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Captain Nemo and Professor Aronnax contemplating measuring instruments in Twenty Thousand Leagues Under the Sea
A Love Meter at a Framingham, Massachusetts Rest Stop.

In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the measurement results in a given number for the relationship between the item under study and the referenced unit of measurement. Measuring instruments, and formal test methods which define their use, are the means by which this translation is made. All measuring instruments are subject to varying degrees of instrument error and measurement uncertainty.

Physicists use a vast range of instruments to perform their measurements. These range from simple objects such as rulers and stopwatches to electron microscopes and particle accelerators. Virtual instrumentation is widely used in the development of modern measuring instruments.


Time, energy, power and action



Time-points in the past can be measured with respect to the present of an observer. Time-points in the future can be fixed. But there seems to exist no device that can set time to a predetermined value (time machine), unlike it is possible with other physical quantities (for example: distance or volume). The time-point called present seems to move in one direction only. Entropy production and cause-and-effect observations of events correlate to this observation.

For more information on time, especially standards, also consult the time portal.

Timeline of time measurement technology

For the ranges of time-values see: Orders of magnitude (time)


Changing energy carriers, linear momentum to angular momentum. No measurement primarily intended.

Example: In a plant that furnishes pumped-storage hydroelectricity, mechanical work and electrical work is done by machines like electric pumps and generators. The pumped water stores mechanical work. The amount of energy put into the system equals the amount of energy which comes out of the system, less that used to overcome friction.

Such examples suggested the derivation of some unifying concepts: Instead of discerning (transferred) forms of work or stored work, there has been introduced one single quantity called energy. Energy is assumed to have substance-like qualities; energy can be apportioned and transferred. Energy cannot be created from nothing, or to be annihilated to nothing, thus energy becomes a conserved quantity, when properly balanced.

For the transfer of energy two directions are used:

(energy carriers exchanging energy) Physical interactions occur by carriers (linear momentum, electric charge, entropy) exchanging energy. For example, a generator transfers energy from angular momentum to electric charge. [1]

(energy forms transforming energy) Energy forms are transformed; for example mechanical energy into electrical energy by a generator. [2]

Often the energy value results from multiplying two related quantities: (a generalized) potential (relative velocity, voltage, temperature difference) times some substance-like quantity (linear momentum, electrical charge, entropy). — Thus energy has to be measured by first choosing a carrier/form. The measurement usually happens indirectly, by obtaining two values (potential and substance-like quantity) and by multiplying their values.

  • (see any measurement device for energy below)

For the ranges of energy-values see: Orders of magnitude (energy)

Power (current of energy)

Power describes energy exchanged by a system at a point in time (current of energy).

  • (see any measurement device for power below)

For the ranges of power-values see: Orders of magnitude (power).


Action describes energy summed up over the time a process lasts (time integral over energy). Its dimension is the same as that of an angular momentum.


This includes basic quantities found in Classical- and continuum mechanics; but strives to exclude temperature-related questions or quantities.

Length (distance)

A steel rule – an instrument for measuring length.

see also Distance measuring equipment

For the ranges of length-values see: Orders of magnitude (length)


For the ranges of area-values see: Orders of magnitude (area)


A measuring cup, a common instrument used to measure volume.

(if the mass density of a solid is known, weighing allows to calculate the volume)

For the ranges of volume-values see: Orders of magnitude (volume)

Mass- or volume flow measurement

Speed (current of length)

For the ranges of speed-values see: Orders of magnitude (speed)



A pair of scales: An instrument for measuring mass in a force field by balancing forces.

For the ranges of mass-values see: Orders of magnitude (mass)

Linear momentum

Force (current of linear momentum)

Measuring absolute pressure in an accelerated reference frame: The principle of a mercury (Hg) barometer in the gravitational field of the earth.

Pressure (current density of linear momentum)

Current density is also called flux.

For the ranges of pressure-values see: Orders of magnitude (pressure)

Timeline of temperature and pressure measurement technology


angular velocity or rotations per time unit

For the value-ranges of angular velocity see: Orders of magnitude (angular velocity)

For the ranges of frequency see: Orders of magnitude (frequency)


Orientation in three-dimensional space

See also the section about navigation below.



Energy carried by mechanical quantities, mechanical work

Electricity, electronics and electrical engineering

Considerations related to electric charge dominate Electricity and Electronics. Electrical charges interact via a field. That field is called electric if the charge doesn't move. If the charge moves, thus realizing an electric current, that field is called magnetic. Electricity can be given a quality — a potential. And electricity has a substance-like property, the electric charge. Energy (or power) in electrodynamics is calculated by multiplying the potential by the amount of charge (or current) found at that potential: potential times charge (or current). (See Classical electromagnetism and its Covariant formulation of classical electromagnetism)

An instrument for detecting net charges, the electroscope.

Electric charge

For the ranges of charge values see: Orders of magnitude (charge)

Electric current (current of charge)

Voltage (electric potential difference)

Electric resistance, electrical conductance (and electrical conductivity)

Electric capacitance

Electric inductance

Energy carried by electricity or electric energy

Power carried by electricity (current of energy)

These are instruments used for measuring electrical properties. Also see meter (electronics).

