Daubenton and Michel Adanson, he made a series of experiments with them, a description of which he gave in a letter to G. L. L. Buffon in 1759. The subject, however, had already engaged the attention of the German philosopher, F. U. T. Aepinus, who published an account of them in 1756. Hitherto nothing had been said respecting the necessity of heat to excite the tourmaline; but it was shown by Aepinus that a temperature between 992° and 212° Fahr. was requisite for the development of its attractive powers. Benjamin Wilson (Phil. Trans., 1763, &c.), J. Priestley, and Canton continued the investigation, but it was reserved for the Abbe flatly to throw a clear light on this curious branch of the science (Traite de mineralogie, 1801). He found that the electricity of the tourmaline decreased rapidly from the summits or poles towards the middle of the crystal, where it was imperceptible; and he discovered that if a tourmaline is broken into any number of fragments, each fragment, when excited, has two opposite poles. flatly discovered the same property in the Siberian and Brazilian topaz, borate of magnesia, mesotype, prehnite, sphene and calamine. He also found that the polarity which minerals receive from heat has a relation to the secondary forms of their crystals - the tourmaline, for example, having its resinous pole at the summit of the crystal which has three faces. In the other pyro-electric crystals above mentioned, Hatly detected the same deviation from the rules of symmetry in their secondary crystals which occurs in tourmaline. C. P. Brard (1788-1838) discovered that pyro-electricity was a property of axinite; and it was afterwards detected in other minerals. In repeating and extending the experiments of Haiiy much later, Sir David Brewster discovered that various artificial salts were pyro-electric, and he mentions the tartrates of potash and soda and tartaric acid as exhibiting this property in a very strong degree. He also made many experiments with the tourmaline when cut into thin slices, and reduced to the finest powder, in which state each particle preserved its pyro-electricity; and he showed that scolezite and mesolite, even when deprived of their water of crystallization and reduced to powder, retain their property of becoming electrical by heat. When this white powder is heated and stirred about by any substance whatever, it collects in masses like new-fallen snow, and adheres to the body with which it is stirred.

For Sir David Brewster's work on pyro-electricity, see Trans. Roy. Soc. Edin., 1845, also Phil. Meg., Dec. 1847. The reader will also find a full discussion on the subject in the Treatise on Electricity, by A. de la Rive, translated by C. V. Walker (London, 1856), vol. ii. part v. ch. i.

Animal electricity

The observation that certain animals could give shocks resembling the shock of a Leyden jar induced a closer examination of these powers. The ancients were acquainted with the benumbing power of the torpedo-fish, but it was not till 1676 that modern naturalists had their attention again drawn to the fact. E. Bancroft was the first person who distinctly suspected that the effects of the torpedo were electrical.

In 1773 John Walsh (d. 1795) and Jan Ingenhousz (1730-1799) proved by many curious experiments that the shock of the torpedo was an electrical one (Phil. Trans., 1 7731 775); and John Hunter (id. 1 773, 1 775) examined and described the anatomical structure of its electrical organs. A. von Humboldt and Gay-Lussac (Ann. Chim., 1805), and Etienne Geoffroy SaintHilaire (Gilb. Ann., 1803) pursued the subject with success; and Henry Cavendish (Phil. Trans., 1776) constructed an artificial torpedo, by which he imitated the actions of the living animal. The subject was also investigated (Phil. Trans., 1812, 1817) by Dr T. J. Todd (1789-1840), Sir Humphry Davy (id. 1829), John Davy (id. 1832, 1834, 1841) and Faraday (Exp. Res., vol. ii.). The power of giving electric shocks has been discovered also in the Gymnotus electricus (electric eel), the Malapterurus electricus, the Trichiurus electricus, and the Tetraodon electricus. The most interesting and the best known of these singular fishes is the Gymnotus or Surinam eel. Humboldt gives a very graphic account of the combats which are carried on in South America between the gymnoti and the wild horses in the vicinity of Calabozo.

Cavendish's Researches

The work of Henry Cavendish (1731-1810) entitles him to a high place in the list of electrical investigators. A considerable part of Cavendish's work was rescued from oblivion in 1879 and placed in an easily accessible form by Professor Clerk Maxwell, who edited the original manuscripts in the possession of the duke of Devonshire.' Amongst Cavendish's important contributions were his exact measurements of electrical capacity. The leading idea which distinguishes his work from that of his predecessors was his use of the phrase " degree of electrification " with a clear scientific definition which shows it to be equivalent in meaning to the modern term " electric potential." Cavendish compared the capacity of different bodies with those of conducting spheres of known diameter and states these capacities in " globular inches," a globular inch being the capacity of a sphere 1 in. in diameter. Hence his measurements are all directly comparable with modern electrostatic measurements in which the unit of capacity is that of a sphere r centimetre in radius. Cavendish measured the capacity of disks and condensers of various forms, and proved that the capacity of a Leyden pane is proportional to the surface of the tinfoil and inversely as the thickness of the glass. In connexion with this subject he anticipated one of Faraday's The Electrical Researches of the Hon. Henry Cavendish 1771-1781, edited from the original manuscripts by J. Clerk Maxwell, F.R.S. (Cambridge, 1879).

greatest discoveries, namely, the effect of the dielectric or in- sulator upon the capacity of a condenser formed with it, in other words, made the discovery of specific inductive capacity (see Electrical Researches, p. 183). He made many measurements of the electric conductivity of different solids and liquids, by comparing the intensity of the electric shock taken through his body and various conductors. He seems in this way to have educated in himself a very precise " electrical sense," making use of his own nervous system as a kind of physiological galvanometer. One of the most important investigations he made in this way was to find out, as he expressed it, " what power of the velocity the resistance is proportional to." Cavendish meant by the term " velocity " what we now call the current, and by " resistance " the electromotive force which maintains the current. By various experiments with liquids in tubes he found this power was nearly unity. This result thus obtained by Cavendish in January 1781, that the current varies in direct proportion to the electromotive force, was really an anticipation of the fundamental law of electric flow, discovered independently by G. S. Ohm in 1827, and since known as Ohm's Law. Cavendish also enunciated in 1776 all the laws of division of electric current between circuits in parallel, although they are generally supposed to have been first given by Sir C. Wheatstone. Another of his great investigations was the determination of the law according to which electric force varies with the distance. Starting from the fact that if an electrified globe, placed within two hemispheres which fit over it without touching, is brought in contact with these hemispheres, it gives up the whole of its charge to them - in other words, that the charge on an electrified body is wholly on the surface - he was able to deduce by most ingenious reasoning the law that electric force varies inversely as the square of the distance. The accuracy of his measurement, by which he established within 2% the above law, was only limited by the sensibility, or rather insensibility, of the pith ball electrometer, which was his only means of detecting the electric charge.2 In the accuracy of his quantitative measurements and the range of his researches and his combination of mathematical and physical knowledge, Cavendish may not inaptly be described as the Kelvin of the 18th century. Nothing but his curious indifference to the publication of his work prevented him from securing earlier recognition for it.

Coulomb's Work. - Contemporary with Cavendish was C. A. Coulomb (1736-1806), who in France addressed himself to the same kind of exact quantitative work as Cavendish in England. Coulomb has made his name for ever famous by his invention and application of his torsion balance to the experimental verification of the fundamental law of electric attraction, in which, however, he was anticipated by Cavendish, namely, that the force of attraction between two small electrified spherical bodies varies as the product of their charges and inversely as the square of the distance of their centres. Coulomb's work received better publication than Cavendish's at the time of its accomplishment, and provided a basis on which mathematicians could operate. Accordingly the close of the 18th century drew into the arena of electrical investigation on its mathematical side P. S. Laplace, J. B. Biot, and above all, S. D. Poisson. Adopting the hypothesis of two fluids, Coulomb investigated experimentally and theoretically the distribution of electricity on the surface of bodies by means of his proof plane. He determined the law of distribution between two conducting bodies in contact; and measured with his proof plane the density of the electricity at different points of two spheres in contact, and enunciated an important law. He ascertained the distribution of electricity among several spheres (whether equal or unequal) placed in contact in a straight line; and he measured the distribution of 2 In 1878 Clerk Maxwell repeated Cavendish's experiments with improved apparatus and the employment of a Kelvin quadrant electrometer as a means of detecting the absence of charge on the inner conductor after it had been connected to the outer case, and was thus able to show that if the law of electric attraction varies inversely as the nth power of the distance, then the exponent n must have a value of 2 t Isua. See Cavendish's Electrical Researches,. P. 419.

electricity on the surface of a cylinder, and its distribution between a sphere and cylinder of different lengths but of the same diameter. His experiments on the dissipation of electricity possess also a high value. He found that the momentary dissipation was proportional to the degree of electrification at the time, and that, when the charge was moderate, its dissipation was not altered in bodies of different kinds or shapes. The temperature and pressure of the atmosphere did not produce any sensible change; but he concluded that the dissipation was nearly proportional to the cube of the quantity of moisture in the air.' In examining the dissipation which takes place along imperfectly insulating substances, he found that a thread of gum-lac was the most perfect of all insulators; that it insulated ten times as well as a dry silk thread; and that a silk thread covered with fine sealing-wax insulated as powerfully as gum-lac when it had four times its length. He found also that the dissipation of electricity along insulators was chiefly owing to adhering moisture, but in some measure also to a slight conducting power. For his memoirs 'see' de math. et phys. de l'acad. de sc., 1785, &c.

Second Period. - We now enter upon the second period of electrical research inaugurated by the epoch-making discovery of Alessandro Volta (1745-1827). L. Galvani had made in 1790 his historic observations on the muscular contraction produced in the bodies of recently killed frogs when an electrical machine was being worked in the same room, and described them in 1791 (De viribus electricitatis in motu musculari commentarius, Bologna, 1791). Volta followed up these observations with rare philosophic insight and experimental skill. He showed that all conductors liquid and solid might be divided into two classes which he called respectively conductors of the first and of the second class, the first embracing metals and carbon in its conducting form, and the second class, water, aqueous solutions of various kinds, and generally those now called electrolytes. In the case of conductors of the first class he proved by the use of the condensing electroscope, aided probably by some form of multiplier or doubler, that a difference of potential (see Electrostatics) was created by the mere contact of two such conductors, one of them being positively electrified and the other negatively. Volta showed, however, that if a series of bodies of the first class, such as disks of various metals, are placed in contact, the potential difference between the first and the last is just the same as if they are immediately in contact. There is no accumulation of potential. If, however, pairs of metallic disks, made, say, of zinc and copper, are alternated with disks of cloth wetted with a conductor of the second class, such, for instance, as dilute acid or any electrolyte, then the effect of the feeble potential difference between one pair of copper and zinc disks is added to that of the potential difference between the next pair, and thus by a sufficiently long series of pairs any required difference of potential can be accumulated.

