A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil. The term has expanded to include uses of the same mechanism in recording, positioning, and servomechanism equipment.
Contents |
The deflection of a magnetic compass needle by current in a wire was first described by Hans Oersted in 1820. The phenomenon was studied both for its own sake and as a means of measuring electrical current. The earliest galvanometer was reported by Johann Schweigger at the University of Halle on 16 September 1820. André-Marie Ampère also contributed to its development. Early designs increased the effect of the magnetic field due to the current by using multiple turns of wire; the instruments were at first called "multipliers" due to this common design feature. The term "galvanometer", in common use by 1836, was derived from the surname of Italian electricity researcher Luigi Galvani, who discovered in 1771 that electric current could make a frog's leg jerk.
Originally the instruments relied on the Earth's magnetic field to provide the restoring force for the compass needle; these were called "tangent" galvanometers and had to be oriented before use. Later instruments of the "astatic" type used opposing magnets to become independent of the Earth's field and would operate in any orientation. The most sensitive form, the Thompson or mirror galvanometer, was invented by William Thomson (Lord Kelvin) and patented by him in 1858. Instead of a compass needle, it used tiny magnets attached to a small lightweight mirror, suspended by a thread; the deflection of a beam of light greatly magnified the deflection due to small currents. Alternatively the deflection of the suspended magnets could be observed directly through a microscope.
The ability to quantitatively measure voltage and current allowed Georg Ohm to formulate Ohm's Law, which states that the voltage across an element is directly proportional to the current through it.
The early moving-magnet form of galvanometer had the disadvantage that it was affected by any magnets or iron masses near it, and its deflection was not linearly proportional to the current. In 1882 Jacques-Arsène d'Arsonval developed a form with a stationary permanent magnet and a moving coil of wire, suspended by coiled hair springs. The concentrated magnetic field and delicate suspension made these instruments sensitive and they could be mounted in any position. By 1888 Edward Weston had brought out a commercial form of this instrument, which became a standard component in electrical equipment. This design is almost universally used in moving-vane meters today.
The most familiar use is as an analog measuring instrument, often called a meter. It is used to measure the direct current (flow of electric charge) through an electric circuit. The D'Arsonval/Weston form used today is constructed with a small pivoting coil of wire in the field of a permanent magnet. The coil is attached to a thin pointer that traverses a calibrated scale. A tiny torsion spring pulls the coil and pointer to the zero position.
When a direct current (DC) flows through the coil, the coil generates a magnetic field. This field acts against the permanent magnet. The coil twists, pushing against the spring, and moves the pointer. The hand points at a scale indicating the electric current. Careful design of the pole pieces ensures that the magnetic field is uniform, so that the angular deflection of the pointer is proportional to the current. A useful meter generally contains provision for damping the mechanical resonance of the moving coil and pointer, so that the pointer settles quickly to its position without oscillation.
The basic sensitivity of a meter might be, for instance, 100 microamperes full scale (with a voltage drop of, say, 50 millivolts at full current). Such meters are often calibrated to read some other quantity that can be converted to a current of that magnitude. The use of current dividers, often called shunts, allows a meter to be calibrated to measure larger currents. A meter can be calibrated as a DC voltmeter if the resistance of the coil is known by calculating the voltage required to generate a full scale current. A meter can be configured to read other voltages by putting it in a voltage divider circuit. This is generally done by placing a resistor in series with the meter coil. A meter can be used to read resistance by placing it in series with a known voltage (a battery) and an adjustable resistor. In a preparatory step, the circuit is completed and the resistor adjusted to produce full scale deflection. When an unknown resistor is placed in series in the circuit the current will be less than full scale and an appropriately calibrated scale can display the value of the previously-unknown resistor.
Because the pointer of the meter is usually a small distance above the scale of the meter, parallax error can occur when the operator attempts to read the scale line that "lines up" with the pointer. To counter this, some meters include a mirror along the markings of the principal scale. The accuracy of the reading from a mirrored scale is improved by positioning one's head while reading the scale so that the pointer and the reflection of the pointer are aligned; at this point, the operator's eye must be directly above the pointer and any parallax error has been minimized.