Electric field (negative gradient of electric potential, voltage per length)

Magnetic field

See also the relevant section in the article about the magnetic field.

For the ranges of magnetic field see: Orders of magnitude (magnetic field)


Temperature-related considerations dominate thermodynamics. There are two distinct thermal properties: A thermal potential — the temperature. For example: A glowing coal has a different thermal quality than a non-glowing one.

And a substance-like property, — the entropy; for example: One glowing coal won't heat a pot of water, but a hundred will.

Energy in thermodynamics is calculated by multipying the thermal potential by the amount of entropy found at that potential: temperature times entropy.

Entropy can be created by friction but not annihilated.

Amount of substance (or Mole number)

Usually determined indirectly. If mass and substance type of the sample are known, then atomic- or molecular masses (taken from a periodic table, masses measured by mass spectrometry) give direct access to the value of the amount of substance. See also the article about molar masses. If specific molar values are given, then the amount of substance of a given sample may be determined by measuring volume, mass or concentration. See also the subsection below about the measurement of the boiling point.


unit overall range
kelvin 0.01–2,000
celsius −273.14–1,700

Imaging technology

See also Temperature measurement and Category:Thermometers. More technically related may be seen thermal analysis methods in materials science.

For the ranges of temperature-values see: Orders of magnitude (temperature)

Energy carried by entropy or thermal energy

An active calorimeter lacking a temperature measurement device.

This includes thermal capacitance or temperature coefficient of energy, reaction energy, heat flow ... Calorimeters are called passive if gauged to measure emerging energy carried by entropy, for example from chemical reactions. Calorimeters are called active or heated if they heat the sample, or reformulated: if they are gauged to fill the sample with a defined amount of entropy.

see also Calorimeter or Calorimetry


Accessible indirectly by measurement of energy and temperature.

Entropy transfer

Phase change calorimeter's energy value divided by absolute temperature give the entropy exchanged. Phase changes produce no entropy and therefore offer themselves as an entropy measurement concept. Thus entropy values occur indirectly by processing energy measurements at defined temperatures, without producing entropy.

Entropy content

The given sample is cooled down to (almost) absolute zero (for example by submerging the sample in liquid helium). At absolute zero temperature any sample is assumed to contain no entropy (see Third law of thermodynamics for further information). Then the following two active calorimeter types can be used to fill the sample with entropy until the desired temperature has been reached: (see also Thermodynamic databases for pure substances)

Entropy production

Processes transferring energy from a non-thermal carrier to heat as a carrier do produce entropy (Example: mechanical/electrical friction, established by Count Rumford). Either the produced entropy or heat are measured (calorimetry) or the transferred energy of the non-thermal carrier may be measured.

  • calorimeter
  • (any device for measuring the work which will or would eventually be converted to heat and the ambient temperature)

Entropy lowering its temperature - without losing energy - produces entropy (Example: Heat conduction in an isolated rod; "thermal friction").

  • calorimeter

temperature coefficient of energy or "heat capacity"

Concerning a given sample, a proportionality factor relating temperature change and energy carried by heat. If the sample is a gas, then this coefficient depends significantly on being measured at constant volume or at constant pressure. (The terminiology preference in the heading indicates that the classical use of heat bars it from having substance-like properties.)

specific temperature coefficient of energy or "specific heat"

The temperature coefficient of energy divided by a substance-like quantity (amount of substance, mass, volume) describing the sample. Usually calculated from measurements by a division or could be measured directly using a unit amount of that sample.

For the ranges of specific heat capacities see: Orders of magnitude (specific heat capacity)

Coefficient of thermal expansion

Melting temperature (of a solid)

Boiling temperature (of a liquid)

See also thermal analysis, Heat.

More on continuum mechanics

This includes mostly instruments which measure macroscopic properties of matter: In the fields of solid state physics; in condensed matter physics which considers solids, liquids and in-betweens exhibiting for example viscoelastic behavior. Furthermore fluid mechanics, where liquids, gases, plasmas and in-betweens like supercritical fluids are studied.


This refers to particle density of fluids and compact(ed) solids like crystals, in contrast to bulk density of grainy or porous solids.

For the ranges of density-values see: Orders of magnitude (density)

Hardness of a solid

Shape and surface of a solid

Deformation of condensed matter

Elasticity of a solid (Elastic moduli)

  • resonant frequency and Damping Analyser (RFDA), using the impulse excitation technique: A small mechanical impulse causes the sample to vibrate. The vibration depends on elastic properties, density, geometry and inner structures (lattice or fissures).

Plasticity of a solid

Measurement results (a) brittle (b) ductile with breaking point (c) ductile without breaking point.