The Voltaic Pile

This led him about 1799 to devise his famous voltaic pile consisting of disks of copper and zinc or other metals with wet cloth placed between the pairs. Numerous examples of Volta's original piles at one time existed in Italy, and were collected together for an exhibition held at Como in 1899, but were unfortunately destroyed by a disastrous fire on the 8th of July 1899. Volta's description of his pile was communicated in a letter to Sir Joseph Banks, president of the Royal Society of London, on the 10th of March 1800, and was printed in the Phil. Trans., vol. 90, pt. I, p. 405. It was then found that when the end plates of Volta's pile were connected to an electroscope the leaves diverged either with positive or negative electricity. Volta also gave his pile another form, the couronne des tasses (crown of cups), in which connected strips of copper and zinc were used to bridge between cups of water or dilute acid. Volta then proved that all metals could be arranged in an electromotive 1 Modern researches have shown that the loss of charge is in fact dependent upon the ionization of the air, and that, provided the atmospheric moisture is prevented from condensing on the insulating supports, water vapour in the air does not per se bestow on it conductance for electricity.

series such that each became positive when placed in contact with the one next below it in the series. The origin of the electromotive force in the pile has been much discussed, and Volta's discoveries gave rise to one of the historic controversies of science. Volta maintained that the mere contact of metals was sufficient to produce the electrical difference of the end plates of the pile. The discovery that chemical action was involved in the process led to the advancement of the chemical theory of the pile and this was strengthened by the growing insight into the principle of the conservation of energy. In 1851 Lord Kelvin (Sir W. Thomson), by the use of his then newly-invented electrometer, was able to confirm Volta's observations on contact electricity by irrefutable evidence, but the contact theory of the voltaic pile was then placed on a basis consistent with the principle of the conservation of energy. A. A. de la Rive and Faraday were ardent supporters of the chemical theory of the pile, and even at the present time opinions of physicists can hardly be said to be in entire accordance as to the source of the electromotive force in a voltaic couple or pile.2 Improvements in the form of the voltaic pile were almost immediately made by W. Cruickshank (1745-1800), Dr W. H. Wollaston and Sir H. Davy, and these, together with other eminent continental chemists, such as A. F. de Fourcroy, L. J. Thenard and J. W. Ritter (1776-1810), ardently prosecuted research with the new instrument. One of the first discoveries made with it was its power to electrolyse or chemically decompose certain solutions. William Nicholson (1753-1815) and Sir Anthony Carlisle (1768-1840) in 1800 constructed a pile of silver and zinc plates, and placing the terminal wires in water noticed the evolution from these wires of bubbles of gas, which they proved to be oxygen and hydrogen. These two gases, as Cavendish and James Watt had shown in 1784, were actually the constituents of water. From that date it was clearly recognized that a fresh implement of great power had been given to the chemist. Large voltaic piles were then constructed by Andrew Crosse (1784-1855) and Sir H. Davy, and improvements initiated by Wollaston and Robert Hare (1781-1858) of Philadelphia. In 1806 Davy communicated to the Royal Society of London a celebrated paper on some " Chemical Agencies of Electricity," and after providing himself at the Royal Institution of London with a battery of several hundred cells, he announced in 1807 his great discovery of the electrolytic decomposition of the alkalis, potash and soda, obtaining therefrom the metals potassium and sodium. In July 1808 Davy laid a request before the managers of the Royal Institution that they would set on foot a subscription for the purchase of a specially large voltaic battery; as a result he was provided with one of 2000 pairs of plates, and the first experiment performed with it was the production of the electric arc light between carbon poles. Davy followed up his initial work with a long and brilliant series of electrochemical investigations described for the most part in the Phil. Trans. of the Royal Society.

Magnetic Action of Electric Current. - Noticing an analogy between the polarity of the voltaic pile and that of the magnet, philosophers had long been anxious to discover a relation between the two, but twenty years elapsed after the invention of the pile before Hans Christian Oersted (1777-1851), professor of natural philosophy in the university of Copenhagen, made in 1819 the discovery which has immortalized his name. In the Annals of Philosophy (1820, 16, p. 273) is to be found an English translation of Oersted's original Latin essay (entitled " Experiments on the Effect of a Current of Electricity on the Magnetic Needle "), dated the 21st of July 1820, describing his discovery. In it Oersted describes the action he considers is taking place around 2 Faraday discussed the chemical theory of the pile and arguments in support of it in the 8th and 16th series of his Experimental Researches on Electricity. De la Rive reviews the subject in his large Treatise on Electricity and Magnestism, vol. ii. ch. iii. The writer made a contribution to the discussion in 1874 in a paper on " The Contact Theory of the Galvanic Cell," Phil. Mag., 18 74, 47, p. 401. Sir Oliver Lodge reviewed the whole position in a paper in 1885, " On the Seat of the Electromotive Force in a Voltaic Cell," Journ. Inst. Elec. Eng., 1885, 14, p. 186.

the conductor joining the extremities of the pile; he speaks of it as the electric conflict, and says: " It is sufficiently evident that the electric conflict is not confined to the conductor, but is dispersed pretty widely in the circumjacent space. We may likewise conclude that this conflict performs circles round the wire, for without this condition it seems impossible that one part of the wire when placed below the magnetic needle should drive its pole to the east, and when placed above it, to the west." Oersted's important discovery was the fact that when a wire joining the end plates of a voltaic pile is held near a pivoted magnet or compass needle, the latter is deflected and places itself more or less transversely to the wire, the direction depending upon whether the wire is above or below the needle, and on the manner in which the copper or zinc ends of the pile are connected to it. It is clear, moreover, that Oersted clearly recognized the existence of what is now called the magnetic field round the conductor. This discovery of Oersted, like that of Volta, stimulated philosophical investigation in a high degree.

Electrodynamics

On the 2nd of October 1820, A. M. Ampere presented to the French Academy of Sciences an important memoir,' in which he summed up the results of his own and D. F. J. Arago's previous investigations in the new science of electromagnetism, and crowned that labour by the announcement of his great discovery of the dynamical action between conductors conveying the electric currents. Ampere in this paper gave an account of his discovery that conductors conveying electric currents exercise a mutual attraction or repulsion on one another, currents flowing in the same direction in parallel conductors attracting, and those in opposite directions repelling. Respecting this achievement when developed in its experimental and mathematical completeness, Clerk Maxwell says that it was " perfect in form and unassailable in accuracy." By a series of well-chosen experiments Ampere established the laws of this mutual action, and not only explained observed facts by a brilliant train of mathematical analysis, but predicted others subsequently experimentally realized. These investigations led him to the announcement of the fundamental law of action between elements of current, or currents in infinitely short lengths of linear conductors, upon one another at a distance; summed up in compact expression this law states that the action is proportional to the product of the current strengths of the two elements, and the lengths of the two elements, and inversely proportional to the square of the distance between the two elements, and also directly proportional to a function of the angles which the line joining the elements makes with the directions of the two elements respectively. Nothing is more remarkable in the history of discovery than the manner in which Ampere seized upon the right clue which enabled him to disentangle the complicated phenomena of electrodynamics and to deduce them all as a consequence of one simple fundamental law, which occupies in electrodynamics the position of the Newtonian law of gravitation in physical astronomy.

In 1821 Michael Faraday (1791-1867), who was destined later on to do so much for the science of electricity, discovered electromagnetic rotation, having succeeded in causing a wire conveying a voltaic current to rotate continuously round the pole of a permanent magnet. 2 This experiment was repeated in a variety of forms by A. A. De la Rive, Peter Barlow (1776-1862), William Ritchie (1 790-1837), William Sturgeon (1783-1850), and others; and Davy (Phil. Trans., 1823) showed that when two wires connected with the pole of a battery were dipped into a cup of mercury placed on the pole of a powerful magnet, the fluid rotated in opposite directions about the two electrodes.

Electromagnetism

In 1820 Arago (Ann. Chim. Phys., 1820, 1 5, p. 94) and Davy (Annals of Philosophy, 1821) discovered independently the power of the electric current to magnetize 1 " Memoire sur la theorie mathematique des phenomenes electrodynamiques," Memoires de l'institut, 1820, 6; see also Ann. de Chim., 1820, 15.

z See M. Faraday, " On some new Electro-Magnetical Motions and on the Theory of Magnetism," Quarterly Journal of Science, 1822, 12, p. 74; or Experimental Researches on Electricity, vol. ii. p. 127.

iron and steel. Felix Savary (1797-1841) made some very curious observations in 1827 on the magnetization of steel needles placed at different distances from a wire conveying the discharge of a Leyden jar (Ann. Chim. Phys., 1827, 34). W. Sturgeon in 1824 wound a copper wire round a bar of iron bent in the shape of a horseshoe, and passing a voltaic current through the wire showed that the iron became powerfully magnetized as long as the connexion with the pile was maintained (Trans. Soc. Arts, 1825). These researches gave us the electromagnet, almost as potent an instrument of research and invention as the pile itself (see Electromagnetism).

Ampere had already previously shown that a spiral conductor or solenoid when traversed by an electric current possesses magnetic polarity, and that two such solenoids act upon one another when traversed by electric currents as if they were magnets. Joseph Henry, in the United States, first suggested the construction of what were then called intensity electromagnets, by winding upon a horseshoe-shaped piece of soft iron many superimposed windings of copper wire, insulated by covering it with silk or cotton, and then sending through the coils the current from a voltaic battery. The dependence of the intensity of magnetization on the strength of the current was subsequently investigated (Pogg. Ann. Phys., 18 39, 47) by H. F. E. Lenz (1804-1865) and M. H. von Jacobi (1801-1874). J. P. Joule found that magnetization did not increase proportionately with the current, but reached a maximum (Sturgeon's Annals of Electricity, 1839, 4). Further investigations on this subject were carried on subsequently by W. E. Weber (1804-1891), J. H. J. Miller (1809-1875), C. J. Dub (1817-1873), G. H. Wiedemann (1826-1899), and others, and in modern times by H. A. Rowland (1848-1901), Shelford Bidwell (b. 1848), John Hopkinson (1849-1898), J. A. Ewing (b. 1855) and many others. Electric magnets of great power were soon constructed in this manner by Sturgeon, Joule, Henry, Faraday and Brewster. Oersted's discovery in 1819 was indeed epoch-making in the degree to which it stimulated other research. It led at once to the construction of the galvanometer as a means of detecting and measuring the electric current in a conductor. In 1820 J. S. C. Schweigger (1779-1857) with his " multiplier " made an advance upon Oersted's discovery, by winding the wire conveying the electric current many times round the pivoted magnetic needle and thus increasing the deflection; and L. Nobili (1784-1835) in 1825 conceived the ingenious idea of neutralizing the directive effect of the earth's magnetism by employing a pair of magnetized steel needles fixed to one axis, but with their magnetic poles pointing in opposite directions. Hence followed the astatic multiplying galvanometer.