Extremely sensitive measuring equipment once used mirror galvanometers that substituted a mirror for the pointer. A beam of light reflected from the mirror acted as a long, massless pointer. Such instruments were used as receivers for early trans-Atlantic telegraph systems, for instance. The moving beam of light could also be used to make a record on a moving photographic film, producing a graph of current versus time, in a device called an oscillograph.
Today the main type of galvanometer mechanism still used is the moving coil D'Arsonval/Weston mechanism, which is used in traditional analog meters.
A tangent galvanometer is an early measuring instrument used for the measurement of electric current. It works by using a compass needle to compare a magnetic field generated by the unknown current to the magnetic field of the Earth. It gets its name from its operating principle, the tangent law of magnetism, which states that the tangent of the angle a compass needle makes is proportional to the ratio of the strengths of the two perpendicular magnetic fields. It was first described by Claude Pouillet in 1837.
A tangent galvanometer consists of a coil of insulated copper wire wound on a circular non-magnetic frame. The frame is mounted vertically on a horizontal base provided with levelling screws. The coil can be rotated on a vertical axis passing through its centre. A compass box is mounted horizontally at the centre of a circular scale. It consists of a tiny, powerful magnetic needle pivoted at the centre of the coil. The magnetic needle is free to rotate in the horizontal plane. The circular scale is divided into four quadrants. Each quadrant is graduated from 0° to 90°. A long thin aluminium pointer is attached to the needle at its centre and at right angle to it. To avoid errors due to parallax a plane mirror is mounted below the compass needle.
In operation, the instrument is first rotated until the magnetic field of the Earth, indicated by the compass needle, is parallel with the plane of the coil. Then the unknown current is applied to the coil. This creates a second magnetic field on the axis of the coil, perpendicular to the Earth's magnetic field. The compass needle responds to the vector sum of the two fields, and deflects to an angle equal to the tangent of the ratio of the two fields. From the angle read from the compass's scale, the current could be found from a table.[1]
The current supply wires have to be wound in a small helix, like a pig's tail, otherwise the field due to the wire will affect the compass needle and an incorrect reading will be obtained.
The galvanometer is oriented so that the plane of the coil is parallel to the local magnetic meridian, that is the horizontal component BH of the Earth's magnetic field. When a current passes through the galvanometer coil, a second magnetic field B perpendicular to the coil is created, of strength:
where I is the current in amperes, n is the number of turns of the coil and r is the radius of the coil. These two perpendicular magnetic fields add vectorially, and the compass needle points along the direction of their resultant, at an angle of:
From tangent law, 
, i.e.
or
or 
, where K is called the Reduction Factor of the tangent galvanometer.
One problem with the tangent galvanometer is that its resolution degrades at both high currents and low currents. The maximum resolution is obtained when the value of θ is 45°. When the value of θ is close to 0° or 90°, a large percentage change in the current will only move the needle a few degrees.
A tangent galvanometer can also be used to measure the magnitude of the horizontal component of the geomagnetic field. When used in this way, a low-voltage power source, such as a battery, is connected in series with a rheostat, the galvanometer, and an ammeter. The galvanometer is first aligned so that the coil is parallel to the geomagnetic field, whose direction is indicated by the compass when there is no current through the coils. The battery is then connected and the rheostat is adjusted until the compass needle deflects 45 degrees from the geomagnetic field, indicating that the magnitude of the magnetic field at the center of the coil is the same as that of the horizontal component of the geomagnetic field. This field strength can be calculated from the current as measured by the ammeter, the number of turns of the coil, and the radius of the coils.
A major early use for galvanometers was for finding faults in telecommunications cables. They were superseded in this application late in the 20th century by time-domain reflectometers.
Probably the largest use of galvanometers was the D'Arsonval/Weston type movement used in analog meters in electronic equipment. Since the 1980s, galvanometer-type analog meter movements have been displaced by analog to digital converters (ADCs) for some uses. A digital panel meter (DPM) contains an analog to digital converter and numeric display. The advantages of a digital instrument are higher precision and accuracy, but factors such as power consumption or cost may still favor application of analog meter movements.