Tensile strength, ductility or malleability of a solid

Granularity of a solid or a suspension

Viscosity of a fluid

Optical activity

Surface tension of liquids

Imaging technology

  • Tomograph, device and method for non-destructive analysis of multiple measurements done on a geometric object, for producing 2- or 3-dimensional images, representing the inner structure of that geometric object.
  • Wind tunnel

This section and the following sections include instruments from the wide field of Category:Materials science, materials science.

More on electric properties of condensed matter, gas

The electrochemical cell: A device for measuring substance potentials.

Permittivity, relative static permittivity, (dielectric constant) or electric susceptibility

Such measurements also allow to access values of molecular dipoles.

Magnetic susceptibility or magnetization

For other methods see the section in the article about magnetic susceptibility.

See also the Category:Electric and magnetic fields in matter

Substance potential or chemical potential or molar Gibbs energy

A reaction transmuting substances, from reactants to products, has an overall energy balance which consists of two parts: A balance that accounts for the changed entropy content of the substances. And another one that accounts for the energy freed or taken by that reaction itself, the Gibbs energy change. The sum of reaction energy and energy associated to the change of entropy content is also called enthalpy. Often the whole enthalpy is carried by entropy and thus measurable calorimetrically. For standard conditions in chemical reactions either molar entropy content and molar Gibbs energy with respect to some chosen zero point are tabulated. Or molar entropy content and molar enthalpy with respect to some chosen zero are tabulated. (See Standard enthalpy change of formation and Standard molar entropy)

The substance potential of a redox reaction is usually determined electrochemically using reversible cells.

See also the article on electrochemistry.

Sub-microstructural properties of condensed matter, gas

Crystal structure

Imaging technology, Microscope

Rays ("waves" and "particles")

Sound, compression waves in matter

Microphones in general, sometimes their sensitivity is increased by the reflection- and concentration principle realized in acoustic mirrors.

Sound pressure

A device for unmixing sun-light: the prism.

Light and radiation without a rest mass, non-ionizing

(for lux meter see the section about human senses and human body)

See also Category:Optical devices

Photon polarization

Pressure (current density of linear momentum)

radiant flux

The measure of the total power of light emitted.

A Cathode ray tube.

Radiation with a rest mass, particle radiation

Cathode ray

Atom polarization and electron polarization

Another visualization of the electromagnetic spectrum.

Ionizing radiation

Ionizing radiation includes rays of "particles" as well as rays of "waves". Especially X-rays and Gamma rays transfer enough energy in non-thermal, (single) collision processes to separate electron(s) from an atom.

A cloud chamber detecting alpha-rays.

particle current

Identification and content

This could include chemical substances, rays of any kind, elementary particles, quasiparticles. Many measurement devices outside this section may be used or at least become part of an identification process.

For identification and content concerning chemical substances see also analytical chemistry especially its List of chemical analysis methods and the List of materials analysis methods.

Substance content in mixtures, substance identification

pH: Concentration of protons in a solution


Human senses and human body


Luminuos flux, photometry

A measure of the perceived power of light, luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.

illuminance, photometry


Loudness in phon


Temperature (sense and body)

Body temperature or Core temperature

circulatory system (mainly heart and blood vessels for distributing substances fast)

Blood-related parameters are listed in a blood test.

Respiratory system (lung and airways controlling the breathing process)

A spirometer, inhaling into pipe a fills volume b, the rest balances forces.

concentration or partial pressure of carbon dioxide in the respiratory gases

nervous system (nerves transmitting and processing information electrically)

musculoskeletal system (muscles and bones for movement)

power, work of muscles

metabolic system

Medical imaging

An echocardiogram processed into a three dimensional representation.

See also: Category:Physiological instruments and Category:Medical testing equipment.


See also Category:Meteorological instrumentation and equipment.

Navigation and Surveying

See also Category:Navigational equipment and Category:Navigation. See also Category:Surveying instruments.


See also Category:Astronomical instruments and Category:Astronomical observatories.


Some instruments, such as telescopes and sea navigation instruments, have had military applications for many centuries. However, the role of instruments in military affairs rose exponentially with the development of technology via applied science, which began in the mid-19th century and has continued through the present day. Military instruments as a class draw on most of the categories of instrument described throughout this article, such as navigation, astronomy, optics and imaging, and the kinetics of moving objects. Common abstract themes that unite military instruments are seeing into the distance, seeing in the dark, knowing an object's geographic location, and knowing and controlling a moving object's path and destination.

Special features of these instruments may include ease of use, speed, reliability and accuracy; nevertheless additionally one might hope seeing them as instruments whose existence, not use, ultimately helps in establishing a humane and humanistic peace between individual humans as well as groups of them.

Uncategorized, specialized, or generalized application

Fictional devices

  • Tricorder, a multipurpose scanning device, originating from the science-fictional Star Trek series.

See also


Note that the alternate spelling "-metre" is never used when referring to a measuring device.


  1. ^ Fuchs, Hans U. (1996). The Dynamics of Heat. Springer. ISBN 0387946039.  
  2. ^ Callen, Herbert (1985). Thermodynamics and an introduction to Thermostatics. John Wiley & Sons, Inc.. ISBN 0471610569.  


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