Electrodynamic Rotation

The study of the relation between the magnet and the circuit conveying an electric current then led Arago to the discovery of the " magnetism of rotation." He found that a vibrating magnetic compass needle came to rest sooner when placed over a plate of copper than otherwise, and also that a plate of copper rotating under a suspended magnet tended to drag the magnet in the same direction. The matter was investigated by Charles Babbage, Sir J. F. W. Herschel, Peter Barlow and others, but did not receive a final explanation until after the discovery of electromagnetic induction by Faraday in 1831. Ampere's investigations had led electricians to see that the force acting upon a magnetic pole due to a current in a neighbouring conductor was such as to tend to cause the pole to travel round the conductor. Much ingenuity had, however, to be expended before a method was found of exhibiting such a rotation. Faraday first succeeded by the simple but ingenious device of using a light magnetic needle tethered flexibly to the bottom of a cup containing mercury so that one pole of the magnet was just above the surface of the mercury. On bringing down on to the mercury surface a wire conveying an electric current, and allowing the current to pass through the mercury and out at the bottom, the magnetic pole at once began to rotate round the wire (Exper. Res., 1822, 2, p. 148). Faraday and others then discovered, as already mentioned, means to make the conductor conveying the current rotate round a magnetic pole, and Ampere showed that a magnet could be made to rotate on its own axis when a current was passed through it. The difficulty in this case consisted in discovering means by which the current could be passed through one half of the magnet without passing it through the other half. This, however, was overcome by sending the current out at the centre of the magnet by means of a short length of wire dipping into an annular groove containing mercury. Barlow, Sturgeon and others then showed that a copper disk could be made to rotate between the poles of a horseshoe magnet when a current was passed through the disk from the centre to the circumference, the disk being rendered at the same time freely movable by making a contact with the circumference by means of a mercury trough. These experiments furnished the first elementary forms of electric motor, since it was then seen that rotatory motion could be produced in masses of metal by the mutual action of conductors conveying electric current and magnetic fields. By his discovery of thermoelectricity in 1822 (Pogg. Ann. Phys., 6), T. J. Seebeck (1770-1831) opened up a new region of research (see Thermo-Electricity). James Cumming (1777-1861) in 1823 (Annals of Philosophy, 1823) found that the thermo-electric series varied with the temperature, and J. C. A. Peltier (1785-1845) in 1814 discovered that a current passed across the junction of two metals either generated or absorbed heat.

Ohm's Law

In 1827 Dr G. S. Ohm (1787-1854) rendered a great service to electrical science by his mathematical investigation of the voltaic circuit, and publication of his paper, Die galvanische Kette mathematisch bearbeitet. Before his time, ideas on the measurable quantities with which we are concerned in an electric circuit were extremely vague. Ohm introduced the clear idea of current strength as an effect produced by electromotive force acting as a cause in a circuit having resistance as its quality, and showed that the current was directly proportional to the electromotive force and inversely as the resistance. Ohm's law, as it is called, was based upon an analogy with the flow of heat in a circuit, discussed by Fourier. Ohm introduced the definite conception of the distribution along the circuit of " electroscopic force " or tension (Spannung), corresponding to the modern term potential. Ohm verified his law by the aid of thermo-electric piles as sources of electromotive force, and Davy, C. S. M. Pouillet (1791-1868), A. C. Becquerel (1788-1878), G. T. Fechner (1801-1887), R. H. A. Kohlrausch (1809-1858) and others laboured at its confirmation. In more recent times, 1876, it was rigorously tested by G. Chrystal (b. 1851) at Clerk Maxwell's instigation (see Brit. Assoc. Report, 1876, p. 36), and although at its original enunciation its meaning was not at first fully apprehended, it soon took its place as the expression of the fundamental law of electrokinetics.

Induction of Electric Currents

In 1831 Faraday began the investigations on electromagnetic induction which proved more fertile in far-reaching practical consequences than any of those which even his genius gave to the world. These advances all centre round his supreme discovery of the induction of electric currents. Fully familiar with the fact that an electric charge upon one conductor could produce a charge of opposite sign upon a neighbouring conductor, Faraday asked himself whether an electric current passing through a conductor could not in any like manner induce an electric current in some neighbouring conductor. His first experiments on this subject were made in the month of November 1825, but it was not until the 29th of August 1831 that he attained success. On that date he had provided himself with an iron ring, over which he had wound two coils of insulated copper wire. One of these coils was connected with the voltaic battery and the other with the galvanometer. He found that at the moment the current in the battery circuit was started or stopped, transitory currents appeared in the galvanometer circuit in opposite directions. In ten days of brilliant investigation, guided by clear insight from the very first into the meaning of the phenomena concerned, he established experimentally the fact that a current may be induced in a conducting circuit simply by the variation in a magnetic field, the lines of force of which are linked with that circuit. The whole of Faraday's investigations on this subject can be summed up in the single statement that if a conducting circuit is placed in a magnetic field, and if either by variation of the field or by movement or variation of the form of the circuit the total magnetic flux linked with the circuit is varied, an electromotive force is set up in that circuit which at any instant is measured by the rate at which the total flux linked with the circuit is changing.

Amongst the memorable achievements of the ten days which Faraday devoted to this investigation was the discovery that a current could be induced in a conducting wire simply by moving it in the neighbourhood of a magnet. One form which this experiment took was that of rotating a copper disk between the poles of a powerful electric magnet. He then found that a conductor, the ends of which were connected respectively with the centre and edge of the disk, was traversed by an electric current. This important fact laid the foundation for all subsequent inventions which finally led to the production of electromagnetic or dynamo-electric machines.

Third Period. - With this supremely important discovery of Faraday's we enter upon'the third period of electrical research, in which that philosopher himself was the leading figure. He not only collected the facts concerning electromagnetic induction so industriously that nothing of importance remained for future discovery, and embraced them all in one law of exquisite simplicity, but he introduced his famous conception of lines of force which changed entirely the mode of regarding electrical phenomena. The French mathematicians, Coulomb, Biot, Poisson and Ampere, had been content to accept the fact that electric charges or currents in conductors could exert forces on other charges or conductors at a distance without inquiring into the means by which this action at a distance was produced. Faraday's mind, however, revolted against this notion; he felt intuitively that these distance actions must be the result of unseen operations in the interposed medium. Accordingly when he sprinkled iron filings on a card held over a magnet and revealed the curvilinear system of lines of force (see Magnetism), he regarded these fragments of iron as simple indicators of a physical state in the space already in existence round the magnet. To him a magnet was not simply a bar of steel; it was the core and origin of a system of lines of magnetic force attached to it and moving with it. Similarly he came to see an electrified body as a centre of a system of lines of electrostatic force. All the space round magnets, currents and electric charges was therefore to Faraday the seat of corresponding lines of magnetic or electric force. He proved by systematic experiments that the electromotive forces set up in conductors by their motions in magnetic fields or by the induction of other currents in the field were due to the secondary conductor cutting lines of magnetic force. He invented the term " electrotonic state " to signify the total magnetic flux due to a conductor conveying a current, which was linked with any secondary circuit in the field or even with itself.

Faraday's Researches

Space compels us to limit our account of the scientific work done by Faraday in the succeeding twenty years, in elucidating electrical phenomena and adding to the knowledge thereon, to the very briefest mention. We must refer the reader for further information to his monumental work entitled Experimental Researches on Electricity, in three volumes, reprinted from the Phil. Trans. between 1831 and 1851. Faraday divided these researches into various series. The 1st and 2nd concern the discovery of magneto-electric induction already mentioned. The 3rd series (1833) he devoted to discussion of the identity of electricity derived from various sources, frictional, voltaic, animal and thermal, and he proved by rigorous experiments the identity and similarity in properties of the electricity generated by these various methods. The 5th series (1833) is occupied with his electrochemical researches. In the 7th series (1834) he defines a number of new terms, such as electrolyte, electrolysis, anode and cathode, &c., in connexion with electrolytic phenomena, which were immediately adopted into the vocabulary of science. His most important contribution at this date was the invention of the voltameter and his enunciation of the laws of electrolysis. The voltameter provided a means of measuring quantity of electricity, and in the hands of Faraday and his successors became an appliance of fundamental importance. The 8th series is occupied with a discussion of the theory of the voltaic pile, in which Faraday accumulates evidence to prove that the source of the energy of the pile must be chemical. He returns also to this subject in the 16th series. In the 9th series (1834) he announced the discovery of the important property of electric conductors, since called their self-induction or inductance, a discovery in which, however, he was anticipated by Joseph Henry in the United States. The 11th series (1837) deals with electrostatic induction and the statement of the important fact of the specific inductive capacity of insulators or dielectrics. This discovery was made in November 1837 when Faraday had no knowledge of Cavendish's previous researches into this matter. The 19th series (1845) contains an account of his brilliant discovery of the rotation of the plane of polarized light by transparent dielectrics placed in a magnetic field, a relation which established for the first time a practical connexion between the phenomena of electricity and light. The 10th series (1845) contains an account of his researches on the universal action of magnetism and diamagnetic bodies. The 22nd series (1848) is occupied with the discussion of magnetocrystallic force and the abnormal behaviour of various crystals in a magnetic field. In the 25th series (1850) he made known his discovery of the magnetic character of oxygen gas, and the important principle that the terms paramagnetic and diamagnetic are relative. In the 26th series (1850) he returned to a discussion of magnetic lines of force, and illuminated the whole subject of the magnetic circuit by his transcendent insight into the intricate phenomena concerned. In 1855 he brought these researches to a conclusion by a general article on magnetic philosophy, having placed the whole subject of magnetism and electromagnetism on an entirely novel and solid basis. In addition to this he provided the means for studying the phenomena not only qualitatively, but also quantitatively, by the profoundly ingenious instruments he invented for that purpose.

Electrical Measurement

Faraday's ideas thus pressed upon electricians the necessity for the quantitative measurement of electrical phenomena.' It has been already mentioned that Schweigger invented in 1820 the " multiplier," and Nobili in 1825 the astatic galvanometer. C. S. M. Pouillet in 1837 contributed the sine and tangent compass, and W. E. Weber effected great improvements in them and in the construction and use of galvanometers. In 1849 H. von Helmholtz devised a tangent galvanometer with two coils. The measurement of electric resistance then engaged the attention of electricians. By his Memoirs in the Phil. Trans. in 1843, Sir Charles Wheatstone gave a great impulse to this study. He invented the rheostat and improved the resistance balance, invented by S. H. Christie (1784-1865) in 1833, and subsequently called the Wheatstone Bridge. (See his Scientific Papers, published by the Physical Society of London, p. 129.) Weber about this date invented the electrodynamometer, and applied the mirror and scale method of reading deflections, and in co-operation with C. F. Gauss introduced a system of absolute measurement of electric and magnetic phenomena. In 1846 Weber proceeded with improved apparatus to test Ampere's laws of electrodynamics. In 1845 H. G. Grassmann (1809-1877) published (Pogg. Ann. vol. 64) his " Neue Theorie der Electrodynamik," in which he gave an elementary law differing from that of Ampere but leading to the same results for closed circuits. In the same year F. E. Neumann published another law. In 1846 Weber announced his famous hypothesis concerning the connexion of electrostatic and electrodynamic phenomena. The work of Neumann and Weber had been stimulated by that of H. F. E. Lenz (1804-1865), 1 Amongst the most important of Faraday's quantitative researches must be included the ingenious and convincing proofs he provided that the production of any quantity of electricity of one sign is always accompanied by the production of an equal quantity of electricity of the opposite sign. See Experimental Researches on Electricity, vol. i. § 1177.

whose researches (Pogg. Ann., 1834, 31; 18 35, 34) among other results led him to the statement of the law by means of which the direction of the induced current can be predicted from the theory of Ampere, the rule being that the direction of the induced current is always such that its electrodynamic action tends to oppose the motion which produces it.