Most modern uses for the galvanometer mechanism are in positioning and control systems. A galvanometer mechanism is used for the head positioning servos in hard disk drives. They are also used in laser marking and projection, and in imaging applications such as Optical Coherence Tomography (OCT) retinal scanning. Mirror galvanometer systems are used as beam positioning elements in laser optical systems. These are typically high power galvanometer mechanisms used with closed loop servo control systems. The newest generation of galvanometers designed for beam steering applications can have frequency responses over 10 kHz with appropriate servo technology. Examples of manufacturers of such systems are Cambridge Technology Inc. (www.camtech.com) and General Scanning (www.gsig.com).
Galvanometer mechanisms are also used to position the pens in analog strip chart recorders such as used in electrocardiographs, electroencephalographs and polygraphs. Strip chart recorders with galvanometer driven pens may have a full scale frequency response of 100 Hz and several centimeters deflection. The writing mechanism may be a heated tip on the needle writing on heat-sensitive paper, or a hollow ink-fed pen. In some types the pen is continuously pressed against the paper, so the galvanometer must be strong enough to move the pen against the friction of the paper. In other types, such as the Rustrak recorders, the needle is only intermittently pressed against the writing medium; at that moment, an impression is made and then the pressure is removed, allowing the needle to move to a new position and the cycle repeats. In this case, the galvanometer need not be especially strong.
GALVANOMETER, an instrument for detecting or measuring electric currents. The term is generally applied to instruments which indicate electric current in scale divisions or arbitrary units, as opposed to instruments called amperemeters (q.v.), which show directly on a dial the value of the current in amperes.
Galvanometers may be divided into direct current and alternating current instruments, according as they are intended to measure one or other of these two classes of currents (see Electro Kinetics) .
The principle on which one type of direct current galvanometer, called a movable needle galvanometer, depends for its action is that a small magnet when suspended in the centre of a coil of wire tends to set its magnetic axis in the direction of the magnetic field of the coil at that point due to the current passing through it. In the other type, or movable coil galvanometer, the coil is suspended and the magnet fixed; hence the coil tends to set itself with its axis parallel to the lines of force of the magnet. The movable system must be constrained in some way to take up and retain a definite position when no current is passing by means which are called the "control." In its simple and original form the movable needle galvanometer consisted of a horizontal magnetic needle suspended within a coil Movable of insulated wire by silk fibres or pivoted on a point like a compass needle. The direction of such a needle is con needleo- trolled by the direction of the terrestrial magnetic force. within the coil. If the needle is so placed that its axis is parallel to the plane of the coil, then when an electric cur rent passes through the coil it is deflected and places itself at an angle to the axis of the coil determined by the strength of the current and of the controlling field. In the early forms of movable needle galvanometer the needle was either a comparatively large magnet several inches in length, or else a smaller magnet was employed carrying a long pointer which moved over a scale of degrees so as to indicate the deflexion. A method of measuring the deflexion by means of a mirror scale and telescope was introduced by K. F. Gauss and W. Weber. The magnet had a mirror attached to it, and a telescope having cross wires in the focus was used to observe the scale divisions of a fixed scale seen reflected in the mirror. Lord Kelvin (Professor W. Thomson) made the important improvement of reducing the size of the needle and attaching it to the back of a very small mirror, the two being suspended by a single fibre of cocoon silk. The mirror was made of silvered microscopic glass about 4 in. in diameter, and the magnetic needle or needles consisted of short fragments of watchspring cemented to its back. A ray of light being thrown on the mirror from a lamp the deflexions of the needle were observed by watching the movements of a spot of light reflected from it upon a fixed scale. This form of mirror galvanometer was first devised in connexion with submarine cable signalling, but soon became an indispensable instrument in the physical laboratory.
