# Measuring instrument

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Captain Nemo and Professor Aronnax contemplating measuring instruments in Twenty Thousand Leagues Under the Sea

A measuring instrument is a device for measuring a physical quantity. 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 process of measurement gives a number relating the item under study and the referenced unit of measurement. Measuring instruments, and formal test methods which define the instrument's use, are the means by which these relations of numbers are obtained. All measuring instruments are subject to varying degrees of instrument error and measurement uncertainty.

Scientists, engineers and other humans use a vast range of instruments to perform their measurements. These instruments may 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

Watch, a time measurement device

In the past, a common time measuring instrument was the sundial. Today, the usual measuring instruments for time are clocks and watches. For highly accurate measurement of time an atomic clock is used.

## Energy

Energy is measured by an energy meter. Examples of energy meters include:

### Electricity meter

An electricity meter measures energy directly in kilowatt hours.

### Gas meter

A gas meter measures energy indirectly by recording the volume of gas used. This figure can then be converted to a measure of energy by multiplying it by the calorific value of the gas.

## Power (flux of energy)

A physical system that exchanges energy may be described by the amount of energy exchanged per time-interval, also called power or flux of energy.

• (see any measurement device for power below)

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

## Action

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.

## Mechanics

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

### Length (distance)

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

### Area

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

### Volume

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)

### Speed (flux of length)

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

### Mass

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)

### Force (flux 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 (flux density of linear momentum)

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

### 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.

## 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, especially in an electrically neutral conductor, 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 elementary 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)

### Power carried by electricity (current of energy)

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

### 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)

### Combination instruments

• Multimeter, combines the functions of ammeter, voltmeter and ohmmeter as a minimum.
• LCR meter, combines the functions of ohmeter, capacitance meter and inductance meter. Also called component bridge due to the bridge circuit method of measurement.

## Thermodynamics

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)

A physical quantity introduced in chemistry; 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.

### Temperature

#### 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

### Entropy

Entropy is 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)

### 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.

### Density

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)

### Deformation of condensed matter

#### Plasticity of a solid

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

### 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

Phase conversions like changes of aggregate state, chemical reactions or nuclear reactions transmuting substances, from reactants to products, or diffusion through membranes have an overall energy balance. Especially at constant pressure and constant temperature molar energy balances define the notion of a substance potential or chemical potential or molar Gibbs energy, which gives the energetic information about whether the process is possible or not - in a closed system.

Energy balances that include entropy consist 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 current-free using reversible cells.

Other values may be determined indirectly by calorimetry. Also by analyzing phase-diagrams.

See also the article on electrochemistry.

## Sub-microstructural properties of condensed matter, gas

### Imaging technology, Microscope

See also the article on spectroscopy and the list of materials analysis methods.

## 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

#### radiant flux

The measure of the total power of light emitted.

A Cathode ray tube.

### Radiation with a rest mass, particle radiation

#### 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.

## 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.

## Human senses and human body

### Sight

#### Brightness: photometry

Photometry is the measurement of light in terms of its perceived brightness to the human eye. Photometric quantities derive from analogous radiometric quantities by weighting the contribution of each wavelength by a luminosity function that models the eye's spectral sensitivity. For the ranges of possible values, see the orders of magnitude in: illuminance, luminance, and luminous flux.

### 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.

### Medical imaging

An echocardiogram processed into a three dimensional representation.

## Navigation and surveying

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

## Military

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.
• Sonic Screwdriver, a multifunctional device used occasionally for scanning, originating from the science-fictional Doctor Who series.

## Notes

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