Neumann in 1845 did for electromagnetic induction what Ampere did for electrodynamics, basing his researches upon the experimental laws of Lenz. He discovered a function, which has been called the potential of one circuit on another, from which he deduced a theory of induction, completely in accordance with experiment. Weber at the same time deduced the mathematical laws of induction from his elementary law of electrical action, and with his improved instruments arrived at accurate verifications of the law of induction which by this time had been developed mathematically by Neumann and himself. In 1849 G. R. Kirchhoff determined experimentally in a certain case the absolute value of the current induced by one circuit in another, and in the same year Erik Edland (1819-1888) made a series of careful experiments on the induction of electric currents which further established received theories. These labours laid the foundation on which was subsequently erected a complete system for the absolute measurement of electric and magnetic quantities, referring them all to the fundamental units of mass, length and time. Helmholtz gave at the same time a mathematical theory of induced currents and a valuable series of experiments in support of them (Pogg. Ann., 1851). This great investigator and luminous expositor just before that time had published his celebrated essay, Die Erhaltung der Kraft (" The Conservation of Energy "), which brought to a focus ideas which had been accumulating in consequence of the work of J. P. Joule, J. R. von Mayer and others, on the transformation of various forms of physical energy, and in particular the mechanical equivalent of heat. Helmholtz brought to bear upon the subject not only the most profound mathematical attainments, but immense experimental skill, and his work in connexion with this subject is classical.

Lord Kelvin's Work. - About 1842 Lord Kelvin (then William Thomson) began that long career of theoretical and practical discovery and invention in electrical science which revolutionized every department of pure and applied electricity. His early contributions to electrostatics and electrometry are to be found described in his Reprint of Papers on Electrostatics and Magnetism (1872), and his later work in his collected Mathematical and Physical Papers. By his studies in electrostatics, his elegant method of electrical images, his development of the theory of potential and application of the principle of conservation of energy, as well as by his inventions in connexion with electrometry, he laid the foundations of our modern knowledge of electrostatics. His work on the electrodynamic qualities of metals, thermo-electricity, and his contributions to galvanometry, were not less massive and profound. From 1842 onwards to the end of the 19th century, he was one of the great master workers in the field of electrical discovery and research. 2 In 1853 he published a paper " On Transient Electric Currents " (Phil. Mag., 18 53 [4], 5, p. 393), in which he applied the principle of the conservation of energy to the discharge of a Leyden jar. He added definiteness to the idea of the self-induction or inductance of an electric circuit, and gave a mathematical expression for the current flowing out of a Leyden jar during its discharge. He confirmed an opinion already previously expressed by Helmholtz and by Henry, that in some circumstances this discharge is oscillatory in nature, consisting of an alternating electric current of high frequency. These theoretical predictions were confirmed and others, subsequently, by the work of B. W. Feddersen (b. 1832), C. A. Paalzow (b. 1823), and it was then seen that the familiar phenomena of the discharge of a Leyden 2 In this connexion the work of George Green (1793-1841) must not be forgotten. Green's Essay on the Application of Mathematical Analysis to the Theories of Electricity and Magnetism, published in 1828, contains the first exposition of the theory of potential. An important theorem contained in it is known as Green's theorem, and is of great value.

jar provided the means of generating electric oscillations of very high frequency.

Telegraphy

Turning to practical applications of electricity, we may note that electric telegraphy took its rise in 1820, beginning with a suggestion of Ampere immediately after Oersted's discovery. It was established by the work of Weber and Gauss at Göttingen in 1836, and that of C. A. Steinheil (1801-1870) of Munich, Sir W. F. Cooke (1806-1879) and Sir C. Wheatstone in England, Joseph Henry and S. F. B. Morse (1791-1872) in the United States in 1837. In 1845 submarine telegraphy was inaugurated by the laying of an insulated conductor across the English Channel by the brothers Brett, and their temporary success was followed by the laying in 1851 of a permanent Dover-Calais cable by T. R. Crampton. In 1856 the project for an Atlantic submarine cable took shape and the Atlantic Telegraph Company was formed with a capital of X350,000, with Sir Charles Bright as engineer-in-chief and E. O. W. Whitehouse as electrician. The phenomena connected with the propagation of electric signals by underground insulated wires had already engaged the attention of Faraday in 1854, who pointed out the Leyden-jar-like action of an insulated subterranean wire. Scientific and practical questions connected with the possibility of laying an Atlantic submarine cable then began to be discussed, and Lord Kelvin was foremost in developing true scientific knowledge on this subject, and in the invention of appliances for utilizing it. One of his earliest and most useful contributions (in 1858) was the invention of the mirror galvanometer. Abandoning the long and somewhat heavy magnetic needles that had been used up to that date in galvanometers, he attached to the back of a very small mirror made of microscopic glass a fragment of magnetized watch-spring, and suspended the mirror and needle by means of a cocoon fibre in the centre of a coil of insulated wire. By this simple device he provided a means of measuring small electric currents far in advance of anything yet accomplished, and this instrument proved not only most useful in pure scientific researches, but at the same time was of the utmost value in connexion with submarine telegraphy. The history of the initial failures and final success in laying the Atlantic cable has been well told by Mr. Charles Bright (see The Story of the Atlantic Cable, London, 1903).1 The first cable laid in 1857 broke on the IIth of August during laying. The second attempt in 1858 was successful, but the cable completed on the 5th of August 1858 broke down on the 10th of October 1858, after 732 messages had passed through it. The third cable laid in 1865 was lost on the 2nd of August 1865, but in 1866 a final success was attained and the 1865 cable also recovered and completed. Lord Kelvin's mirror galvanometer was first used in receiving signals through the short-lived 1858 cable. In 1867 he invented his beautiful siphon-recorder for receiving and recording the signals through long cables. Later, in conjunction with Prof. Fleeming Jenkin, he devised his automatic curb sender, an appliance for sending signals by means of punched telegraphic paper tape. Lord Kelvin's contributions to the science of exact electric measurement 2 were enormous. His ampere-balances, voltmeters and electrometers, and double bridge, are elsewhere described in detail (see Amperemeter; Electrometer, and Wheatstone'S Bridge).

Dynamo

The work of Faraday from 1831 to 1851 stimulated and originated an immense mass of scientific research, but at the same time practical inventors had not been slow to perceive that it was capable of purely technical application. Faraday's copper disk rotated between the poles of a magnet, and producing thereby an electric current, became the parent of 1 See also his Submarine Telegraphs (London, 1898).

2 The quantitative study of electrical phenomena has been enormously assisted by the establishment of the absolute system of electrical measurement due originally to Gauss and Weber. The British Association for the advancement of science appointed in 1861 a committee on electrical units, which made its first report in 1862 and has existed ever since. In this work Lord Kelvin took a leading part. The popularization of the system was greatly assisted by the publication by Prof. J. D. Everett of The C.G.S. System of Units (London, 1891).

innumerable machines in which mechanical energy was directly converted into the energy of electric currents. Of these machines, originally called magneto-electric machines, one of the first was devised in 1832 by H. Pixii. It consisted of a fixed horseshoe armature wound over with insulated copper wire in front of which revolved about a vertical axis a horseshoe magnet. Pixii, who invented the split tube commutator for converting the alternating current so produced into a continuous current in the external circuit, was followed by J. Saxton, E. M. Clarke, and many others in the development of the above-described magneto-electric machine. In 18J7 E. W. Siemens effected a great improvement by inventing a shuttle armature and improving the shape of the field magnet. Subsequently similar machines with electromagnets were introduced by Henry Wilde (b. 1833), Siemens, Wheatstone, W. Ladd and others, and the principle of self-excitation was suggested by Wilde, C. F. Varley (1828-1883), Siemens and Wheatstone (see Dynamo). ,,These machines about 1866 and 1867 began to be constructed on a commercial scale and were employed in the production of the electric light. The discovery of electric-current induction also led to the production of the induction coil, improved and brought to its present perfection by W. Sturgeon, E. R. Ritchie, N. J. Callan, H. D. Riihmkorff (1803-1877), A. H. L. Fizeau, and more recently by A. Apps and modern inventors. About the same time Fizeau and J. B. L. Foucault devoted attention to the invention of automatic apparatus for the production of Davy's electric arc (see Lighting: Electric), and these appliances in conjunction with magneto-electric machines were soon employed in lighthouse work. With the advent of large magneto-electric machines the era of electrotechnics was fairly entered, and this period, which may be said to terminate about 1867 to 1869, was consummated by the theoretical work of Clerk Maxwell.

Maxwell's Researches

James Clerk Maxwell (1831-1879) entered on his electrical studies with a desire to ascertain if the ideas of Faraday, so different from those of Poisson and the French mathematicians, could be made the foundation of a mathematical method and brought under the power of analysis.3 Maxwell started with the conception that all electric and magnetic phenomena are due to effects taking place in the dielectric or in the ether if the space be vacuous. The phenomena of light had compelled physicists to postulate a space-filling medium, to which the name ether had been given, and Henry and Faraday had long previously suggested the idea of an electromagnetic medium. The vibrations of this medium constitute the agency called light. Maxwell saw that it was unphilosophical to assume a multiplicity of ethers or media until it had been proved that one would not fulfil all the requirements. He formulated the conception, therefore, of electric charge as consisting in a displacement taking place in the dielectric or electromagnetic medium (see Electrostatics). Maxwell never committed himself to a precise definition of the physical nature of electric displacement, but considered it as defining that which Faraday had called the polarization in the insulator, or, what is equivalent, the number of lines of electrostatic force passing normally through a unit of area in the dielectric. A second fundamental conception of Maxwell was that the electric displacement whilst it is changing is in effect an electric current, and creates, therefore, magnetic force. The total current at any point in a dielectric must be considered as made up of two parts: first, the true conduction current, if it exists; and second, the rate of change of dielectric displacement. The fundamental fact connecting electric currents and magnetic fields is that the line integral of magnetic force taken once round a conductor conveying an electric current is equal to 4 7r-times the surface integral of the current density, or to 4 7r-times the total current flowing through the closed line round which the integral is taken (see Electrokinetics). A second relation connecting magnetic and electric force is 3 The first paper in which Maxwell began to translate Faraday's conceptions into mathematical language was " On Faraday's Lines of Force," read to the Cambridge Philosophical Society on the 10th of December 1855 and the I ith of February 1856. See Maxwell's Collected Scientific Papers, i. 155.

based upon Faraday's fundamental law of induction, that the rate of change of the total magnetic flux linked with a conductor is a measure of the electromotive force created in it (see Electrokinetics). Maxwell also introduced in this connexion the notion of the vector potential. Coupling together these ideas he was finally enabled to prove that the propagation of electric and magnetic force takes place through space with a certain velocity determined by the dielectric constant and the magnetic permeability of the medium. To take a simple instance, if we consider an electric current as flowing in a conductor it is, as Oersted discovered, surrounded by closed lines of magnetic force. If we imagine the current in the conductor to be instantaneously reversed in direction, the magnetic force surrounding it would not be instantly reversed everywhere in direction, but the reversal would be propagated outwards through space with a certain velocity which Maxwell showed was inversely as the square root of the product of the magnetic permeability and the dielectric constant or specific inductive capacity of the medium.