In course of time both the original form of single needle galvanometer and mirror galvanometer were improved by introducing the Asiatic astatic principle and weakening the external controlling magnetic field. If two magnetic needles of equal size and meters. moment are attached rigidly to one stem parallel to each m other but with poles placed in opposite directions an astatic system results; that is, if the needles are so suspended as to be free to move in a horizontal plane, and if they are made exactly equal in magnetic strength, the system will have no directive power. If one needle is slightly weaker than the other, the suspended system will set itself with some axis parallel to the lines of force of a field in which it is placed. In a form of astatic needle galvanometer devised by Professor A. Broca of Paris, the pair of magnetized needles are suspended vertically and parallel to each other with poles in opposite directions. The upper poles are included in one coil and the lower poles within another coil, so connected that the current circulates in the right direction in each coil to displace the pairs of poles in the same direction. By this mode of arrangement a greater magnetic moment can be secured, together with more perfect astaticity and freedom from disturbance by external fields. The earth's magnetic field can be weakened by means of a controlling magnet arranged to create in the space in the interior of the galvanometer coils an extremely feeble controlling magnetic field. In instruments having a coil for each needle and designed so that the current in both coils passes so as to turn both needles in the same direction, the controlling magnet is so adjusted that the normal position of the needles is with the magnetic axis parallel to the plane of the coil. An astatic magnetic system used in conjunction with a mirror galvanometer gives a highly sensitive form of instrument (fig. I); it is, however, easily disturbed by stray magnetic fields caused by neighbouring magnets or currents through conductors, and therefore is not suitable for use in many places.
This fact led to the introduction of the movable coil galvanometer which was first devised by Lord Kelvin as a telegraphic signalling instrument but subsequently modified by A. d'Arsonval and others into a laboratory galvanometer (fig. 2). In this instrument a permanent magnet, generally of the horseshoe shape, is employed to create a strong magnetic field, in which a light movable coil is suspended. The suspension is bifilar, consisting of two fine wires which are connected to the ends of the coil and serve to lead the current in and out. If such a coil is placed with its plane parallel to the lines of force of the permanent magnet, then when a current is passing through it it displaces itself in the field, so as to set with its axis more nearly parallel to the lines of force of the field. The movable coil may carry a pointer or a mirror; in the latter form it is well represented by several much used laboratory instruments. The movable coil galvanometer has the great advantage that it is not easily disturbed by the magnetic fields caused by neighbouring magnets or electric currents, and thus is especially useful in the electrical workshop and factory.
In the practical construction of the suspended needle fixed coil galvanometer great care must be taken with the insulation of the wire of the coil. This wire is generally silk-covered, wound on a frame, the whole being thoroughly saturated with paraffin wax. In some cases two wires are wound on in parallel, constituting a "differential galvanometer." When properly adjusted this instrument can be used for the exact comparison of electric currents by a null method, because if an electric current is passed through one wire and creates certain deflexions of the needle, the current which annuls this deflexion when passed through the other wire must be equal to the first current. In the construction of a movable coil galvanometer, it is usual to intensify the magnetic field by inserting a fixed soft iron core in the interior of the movable coil. If the current to be measured is too large to be passed entirely through the galvanometer, a portion is allowed to flow through a circuit connecting the two terminals of the instrument. This circuit is called a shunt and is generally arranged so as to take 0.9, 0.99, or 0.999 of the total current, leaving o. I, o oi or o ooi to flow through the galvanometer.
W. E. Ayrton and T. Mather have designed a universal shunt box or resistance which can be applied to any galvanometer and by which a known fraction of any current can be sent through the galvanometer when we know its resistance (see Jour. Inst. Elec. Eng. Lond., 1894, 23, p. 314). A galvanometer can be calibrated, or the meaning of its deflexion determined, by passing through it an electric current of known value and observing the deflexion of the needle or coil. The known current can be provided in the following manner: - a single secondary cell of any kind can have its electromotive force measured by the potentiometer, and compared with that of a standard voltaic cell. If the secondary cell is connected with the galvanometer through a known high resistance R, and if the galvanometer is shunted, that is, has its terminals connected by another resistance S, then if the resistance of the galvanometer itself is denoted by G, FIG. 2. - Movable Coil Galvanometer.