These great results were announced by him for the first time in a paper presented in 1864 to the Royal Society of London and printed in the Phil. Trans. for 1865, entitled " A Dynamical Theory of the Electromagnetic Field." Maxwell showed in this paper that the velocity of propagation of an electromagnetic impulse through space could also be determined by certain experimental methods which consisted in measuring the same electric quantity, capacity, resistance or potential in two ways. W. E. Weber had already laid the foundations of the absolute system of electric and magnetic measurement, and proved that a quantity of electricity could be measured either by the force it exercises upon another static or stationary quantity of electricity, or magnetically by the force this quantity of electricity exercises upon a magnetic pole when flowing through a neighbouring conductor. The two systems of measurement were called respectively the electrostatic and the electromagnetic systems (see Physical Units). Maxwell suggested new methods for the determination of this ratio of the electrostatic to the electromagnetic units, and by experiments of great ingenuity was able to show that this ratio, which is also that of the velocity of the propagation of an electromagnetic impulse through space, is identical with that of light. This great fact once ascertained, it became clear that the notion that electric phenomena are affections of the luminiferous ether was no longer a mere speculation but a scientific theory capable of verification. An immediate deduction from Maxwell's theory was that in transparent dielectrics, the dielectric constant or specific inductive capacity should be numerically equal to the square of the refractive index for very long electric waves. At the time when Maxwell developed his theory the dielectric constants of only a few transparent insulators were known and these were for the most part measured with steady or unidirectional electromotive force. The only refractive indices which had been measured were the optical refractive indices of a number of transparent substances. Maxwell made a comparison between the optical refractive index and the dielectric constant of paraffin wax, and the approximation between the numerical values of the square of the first and that of the last was sufficient to show that there was a basis for further work. Maxwell's electric and magnetic ideas were gathered together in a great mathematical treatise on electricity and magnetism which was published in 1873.1 This book stimulated in a most remarkable degree theoretical and practical research into the phenomena of electricity and magnetism. Experimental methods were devised for the further exact measurements of the electromagnetic velocity and numerous determinations of the dielectric constants of various solids, liquids and gases, and comparisons of these with the corresponding optical refractive indices were conducted. This early work indicated that whilst there were a number of cases in which the square 1 A Treatise on Electricity and Magnetism (2 vols.), by James Clerk Maxwell, sometime professor of experimental physics in the university of Cambridge. A second edition was edited by Sir W. D. Niven in 1881 and a third by Prof. Sir J. J. Thomson in 1891.

of optical refractive index for long waves and the dielectric constant of the same substance were sufficiently close to afford an apparent confirmation of Maxwell's theory, - yet in other cases there were considerable divergencies. L. Boltzmann (1844-1907) made a large number of determinations for solids and for gases, and the dielectric constants of many solid and liquid substances were determined by N. N. Schiller (b. 1848), P. A. Silow (b. 1850), J. Hopkinson and others. The accumulating determinations of the numerical value of the electromagnetic velocity (v) from the earliest made by Lord Kelvin (Sir W. Thomson) with the aid of King and M'Kichan, or those of Clerk Maxwell, W. E. Ayrton and J. Perry, to more recent ones by J. J. Thomson, F. Himstedt, H. A. Rowland, E. B. Rosa, J. S. H. Pellat and H. A. Abraham, showed it to be very close to the best determinations of the velocity of light (see Physical Units). On the other hand, the divergence in some cases between the square of the optical refractive index and the dielectric constant was very marked. Hence although Maxwell's theory of electrical action when first propounded found many adherents in Great Britain, it did not so much dominate opinion on the continent of Europe.

Fourth Period. -With the publication of Clerk Maxwell's treatise in 1873, we enter fully upon the fourth and modern period of electrical research. On the technical side the invention of a new form of armature for dynamo electric machines by Z. T. Gramme (1826-1901) inaugurated a departure from which we may date modern electrical engineering. It will be convenient to deal with technical development first.

Technical Development

As far back as 1841 large magnetoelectric machines driven by steam power had been constructed, and in 1856 F. H. Holmes had made a magneto machine with multiple permanent magnets which was installed in 1862 in Dungeness lighthouse. Further progress was made in 1867 when H. Wilde introduced the use of electromagnets for the field magnets. In 1860 Dr Antonio Pacinotti invented what is now called the toothed ring winding for armatures and described it in an Italian journal, but it attracted little notice until reinvented in 1870 by Gramme. In this new form of bobbin, the armature consisted of a ring of iron wire wound over with an endless coil of wire and connected to a commutator consisting of copper bars insulated from one another. Gramme dynamos were then soon made on the self-exciting principle. In 1873 at Vienna the fact was discovered that a dynamo machine of the Gramme type could also act as an electric motor and jwas set in rotation when a current was passed into it from another similar machine. Henceforth the electric transmission of power came within the possibilities of engineering.

Electric Lighting

In 1876, Paul Jablochkov (1847-1894), a Russian officer, passing through Paris, invented his famous electric candle, consisting of two rods of carbon placed side by side and separated from one another by an insulating material. This invention in conjunction with an alternating current dynamo provided a new and simple form of electric arc lighting. Two years afterwards C. F. Brush, in the United States, produced another efficient form of dynamo and electric arc lamp suitable for working in series (see Lighting: Electric), and these inventions of Brush and Jablochkov inaugurated commercial arc lighting. The so-called subdivision of electric light by incandescent lighting lamps then engaged attention. E. A. King in 1845 and W. E. Staite in 1848 had made incandescent electric lamps of an elementary form, and T. A. Edison in 1878 again attacked the problem of producing light by the incandescence of platinum. It had by that time become clear that the most suitable material for an incandescent lamp was carbon contained in a good vacuum, and St G. Lane Fox and Sir J. W. Swan in England, and T. A. Edison in the United States, were engaged in struggling with the difficulties of producing a suitable carbon incandescence electric lamp. Edison constructed in 1879 a successful lamp of this type consisting of a vessel wholly of glass containing a carbon filament made by carbonizing paper or some other carbonizable material, the vessel being exhausted and the current led into the filament through platinum wires.

In 1879 and 1880, Edison in the United States, and Swan in conjunction with C. H. Stearn in England, succeeded in completely solving the practical problems. From and after that date incandescent electric lighting became commercially possible, and was brought to public notice chiefly by an electrical exhibition held at the Crystal Palace, near London, in 1882. Edison, moreover, as well as Lane-Fox, had realized the idea of a public electric supply station, and the former proceeded to establish in Pearl Street, New York, in 1881, the first public electric supply station. A similar station in England was opened in the basement of a house in Holborn Viaduct, London, in March 1882. Edison, with copious ingenuity, devised electric meters, electric mains, lamp fittings and generators complete for the purpose. In 1881 C. A. Faure made an important improvement in the lead secondary battery which G. Plante (1834-1889) had invented in 1859, and storage batteries then began to be developed as commercial appliances by Faure, Swan, J. S. Sellon and many others (see Accumulator). In 1882, numerous electric lighting companies were formed for the conduct of public and private lighting, but an electric lighting act passed in that year greatly hindered commercial progress in Great Britain. Nevertheless the delay was utilized in the completion of inventions necessary for the safe and economical distribution of electric current for the purpose of electric lighting.

Telephone

Going back a few years we find the technical applications of electrical invention had developed themselves in other directions. Alexander Graham Bell in 1876 invented the speaking telephone, and Edison and Elisha Gray in the United States followed almost immediately with other telephonic inventions for electrically transmitting speech. About the same time D. E. Hughes in England invented the microphone. In 1879 telephone exchanges began to be developed in the United States, Great Britain and other countries.

Electric Power. - Following on the discovery in 1873 of the reversible action of the dynamo and its use as a motor, efforts began to be made to apply this knowledge to transmission of power, and S. D. Field, T. A. Edison, Leo Daft, E. M. Bentley and W. H. Knight, F. J. Sprague, C. J. Van Depoele and others between 1880 and 1884 were the pioneers of electric traction. One of the earliest electric tram cars was exhibited by E. W. and W. Siemens in Paris in 1881. In 1883 Lucien Gaulard, following a line of thought opened by Jablochkov, proposed to employ high pressure alternating currents for electric distributions over wide areas by means of transformers. His ideas were improved by Carl Zipernowsky and O. T. Blathy in Hungary and by S. Z. de Ferranti in England, and the alternating current transformer (see Transformers) came into existence. Polyphase alternators were first exhibited at the Frankfort electrical exhibition in 1891, developed as a consequence of scientific researches by Galileo Ferraris (1847-1897),Nikola Tesla,M. O.von Dolivo-Dobrowolsky and C. E. L. Brown, and long distance transmission of electrical power by polyphase electrical currents (see Power Transmission: Electric) was exhibited in operation at Frankfort in 1891. Meanwhile the early continuous current dynamos devised by Gramme, Siemens and others had been vastly improved in scientific principle and practical construction by the labours of Siemens, J. Hopkinson, R. E. B. Crompton, Elihu Thomson, Rudolf Eickemeyer, Thomas Parker and others, and the theory of the action of the dynamo had been closely studied by J. and E. Hopkinson, G. Kapp, S. P. Thompson, C. P. Steinmetz and J. Swinburne, and great improvements made in the alternating current dynamo by W. M. Mordey, S. Z. de Ferranti and Messrs Ganz of Budapest. Thus in twenty years from the invention of the Gramme dynamo, electrical engineering had developed from small beginnings into a vast industry. The amendment, in 1888, of the Electric Lighting Act of 1882, before long caused a huge development of public electric lighting in Great Britain. By the end of the 19th century every large city in Europe and in North and South America was provided with a public electric supply fcr the purposes of electric lighting. The various improvements in electric illuminants, such as the Nernst oxide lamp, the tantalum and osmium incandescent lamps, and improved forms of arc lamp, enclosed, inverted and flame arcs, are described under Lighting: Electric. Between 1890 and 1900, electric traction advanced rapidly in the United States of America but more slowly in England. In 1902 the success of deep tube electric railways in Great Britain was assured, and in 1904 main line railways began to abandon, at least experimentally, the steam locomotive and substitute for it the electric transmission of power. Long distance electrical transmission had been before that time exemplified in the great scheme of utilizing the falls of Niagara. The first projects were discussed in 1891 and 1892 and completed practically some ten years later. In this scheme large turbines were placed at the bottom of hydraulic fall tubes 150 ft. deep, the turbines being coupled by long shafts with 5000 H.P. alternating, current dynamos on the surface. By these electric current was generated and transmitted to towns and factories around, being sent overhead as far as Buffalo, a distance of 18 m. At the end of the 19th century electrochemical industries began to be developed which depended on the possession of cheap electric energy. The production of aluminium in Switzerland and Scotland, carborundum and calcium carbide in the United States, and soda by the Castner-Kellner process, began to be conducted on an immense scale. The early work of Sir W. Siemens on the electric furnace was continued and greatly extended by Henri Moissan and others on its scientific side, and electro-chemistry took its place as one of the most promising departments of technical research and invention. It was. stimulated and assisted by improvements in the construction of large dynamos and increased knowledge concerning the control of powerful electric currents.

In the early part of the 10th century the distribution in bulk of electric energy for power purposes in Great Britain began to assume important proportions. It was seen to be uneconomical for each city and town to manufacture its own supply since, owing to the intermittent nature of the demand for current for lighting, the price had to be kept up to 4d. and 6d. per unit.. It was found that by the manufacture in bulk, even by steam engines, at primary centres the cost could be considerably reduced, and in numerous districts in England large power stations began to be erected between 1903 and 1905 for the supply of current for power purposes. This involved almost a revolution in the nature of the tools used, and in the methods of working, and may ultimately even greatly affect the factory system and the concentration of population in large towns which was brought about in the early part of the 19th century by the invention of the steam engine.

Development of Electric Theory. Turning now to the theory of electricity, we may note the equally remarkable progress made in 300 years in scientific insight into the nature of the agency which has so recast the face of human society. There is no need to dwell upon the early crude theories of the action of amber and lodestone. In a true scientific sense no hypothesis was possible, because few facts had been accumulated. The discoveries of Stephen Gray and C. F. de C. du Fay on the conductivity of some bodies for the electric agency and the dual character of electrification gave rise to the first notions of ., electricity as an imponderable fluid, or non-gravitative subtile matter, of a more refined and penetrating kind than ordinary liquids and gases. Its duplex character, and the fact that the electricity produced by rubbing glass and vitreous substances was different from that produced by rubbing sealing-wax and resinous substances, seemed to necessitate the assumption of two kinds of electric fluid; hence there arose the conception of positive and negative electricity, and the two-fluid theory came into existence.

Single fluid Theory

The study of the phenomena of the Leyden jar and of the fact that the inside and outside coatings. possessed opposite electricities, so that in charging the jar as much positive electricity is added to one side as negative to the other, led Franklin about 1750 to suggest a modification called the single fluid theory, in which the two states of electrification were regarded as not the results of two entirely different fluids but of the addition or subtraction of one electric fluid from matter, so that positive electrification was to be looked upon as the result of increase or addition of something to ordinary matter and negative as a subtraction. The positive and negative electrifications of the two coatings of the Leyden jar were therefore to be regarded as the result of a transformation of something called electricity from one coating to the other, by which process a certain measurable quantity became so much less on one side by the same amount by which it became more on the other. A modification of this single fluid theory was put forward by F. U. T. Aepinus which was explained and illustrated in his Tentamen theoriae electricitatis et magnetismi, published in St Petersburg in 1759. This theory was founded on the following principles: - (I) the particles of the electric fluid repel each other with a force decreasing as the distance increases; (2) the particles of the electric fluid attract the atoms of all bodies and are attracted by them with a force obeying the same law; (3) the electric fluid exists in the pores of all bodies, and while it moves without any obstruction in conductors such as metals, water, &c., it moves with extreme difficulty in so-called non-conductors such as glass, resin, &c.; (4) electrical phenomena are produced either by the transference of the electric fluid of a body containing more to one containing less, or from its attraction and repulsion when no transference takes place. Electric attractions and repulsions were, however, regarded as differential actions in which the mutual repulsion of the particles of electricity operated, so to speak, in antagonism to the mutual attraction of particles of matter for one another and of particles of electricity for matter. Independently of Aepinus, Henry Cavendish put forward a single-fluid theory of electricity (Phil. Trans., 1771, 61, p. 584), in which he considered it in more precise detail.

Two fluid Theory

In the elucidation of electrical phenomena, however, towards the end of the 18th century, a modification of the two-fluid theory seems to have been generally preferred. The notion then formed of the nature of electrification was something as follows: All bodies were assumed to contain a certain quantity of a so-called neutral fluid made up of equal quantities of positive and negative electricity, which when in this state of combination neutralized one another's properties. The neutral fluid could, however, be divided up or separated into its two constituents, and these could be accumulated on separate conductors or non-conductors. This view followed from the discovery of the facts of electric induction of J. Canton (1 753, 1 754) When, for instance, a positively electrified body was found to induce upon another insulated conductor a charge of negative electricity on the side nearest to it, and a charge of positive electricity on the side farthest from it, this was explained by saying that the particles of each of the two electric fluids repelled one another but attracted those of the positive fluid. Hence the operation of the positive charge upon the neutral fluid was to draw towards the positive the negative constituent ,of the neutral charge and repel to the distant parts of the conductor the positive constituent.

C. A. Coulomb experimentally proved that the law of attraction and repulsion of simple electrified bodies was that the force between them varied inversely as the square of the distance and thus gave mathematical definiteness to the two-fluid hypothesis. It was then assumed that each of the two constituents of the neutral fluid had an atomic structure and that the so-called particles of one of the electric fluids, say positive, repelled similar particles with a force varying inversely as a square of the distance and attracted those of the opposite fluid according to the same law. This fact and hypothesis brought electrical phenomena within the domain of mathematical analysis and, as already mentioned, Laplace, Biot, Poisson, G. A. A. Plana (1781-1846), and later Robert Murphy (1806-1843), made them the subject of their investigations on the mode in which electricity distributes itself on conductors when in equilibrium.

Faraday's Views

The two-fluid theory may be said to have held the field until the time when Faraday began his researches on electricity. After he had educated himself by the study of the phenomena of lines of magnetic force in his discoveries on electromagnetic induction, he applied the same conception to electrostatic phenomena, and thus created the notion of lines of electrostatic force and of the important function of the dielectric or non-conductor in sustaining them. Faraday's notion as to the nature of electrification, therefore, about the middle of the 19th century came to be something as follows: - He considered that the so-called charge of electricity on a conductor was in reality nothing on the conductor or in the conductor itself, but consisted in a state of strain or polarization, or a physical change of some kind in the particles of the dielectric surrounding the conductor, and that it was this physical state in the dielectric which constituted electrification. Since Faraday was well aware that even a good vacuum can act as a dielectric, he recognized that the state he called dielectric polarization could not be wholly dependent upon the presence of gravitative matter, but that there must be an electromagnetic medium of a supermaterial nature. In the 13th series of his Experimental Researches on Electricity he discussed the relation of a vacuum to electricity. Furthermore his electrochemical investigations, and particularly his discovery of the important law of electrolysis, that the movement of a certain quantity of electricity through an electrolyte is always accompanied by the transfer of a certain definite quantity of matter from one electrode to another and the liberation at these electrodes of an equivalent weight of the ions, gave foundation for the idea of a definite atomic charge of electricity. In fact, long previously to Faraday's electrochemical researches, Sir H. Davy and J. J. Berzelius early in the 19th century had advanced the hypothesis that chemical combination was due to electric attractions between the electric charges carried by chemical atoms. The notion, however, that electricity is atomic in structure was definitely put forward by Hermann von Helmholtz in a well-known Faraday lecture. Helmholtz says: " If we accept the hypothesis that elementary substances are composed of atoms, we cannot well avoid concluding that electricity also is divided into elementary portions which behave like atoms of electricity." 1 Clerk Maxwell had already used in 1873 the phrase, " a molecule of electricity." 2 Towards the end of the third quarter of the 19th century it therefore became clear that electricity, whatever be its nature, was associated with atoms of matter in the form of exact multiples of an indivisible minimum electric charge which may be considered to be " Nature's unit of electricity." This ultimate unit of electric quantity Professor Johnstone Stoney called an electron.' The formulation of electrical theory as far as regards operations in space free from matter was immensely assisted by Maxwell's mathematical theory. Oliver Heaviside after 1880 rendered much assistance by reducing Maxwell's mathematical analysis to more compact form and by introducing greater precision into terminology (see his Electrical Papers, 1892). This is perhaps the place to refer also to the great services of Lord Rayleigh to electrical science. Succeeding Maxwell as Cavendish professor of physics at Cambridge in 1880, he soon devoted himself especially to the exact redetermination of the practical electrical units in absolute measure. He followed up the early work of the British Association Committee on electrical units by a fresh determination of the ohm in absolute measure, and in conjunction with other work on the electrochemical equivalent of silver and the absolute electromotive force of the Clark cell may be said to have placed exact electrical measurement on a new basis. He also made great additions to the theory of alternating electric currents, and provided fresh appliances for other electrical measurements (see his Collected Scientific Papers, Cambridge, 1900) .

Electro-optics

For a long time Faraday's observation on the rotation of the plane of polarized light by heavy glass in a 1 H. von Helmholtz, " On the Modern Development of Faraday's Conception of Electricity," Journ. Chem. Soc., 1881, 39, p. 277.

2 See Maxwell's Electricity and Magnetism, vol. i. p. 350 (2nd ed., 1881).

" On the Physical Units of 'Nature," Phil. Mag., 1881, [s], 11, p. 381. Also Trans. Roy. Soc. (Dublin, 18 9 1), 4, p. 583.

magnetic field remained an isolated fact in electro-optics. Then M. E. Verdet (1824-1860) made a study of the subject and discovered that a solution of ferric perchioride in methyl alcohol rotated the plane of polarization in an opposite direction to heavy glass (Ann. Chico. Phys., 18 54, 41, p. 37 0; 18 55, 43, p. 37; Com. Rend., 18 54, 39, p. 548). Later A. A. E. E. Kundt prepared metallic films of iron, nickel and cobalt, and obtained powerful negative optical rotation with them (Wied. Ann., 1884, 23, p. 228; 1886, 27, p. 191). John Kerr (1824-1907) discovered that a similar effect was produced when plane polarized light was reflected from the pole of a powerful magnet (Phil. Mag., 1877, [5], 3, p. 321, and 1878, 5, p. 161). Lord Kelvin showed that Faraday's discovery demonstrated that some form of rotation was taking place along lines of magnetic force when passing through a medium.' Many observers have given attention to the exact determination of Verdet's constant of rotation for standard substances, e.g. Lord Rayleigh for carbon bisulphide,2 and Sir W. H. Perkin for an immense range of inorganic and organic bodies. 3 Kerr also discovered that when certain homogeneous dielectrics were submitted to electric strain, they became birefringent (Phil. Mag., 18 75, 5 0, pp. 337 and 446). The theory of electro-optics received great attention from Kelvin, Maxwell, Rayleigh, G. F. Fitzgerald, A. Righi and P. K. L. Drude, and experimental contributions from innumerable workers, such as F. T. Trouton, O. J. Lodge and J. L. Howard, and many others.