the whole resistance of the shunted galvanometer and high resistance has a value represented by R+G +S, and therefore the current through the galvanometer produced by an electromotive force E of the cell is represented by SE R(G +GS' Suppose this current produces a deflexion of the needle or coil or spot of light equal to X scale divisions, we can then alter the value of the resistances R and S, and so determine the relation between the deflexion and the current. By the sensitiveness of the FIG. I. - Kelvin Astatic Mirror Galvanometer. Elliott square pattern.
galvanometer is meant the deflexion produced by a known electromotive force put upon its terminals or a known current sent through it. It is usual to specify the sensitiveness of a mirror galvanometer by requiring a certain deflexion, measured in millimetres, of a spot of light thrown on the scale placed at one metre from the mirror, when an electromotive force of one-millionth of a volt (microvolt) is applied to the terminals of the galvanometer; it may be otherwise expressed by stating the deflexion produced under the same conditions when a current of one microampere is passed through the coil. In modern mirror galvanometers a deflexion of i mm. of the spot of light upon a scale at I metre distance can be produced by a current as small as one hundred millionth (t08) or even one ten thousand millionth (1010) of an ampere. It is easy to produce considerable sensitiveness in the galvanometer, but for practical purposes it must always be controlled by the condition that the zero remains fixed, that is to say, the galvanometer needle or coil must come back to exactly the same position when no current is passing through the instrument. Other important qualifications of a galvanometer are its time-period and its dead-beatness. For certain purposes the needle or coil should return as quickly as possible to the zero position and with either no, or very few, oscillations. If the latter condition is fulfilled the galvanometer is said to be "dead-beat." On the other hand, for some purposes the galvanometer is required with the opposite quality, that is to say, there must be as little retardation as possible to the needle or coil when set in motion under an impulsive blow. Such a galvanometer is called "ballistic." The quality of a galvanometer in this respect is best estimated by taking the logarithmic decrement of the oscillations when the movable system is set swinging. This last term is defined as the logarithm of the ratio of one swing to the next succeeding swing, and a galvanometer of which the logarithmic decrement is large, is said to be highly damped. For many purposes, such as for resistance measurement, it is desirable to have a galvanometer which is highly damped; this result can be obtained by affixing to the needles either light pieces of mica, when it is a movable needle galvanometer, or by winding the coil on a silver frame when it is a movable coil galvanometer. On the other hand, for the comparison of capacities of condensers and for other purposes, a galvanometer is required wlich is as little damped as possible, and for this purpose the coil must have the smallest possible frictional resistance to its motion through the air. In this case the moment of inertia of the movable system must be decreased or the control strengthened.
The Einthoven string galvanometer is another form of sensitive instrument for the measurement of small direct currents. It consists of a fine wire or silvered quartz fibre stretched in a strong magnetic field. When a current passes through the wire it is displaced across the field and the displacement is observed with a microscope.
For the measurement of large currents a "tangent galvanometer" is employed (fig. 3). Two fixed circular coils are placed apart at a distance equal to the radius of either coil, so that a current passing through them creates in the central region between them a nearly uniform magnetic field. m At the centre of the coils is suspended a small magnetic needle the length of which should not be greater than the radius of either coil. The normal position of the needle is at right angles to the line joining the centre of the coils. If a current is passed through the coils, the needle will be deflected, and the tangent of the angle of its deflexion will be nearly proportional to the current passing through the coil, provided that the controlling field is uniform in strength and direction, and that the length of the magnetic needle is so short that the space in which it rotates is a practically uniform magnetic field.