Electric Waves.-In the decade 1880-1890, the most important advance in electrical physics was, however, that which originated with the astonishing researches of Heinrich Rudolf Hertz (1857-1894). This illustrious investigator was stimulated, by a certain problem brought to his notice by H. von Helmholtz, to undertake investigations which had for their object a demonstration of the truth of Maxwell's principle that a variation in electric displacement was in fact an electric current and had magnetic effects. It is impossible to describe here the details of these elaborate experiments; the reader must be referred to Hertz's own papers, or the English translation of them by Prof. D. E. Jones. Hertz's great discovery was an experimental realization of a suggestion made by G. F. Fitzgerald (1851-1901) in 1883 as to a method of producing electric waves in space. He invented for this purpose a radiator consisting of two metal rods placed in one line, their inner ends being provided with poles nearly touching and their outer ends with metal plates. Such an arrangement constitutes in effect a condenser, and when the two plates respectively are connected to the secondary terminals of an induction coil in operation, the plates are rapidly and alternately charged, and discharged across the spark gap with electrical oscillations (see Electrokinetics). Hertz then devised a wave detecting apparatus called a resonator. This in its simplest form consisted of a ring of wire nearly closed terminating in spark balls very close together, adjustable as to distance by a micrometer screw. He found that when the resonator was placed in certain positions with regard to the oscillator, small sparks were seen between the micrometer balls, and when the oscillator was placed at one end of a room having a sheet of zinc fixed against the wall at the other end, symmetrical positions could be found in the room at which, when the resonator was there placed, either no sparks or else very bright sparks occurred at the poles. These effects, as Hertz showed, indicated the establishment of stationary electric waves in space and the propagation of electric and magnetic force through space with a finite velocity. The other additional phenomena he observed finally contributed an all but conclusive proof of the truth of Maxwell's views. By profoundly ingenious methods Hertz showed that these invisible electric waves could be reflected and refracted like waves of light by mirrors and 1 See Sir W. Thomson, Proc. Roy. Soc. Lond., 1856, 8, p. 152; or Maxwell, Elect. and Mag., vol. ii. p. 831.

2 See Lord Rayleigh, Proc. Roy. Soc. Lond., 1884, 37, p. 146; Gordon, Phil. Trans., 1877, 167, p. 1; H. Becquerel, Ann. Chim. Phys., 1882, [3], 27, p. 312.

a Perkin's Papers are to be found in the Journ. Chem. Soc. Lond., 1884, p. 421; 1886, p. 177; 1888, p. 561; 1889, p. 680; 1891, p. 981; 1892, p. Boo; 18 93, p. 75.

prisms, and that familiar experiments in optics could be repeated with electric waves which could not affect the eye. hence there arose a new science of electro-optics, and in all parts of Europe and the United States innumerable investigators took possession of the novel field of research with the greatest delight.

O. J. Lodge, 4 A. Righi, 5 J. H. Poincare, 6 V. F. K. Bjerknes, P. K. L. Drude, J. J. Thomson,' John Trowbridge, Max Abraham, and many others, contributed to its elucidation.

In 1892, E. Branly of Paris devised an appliance for detecting these waves which subsequently proved to be of immense importance. He discovered that they had the power of affecting the electric conductivity of materials when in a state of powder, the majority of metallic filings increasing in conductivity. Lodge devised a similar arrangement called a coherer, and E. Rutherford invented a magnetic detector depending on the power of electric oscillations to demagnetize iron or steel. The sum total of all these contributions to electrical knowledge had the effect of establishing Maxwell's principles on a firm basis, but they also led to technical inventions of the very greatest utility. In 1896 G. Marconi applied a modified and improved form of Branly's wave detector in conjunction with a novel form of radiator for the telegraphic transmission of intelligence through space without wires, and he and others developed this new form of telegraphy with the greatest rapidity and success into a startling and most useful means of communicating through space electrically without connecting wires.

Electrolysis.-The study of the transfer of electricity through liquids had meanwhile received much attention. The general facts and laws of electrolysis were determined experimentally by Davy and Faraday and confirmed by the researches of J. F. Daniell, R. W. Bunsen and Helmholtz. The modern theory of electrolysis grew up under the hands of R. J. E. Clausius, A. W. Williamson and F. W. G. Kohlrausch, and received a great impetus from the work of Svante Arrhenius, J. H. Van't Hoff, W. Ostwald, H. W. Nernst and many others. The theory of the ionization of salts in solution has raised much discussion amongst chemists, but the general fact is certain that electricity only moves through liquids in association with matter, and simultaneously involves chemical dissociation of molecular groups.

Discharge through Gases.-Many eminent physicists had an instinctive feeling that the study of the passage of electricity through gases would shed much light on the intrinsic nature of electricity. Faraday devoted to a careful examination of the phenomena the XIII th series of his Experimental Researches, and among the older workers in this field must be particularly mentioned J. Plucker, J. W. Hittorf, A. A. de la Rive, J. P. Gassiot, C. F. Varley, and W. Spottiswoode and J. Fletcher Moulton. It has long been known that air and other gases at the pressure of the atmosphere were very perfect insulators, but that when they were rarefied and contained in glass tubes with platinum electrodes sealed through the glass, electricity could be passed through them under sufficient electromotive force and produced a luminous appearance known as the electric glow discharge. The so-called vacuum tubes constructed by H. Geissler (1815-1879) containing air, carbonic acid, hydrogen, &c., under a pressure of one or two millimetres, exhibit beautiful appearances when traversed by the high tension current produced by the secondary circuit of an induction coil. Faraday discovered the existence of a dark space round the negative electrode which is usually known as the " Faraday dark space." De la Rive added much to our knowledge of the subject, and J. Plucker and his disciple J. W. Hittorf examined the phenomena exhibited in so-called high vacua, that is, in exceedingly rarefied gases. C. F. Varley discovered the interesting fact that no current could be sent through the rarefied gas unless a certain minimum potential difference of the electrodes was excited. Sir William Crookes took up in 1872 the study of electric discharge through The Work of Hertz (London, 1894).

5 L'Ottica delle oscillazioni elettriche (Bologna, 1897).

6 Les Oscillations e'lectriques (Paris, 1894).

Recent Researches in Electricity and Magnetism (Oxford, 1892).

high vacua, having been led to it by his researches on the radiometer. The particular details of the phenomena observed will be found described in the article Electric conduction (§ The main fact discovered by researches of Plucker, Hittorf and Crookes was that in a vacuum tube containing extremely rarefied air or other gas, a luminous discharge takes place from the negative electrode which proceeds in lines normal to the surface of the negative electrode and renders phosphorescent both the glass envelope and other objects placed in the vacuum tube when it falls upon them. Hittorf made in 1869 the discovery that solid objects could cast shadows or intercept this cathode discharge. The cathode discharge henceforth engaged the attention of many physicists. Varley had advanced tentatively the hypothesis that it consisted in an actual projection of electrified matter from the cathode, and Crookes was led by his researches in 1870, 1871 and 1872 to embrace and confirm this hypothesis in a modified form and announce the existence of a fourth state of matter, which he called radiant matter, demonstrating by many beautiful and convincing experiments that there was an actual projection of material substance of some kind possessing inertia from the surface of the cathode. German physicists such as E. Goldstein were inclined to take another view. Sir J. J. Thomson, the successor of Maxwell and Lord Rayleigh in the Cavendish chair of physics in the university of Cambridge, began about the year 1899 a remarkable series of investigations on the cathode discharge, which finally enabled him to make a measurement of the ratio of the electric charge to the mass of the particles of matter projected from the cathode, and to show that this electric charge was identical with the atomic electric charge carried by a hydrogen ion in the act of electrolysis, but that the mass of the cathode particles, or " corpuscles " as he called them, was far less, viz. about drcith part of the mass of a hydrogen atom." The subject was pursued by Thomson and the Cambridge physicists with great mathematical and experimental ability, and finally the conclusion was reached that in a high vacuum tube the electric charge is carried by particles which have a mass only a fraction, as above mentioned, of that of the hydrogen atom, but which carry a charge equal to the unit electric charge of the hydrogen ion as found by electrochemical researches. 2 P. E. A. Lenard made in 1894 (Wied. Ann. Phys., 51, p. 225) the discovery that these cathode particles or corpuscles could pass through a window of thin sheet aluminium placed in the wall of the vacuum tube and give rise to a class of radiation called the Lenard rays. W. C. Röntgen of Munich made in 1896 his remarkable discovery of the so-called X or Röntgen rays, a class of radiation produced by the impact of the cathode particles against an impervious metallic screen or anticathode placed in the vacuum tube. The study of Röntgen rays was ardently pursued by the principal physicists in Europe during the years 1897 and 1898 and subsequently. The principal property of these Röntgen rays which attracted public attention was their power of passing through many solid bodies and affecting a photographic plate. Hence some substances were opaque to them and others transparent. The astonishing feat of photographing the bones of the living animal within the tissues soon rendered the Röntgen rays indispensable in surgery and directed an army of investigators to their study.

Radioactivity

One outcome of all this was the discovery by H. Becquerel in 1896 that minerals containing uranium, and particularly the mineral known as pitchblende, had the power of affecting sensitive photographic plates enclosed in a black paper envelope when the mineral was placed on the outside, as 1 See J. J. Thomson, Proc. Roy. Inst. Lond., 18 97, 15, p. 419; also Phil. Mag., 18 99, [5], 4 8, P. 547.

2 Later results show that the mass of a hydrogen atom is not far from I. 3 X 10 24 gramme and that the unit atomic charge or natural unit of electricity is 1.3 X 1020 of an electromagnetic C.G.S. unit. The mass of the electron or corpuscle is 7 0 X1028 gramme and its diameter is 3 X I 013 centimetre. The diameter of a chemical atom is of the order of z07 centimetre.

See H. A. Lorentz, " The Electron Theory," Elektrotechnische Zeitschrift, 1905, 26, p. 584; or Science Abstracts, 1905, 8, A, p. 603.

well as of discharging a charged electroscope (Com. Rend., 1896, 122, p. 420). This research opened a way of approach to the phenomena of radioactivity, and the history of the steps by' hich P. Curie and Madame Curie were finally led to the discovery of radium is one of the most fascinating chapters in the history of science. The study of radium and radioactivity (see Radioactivity) led before long to the further remarkable knowledge that these so-called radioactive materials project into surrounding space particles or corpuscles, some of which are identical with those projected from the cathode in a high vacuum tube, together with others of a different nature. The study of radioactivity was pursued with great ability not only by the Curies and A. Debierne, who associated himself with them, in France, but by E. Rutherford and F. Soddy in Canada, and by J. J. Thomson, Sir William Crookes, Sir William Ramsay and others in England.

Electronic Theory

The final outcome of these investigations was the hypothesis that Thomson's corpuscles or particles composing the cathode discharge in a high vacuum tube must be looked upon as the ultimate constituent of what we call negative electricity; in other words, they are atoms of negative electricity, possessing, however, inertia, and these negative electrons are components at any rate of the chemical atom. Each electron is a point-charge of negative electricity equal to 3.9 X Io 1 ° of an electrostatic unit or to. 3 X Io 2 ° of an electromagnetic unit, and the ratio of its charge to its mass is nearly 2 X 10 7 using E.M. units. For the hydrogen atom the ratio of charge to mass as deduced from electrolysis is about Io 1. Hence the mass of an electron is y ff l iT uth of that of a hydrogen atom. No one has yet been able to isolate positive electrons, or to give a complete demonstration that the whole inertia of matter is only electric inertia due to what may be called the inductance of the electrons. Prof. Sir J. Larmor developed in a series of very able papers (Phil. Trans., 1894, 185; 1895, 186; 1897, 190), and subsequently in his book Aether and Matter (1900), a remarkable hypothesis of the structure of the electron or corpuscle, which he regards as simply a strain centre in the aether or electromagnetic medium, a chemical atom being a collection of positive and negative electrons or strain centres in stable orbital motion round their common centre of mass (see Aether). J. J. Thomson also developed this hypothesis in a profoundly interesting manner, and we may therefore summarize very briefly the views held on the nature of electricity and matter at the beginning of the 10th century by saying that the term electricity had come to be regarded, in part at least, as a collective name for electrons, which in turn must be considered as constituents of the chemical atom, furthermore as centres of certain lines of self-locked and permanent strain existing in the universal aether or electromagnetic medium. Atoms of matter are composed of congeries of electrons and the inertia of matter is probably therefore only the inertia of the electromagnetic medium.' Electric waves are produced wherever electrons are accelerated or retarded, that is, whenever the velocity of an electron is changed or accelerated positively or negatively. In every solid body there is a continual atomic dissociation, the result of which is that mixed up with the atoms of chemical matter composing them we have a greater or less percentage of free electrons. The operation called an electric current consists in a diffusion or movement of these electrons through matter, and this is controlled by laws of diffusion which are similar to those of the diffusion of liquids or gases. Electromotive force is due to a difference in the density of the electronic population in different or identical conducting bodies, and whilst the electrons can move freely through so-called conductors their motion is much more hindered or restricted in non-conductors. Electric charge consists, therefore, in an excess or deficit of negative electrons in a body. In the hands of H. A. Lorentz, P. K. L. Drude, J. J. Thomson, J. Larmor and many others, the electronic hypothesis of matter and of electricity has been developed in great detail and may be said to represent the outcome of modern researches upon electrical phenomena.

1 See J. J. Thomson, Electricity and Matter (London, 1904).

The reader may be referred for an admirable summary of the theories of electricity prior to the advent of the electronic hypothesis to J. J. Thomson's " Report on Electrical Theories " (Brit. Assoc. Report, 1885), in which he divides electrical theories enunciated during the 10th century into four classes, and summarizes the opinions and theories of A. M. Ampere, H. G. Grassman, C. F. Gauss, W. E. Weber, G. F. B. Riemann, R. J. E. Clausius, F. E. Neumann and H. von Helmholtz.

BIBLIOGRAPHY.-M. Faraday, Experimental Researches in Electricity (3 vols., London, 1839, 1844, 1855); A. A. De la Rive, Treatise on Electricity (3 vols., London, 1853, 1858); J. Clerk Maxwell, A Treatise on Electricity and Magnetism (2 vols., 3rd ed., 1892); id., Scientific Papers (2 vols., edited by Sir W. J. Niven, Cambridge, 1890); H. M. Noad, A Manual of Electricity (2 vols., London, 1855, 1857); J. J. Thomson, Recent Researches in Electricity and Magnetism (Oxford, 1893); id., Conduction of Electricity through Gases (Cambridge, 1903); id., Electricity and Matter (London, 1904); O. Heaviside, Electromagnetic Theory (London, 1893); O. J. Lodge, Modern Views of Electricity (London, 1889); E. Mascart and J. Joubert, A Treatise on Electricity and Magnetism, English trans. by E. Atkinson (2 vols., London, 1883); Park Benjamin, The Intellectual Rise in Electricity (London, 1895); G. C. Foster and A. W. Porter, Electricity and Magnetism (London, 1903); A. Gray, A Treatise on Magnetism and Electricity (London, 1898); H. W. Watson and S. H. Burbury, The Mathematical Theory of Electricity and Magnetism (2 vols., 1885); Lord Kelvin (Sir William Thomson), Mathematical and Physical Papers (3 vols., Cambridge, 1882); Lord Rayleigh, Scientific Papers (q. vols., Cambridge, 1903); A. Winkelmann, Handbuch der Physik, vols. iii. and iv. (Breslau, 1903 and 1905; a mine of wealth for references to original papers on electricity and magnetism from the earliest date up to modern times). For particular information on the modern Electronic theory the reader may consult W. Kaufmann, " The Developments of the Electron Idea." Physikalische Zeitschrift (1st of Oct. 1901), or The Electrician (1901), 4 8, p. 95; H. A. Lorentz, The Theory of Electrons (1909); E. E. Fournier d'Albe, The Electron Theory (London, 1906); H. Abraham and P. Langevin, Ions, Electrons, Corpuscles (Paris, 1905); J. A. Fleming, " The Electronic Theory of Electricity," Popular Science Monthly (May 1902); Sir Oliver J. Lodge, Electrons, or the Nature and Properties of Negative Electricity (London, 1907). (J.A.F.)


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Simple English

[[File:|thumb|200px|right|Lightning is the most visible way we see electricity in nature]]

Electricity is a type of energy found in nature. We see it in nature when we see a lightning flash or when we stroke an animal and feel sparks.

Scientists have also found we can make electricity if we pass a magnet close to a metal wire or if we put the right chemicals in a jar with two different kinds of metal rod.

Scientists have observed that electricity seems to flow like water from one place to another, either as a spark or as a current in a metal. They now know that all matter has electric charge but this is mostly cancelled out by the presence of matter with an opposite charge. We only see an effect when there is too much or too little electric charge in one place so that it is not cancelled out.

Since the nineteenth century, electricity has been made into a useful creation that affects every part of our lives. Until then it was just a curiosity or a force of nature seen in a thunderstorm.

To be useful electricity has to be made from different sources of energy such as by burning coal or oil or from wind or flowing water in a power station.

Electricity arrives at our homes through wires from the places where it is made. It is used by Electric Lamps for producing light, Electric Heaters to produce heat etc. It is also used by many devices like washing machines, Electric Cookers, etc for doing work. In factories, electricity is used for running machines and computers.

The people who deal with electricity and electrical devices in our homes and factories are called "Electricians".

Contents

Electricity in physics

Electricity works because electric charges push and pull on each other. There are two types of electric charge, positive charge and negative charge. Two positive charges repel each other. This means that if you put two positive charges close together and let them go, they would fly apart. Two negative charges also repel. But a negative charge and a positive charge attract each other. This means that if you put a positive charge and a negative charge close together, they would smack together. A short way to remember this is the phrase opposites attract, likes repel.

Electric charges can push or pull on each other even though they are not touching. This is possible because each charge makes an electric field around itself. An electric field is an area that surrounds a charge. At each point near a charge, the electric field points in a certain direction. If a positive charge is put at that point, it will be pushed in that direction. If a negative charge is put at that point, it will be pushed in the exact opposite direction.

All the matter in the world is made of tiny positive and negative charges. The positive charges are called protons, and the negative charges are called electrons. Protons are much bigger and heavier than electrons, but they both have the same amount of electric charge, except that protons are positive and electrons are negative. Because "opposites attract," protons and electrons stick together. A few protons and electrons can form bigger particles called atoms and molecules. Atoms and molecules are still very tiny. It is impossible to see them without a very powerful microscope. Any big object, like your body, has more atoms and molecules in it than anyone could count.

Because negative electrons and positive protons stick together to make big objects, all big objects that we can see and feel are electrically neutral. Electrically is a word meaning "describing electricity" and neutral is a word meaning, "balanced." That is why we do not feel objects pushing and pulling on us from a distance, like they would if everything was electrically charged. All big objects are electrically neutral because there is exactly the same amount of positive and negative charge in the world. We could say that the world is exactly balanced, or neutral. This seems very surprising and lucky. Scientists still do not know why this is so, even though they have been studying electricity for a long time.

Electric current

File:Ampe ke.gif
A drawing of an electric circuit. The current (I) flows from + round the circuit back to -
File:Electric wiring near
Electricity is sent on wires.

In some materials, electrons are stuck tightly in place, while in other materials, electrons can move all around the material. Protons never move around a solid object because they are so heavy, at least compared to the electrons. A material that lets electrons move around is called a conductor. A material that keeps each electron tightly in place is called an insulator. Examples of conductors are copper, aluminum, silver, and gold. Examples of insulators are rubber, plastic, and wood. Copper is used very often as a conductor because it is a very good conductor and there is so much of it in the world. But sometimes other materials are used.

Inside a conductor, electrons bounce around, but they do not keep going in one direction for long. But if an electric field is set up inside the conductor, the electrons will all start to move in the direction opposite to the direction the field is pointing (because electrons are negatively charged). A battery can make an electric field inside a conductor. If both ends of a piece of wire are connected to the two ends of a battery (called the electrodes) the loop that was made is called a circuit. Electrons will flow around and around the circuit as long as the battery is making an electric field inside the wire. This flow of electrons around the circuit is called electric current.

A conducting wire used to carry electric current is often wrapped in an insulator like rubber. This is because wires that carry current are very dangerous. If a person or an animal touched a bare wire carrying current, they could get hurt or even die depending on how strong the current was. You should be careful around electrical sockets and bare wires that might be carrying current.

It is possible to connect an electrical device to a circuit so that electrical current will flow through a device. This current will make the device do something that we want it to do. Electrical devices can be very simple. For example, in a light bulb, current flows through a special wire called a filament, which makes it glow. Electrical devices can also be very complicated. Electricity can be used to drive a motor inside a tool like a drill or a pencil sharpener. Electricity is also used to power modern electronic devices, including telephones, computers, and televisions.

Some terms related to electricity

  • Electric voltage is the "push" behind the current. It is the amount of work per electric charge that an electric source can do. If a source can do 1 joule of work on 1 coulomb of charge, it has a 1 volt of electric potential.
  • Electric resistance is the ability of a substance to resist the flowing of the current, i.e. to reduce the amount of current that flows through the substance. If an electric voltage of 1 volt maintains a current of 1 ampere through a wire, the resistance of the wire is 1 ohm. When the flow of current is opposed (resisted) energy gets used or gets converted to other forms (e.g. heat, etc.).
  • Electric energy is the ability to do work by means of electric devices. Electric energy is a "conserved" property, meaning that it behaves like a substance and can be moved from place to place. Electric energy is measured in joules or kilowatt-hours (kW h). Electric energy is the amount of energy that an amount of electricity can produce at a time.
  • Electric power is the rate at which electric energy is being used, stored, or transferred. Flows of electrical energy along power lines are measured in watts. If the electric energy is being converted to another form of energy, it is measured in watts. If it is stored (as in electric or magnetic fields), it is measured in volt-amperes reactive. If some of it is converted and some of it is stored, it is measured in volt-amperes.

Methods of generating electricity

File:Nuclear Power Plant
Electricity is made in power stations

Electricity is mostly generated in places called power stations. Power stations use heat to turn water into steam. The force of the steam pressure turns giant fan-like structures called turbines, which are linked to machines called 'generators'. Generators pass wires through strong magnetic fields to generate electricity.

Sometimes a natural flow such as wind or water can be used to directly turn a generator so no heat is needed.

There are many sources of heat which can be used to generate electricity. Heat sources can be classified into two types: renewable energy resources in which the supply of heat energy never runs out and non-renewable energy resources in which the supply will be eventually used up.

Renewable energy resources

Renewable heat energy
  • Solar thermal power
  • Geothermal power
Renewable flow energy
  • Hydro-electric power
  • Wind power
  • Wave power
  • Tidal power

Non-renewable energy resources These all use heat as a source of energy.

  • Coal
  • Oil
  • Natural gas
  • Nuclear power
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