For the detection of small alternating currents a magnetic needle or movable coil galvanometer is of no utility. We can, however, construct an instrument suitable for the purpose by suspending within a coil of insulated wire a small needle of soft iron placed with its axis at an angle of 45° to the axis of the coil. When an alternating current passes through the coil the soft iron needle tends to set itself in the direction of the axis of the coil, and if it is suspended by a quartz fibre or metallic wire so as to afford a control, it can become a metrical instrument. Another arrangement, devised by J. A. Fleming in 1887, consists of a silver or copper disk suspended within a coil, the plane of the disk being held at 45° to that of the coil. When an alternating current is passed through the coil, induced currents are set up in the disk and the mutual action causes the disk to endeavour to set itself so that these currents are a minimum. This metal disk galvanometer has been made sufficiently sensitive to detect the feeble oscillatory electric currents set up in the receiving wire of a wireless telegraph apparatus. The Duddell thermal ammeter is another very sensitive form of alternating current galvanometer. In it the current to be detected or measured is passed through a high resist ance wire or strip of metal leaf mounted on glass, over which is suspended a closed loop of bismuth and antimony, forming a thermoelectric couple. This loop is suspended by a quartz fibre in a strong magnetic field, and one junction of the couple is held just over the resistance wire and as near it as possible without touching. When an alternating current passes through the resistance it creates heat which in turn acts on the thermo-j unction and generates a continuous current in the loop, thus deflecting it in the magnetic field. The sensitiveness of such a thermal ammeter can be made sufficiently great to detect a current of a few microamperes.
REFERENCES. - J. A. Fleming, A Handbook for the Electrical Laboratory and Testing Room, vol. i. (London, 1901); W. E. Ayrton, T. Mather and W. E. Sumpner, "On Galvanometers," Proc. Phys. Soc. London (1890), 10, 393; H. R. Kempe, A Handbook of Electrical Testing (London, 1906); A. Gray, Absolute Measurements in Electricity and Magnetism, vol. ii. part ii. (London, 1893). Useful information is also contained in the catalogues of all the principal electrical instrument makers - Messrs. Elliott Bros., Nalder, The Cambridge Scientific Instrument Company, Pitkin, Hartmann and Braun, Queen and others. (J. A. F.)
|
Galveston >> |
Categories: G-GAN
[[File:|thumb|D'Arsonval galvanometer movement.]]
A galvanometer is a type of ammeter. It is an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection, through a limited arc, in response to electric current flowing through its coil. The term has been expanded to include uses of the same mechanism in recording, positioning, and servomechanism equipment.
The most familiar use is as an analog measuring instrument, often called a meter. It is used to measure the direct current (flow of electric charge) through an electric circuit. The D'Arsonval/Weston form used today is constructed with a small pivoting coil of wire in the field of a permanent magnet. The coil is attached to a thin pointer that traverses a calibrated scale. A tiny torsion spring pulls the coil and pointer to the zero position.
When a direct current (DC) flows through the coil, the coil generates a magnetic field. This field acts against the permanent magnet. The coil twists, pushing against the spring, and moves the pointer. The hand points at a scale indicating the electric current. Careful design of the pole pieces ensures that the magnetic field is uniform, so that the angle of the pointer is proportional to the current. A useful meter generally contains provision for damping the mechanical resonance of the moving coil and pointer, so that the pointer settles quickly to its position without oscillation.
[[File:|thumb|150px|right|Tangent galvanometer made by J.H.Bunnell Co. around 1890.]]
A tangent galvanometer is an early measuring instrument used for the measurement of electric current. It works by using a compass needle to compare a magnetic field generated by the unknown current to the magnetic field of the Earth. It gets its name from its operating principle, the tangent law of magnetism, which states that the tangent of the angle a compass needle makes is proportional to the ratio of the strengths of the two perpendicular magnetic fields. It was first described by Claude Servais Mathias Pouillet in 1837.
A tangent galvanometer consists of a coil of insulated copper wire wound on a circular non-magnetic frame. The frame is mounted vertically on a horizontal base provided with levelling screws. The coil can be rotated on a vertical axis passing through its centre. A compass box is mounted horizontally at the centre of a circular scale. It consists of a tiny, powerful magnetic needle pivoted at the centre of the coil. The magnetic needle is free to rotate in the horizontal plane. The circular scale is divided into four quadrants. Each quadrant is graduated from 0° to 90°. A long thin aluminium pointer is attached to the needle at its centre and at right angle to it. To avoid errors due to parallax a plane mirror is mounted below the compass needle.
   | Selection of historic galvanometer - VL Technology: Galvanometer |
|
|
Wikis
Quiz
Search
