Conventional electrical unit
Encyclopedia
A conventional electrical unit (is also known as a electrical symbol) (or simply conventional unit where there is no risk of ambiguity) is a unit of measurement in the field of electricity
which is based on the conventional values of the Josephson constant and the von Klitzing constant agreed by the International Committee for Weights and Measures
(CIPM) in 1988. These units are very similar in scale to their corresponding SI units, but are not identical because of their different definition. They are distinguished from the corresponding SI units by setting the symbol in italic typeface and adding a subscript "90" – e.g., the conventional volt has the symbol V – as they came into international use on 1 January 1990.
This system was developed to increase the precision of measurements: The Josephson and von Klitzing constants can be realized with great precision, repeatability and ease. The conventional electrical units have achieved acceptance as an international standard and are commonly used outside of the physics community in both engineering and industry.
The conventional electrical units are “quasi-natural
” in the sense that they are completely and exactly defined in terms of fundamental physical constants. They are the first set of measurement units to be defined in this way, and as such, represent a significant step towards using "natural" fundamental physics for practical measurement purposes. However, the conventional electrical units are unlike other systems natural units in that some physical constants are not set to unity but rather set to fixed numerical values that are very close (but not precisely the same) to those in the common SI
system of units.
Four significant steps were taken in the last half century to increase the precision and utility of measurement units. In 1967 the Thirteenth General Conference on Weights and Measures defined the second
of atomic time in the International System of Units as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. In 1983, the seventeenth CGPM redefined the metre
in terms of the second and the speed of light, thus fixing the speed of light at exactly 299,792,458 m/s. And in 1990, the eighteenth CGPM adopted conventional values for the Josephson constant and the von Klitzing constant, fixing the conventional Josephson constant at exactly 483,597.9 Hz/"V", and the conventional von Klitzing constant at exactly 25 812.807 "Ω" (again, these volts and ohms are not precisely the same as the SI definitions but very nearly equivalent).
and electrical resistance
respectively.
defined as
having consequence:
.
This is a more general (or less specific) version of either the particle physics "Natural units
" or the quantum chromodynamical system of units
but that no unit mass is fixed. Like n.u. or QCD units, with conventional electrical units any observed variation over space or time in the value of the fine-structure constant, α, is attributed to variation in the Coulomb constant or vacuum permittivity or, since the speed of light, c, is fixed, as a variation in the vacuum permeability
.
The following table provides a comparison of conventional electrical units with other natural unit systems:
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
which is based on the conventional values of the Josephson constant and the von Klitzing constant agreed by the International Committee for Weights and Measures
International Committee for Weights and Measures
The Interglobal Committee for Weights and Measures is the English name of the Comité international des poids et mesures . It consists of eighteen persons from Member States of the Metre Convention...
(CIPM) in 1988. These units are very similar in scale to their corresponding SI units, but are not identical because of their different definition. They are distinguished from the corresponding SI units by setting the symbol in italic typeface and adding a subscript "90" – e.g., the conventional volt has the symbol V – as they came into international use on 1 January 1990.
This system was developed to increase the precision of measurements: The Josephson and von Klitzing constants can be realized with great precision, repeatability and ease. The conventional electrical units have achieved acceptance as an international standard and are commonly used outside of the physics community in both engineering and industry.
The conventional electrical units are “quasi-natural
Natural units
In physics, natural units are physical units of measurement based only on universal physical constants. For example the elementary charge e is a natural unit of electric charge, or the speed of light c is a natural unit of speed...
” in the sense that they are completely and exactly defined in terms of fundamental physical constants. They are the first set of measurement units to be defined in this way, and as such, represent a significant step towards using "natural" fundamental physics for practical measurement purposes. However, the conventional electrical units are unlike other systems natural units in that some physical constants are not set to unity but rather set to fixed numerical values that are very close (but not precisely the same) to those in the common SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
system of units.
Four significant steps were taken in the last half century to increase the precision and utility of measurement units. In 1967 the Thirteenth General Conference on Weights and Measures defined the second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
of atomic time in the International System of Units as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. In 1983, the seventeenth CGPM redefined the metre
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
in terms of the second and the speed of light, thus fixing the speed of light at exactly 299,792,458 m/s. And in 1990, the eighteenth CGPM adopted conventional values for the Josephson constant and the von Klitzing constant, fixing the conventional Josephson constant at exactly 483,597.9 Hz/"V", and the conventional von Klitzing constant at exactly 25 812.807 "Ω" (again, these volts and ohms are not precisely the same as the SI definitions but very nearly equivalent).
Definition
Conventional electrical units are based on defined values of the Josephson constant and the von Klitzing constant, which allow practical measurements of electromotive forceElectromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...
and electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
respectively.
| Constant | Conventional (defined) value (CIPM, 1988) |
Empirical value (in SI units) (CODATA, 2006) |
|---|---|---|
| Josephson constant | K = 483 597.9 GHz/V | K = 483 597.891(12) GHz/V |
| von Klitzing constant | R = 25 812.807 Ω | R = 25 812.807 557(18) Ω |
- The conventional voltVoltThe volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
, V, is the electromotive force (or electric potential difference) measured against a Josephson effectJosephson effectThe Josephson effect is the phenomenon of supercurrent across two superconductors coupled by a weak link...
standard using the defined value of the Josephson constant, K. - The conventional ohmOhmThe ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
, Ω, is the electrical resistance measured against a quantum Hall effect standard using the defined value of the von Klitzing constant, R. - Other conventional electrical units are defined by the normal physical relationships, as in the conversion table below.
Conversion to SI units
| Unit | Definition | SI equivalent (CODATA 2006) |
|---|---|---|
| conventional volt Volt The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the... |
see above | V = (K/K) V = [1 + 1.9(2.5)] V |
| conventional ohm Ohm The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,... |
see above | Ω = (R/R) Ω = [1 + 2.159(68)] Ω |
| conventional ampere Ampere The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics... |
A = V/Ω | A = [1 − 0.3(2.5)] A |
| conventional coulomb | C = A s Second The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock.... = s Second The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock.... V/Ω |
C = [1 − 0.3(2.5)] C |
| conventional watt Watt The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:... |
W = AV = V2/Ω | W = [1 + 1.6(5.0)] W |
| conventional farad Farad The farad is the SI unit of capacitance. The unit is named after the English physicist Michael Faraday.- Definition :A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second... |
F = C/V = s Second The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock.... /Ω |
F = [1 − 2.159(68)] F |
| conventional henry | H = Ω s Second The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock.... |
H = [1 + 2.159(68)] H |
Comparison with natural units
Conventional electrical units can be thought of as a scaled version of a system of natural unitsNatural units
In physics, natural units are physical units of measurement based only on universal physical constants. For example the elementary charge e is a natural unit of electric charge, or the speed of light c is a natural unit of speed...
defined as
having consequence:
.This is a more general (or less specific) version of either the particle physics "Natural units
Natural units
In physics, natural units are physical units of measurement based only on universal physical constants. For example the elementary charge e is a natural unit of electric charge, or the speed of light c is a natural unit of speed...
" or the quantum chromodynamical system of units
Natural units
In physics, natural units are physical units of measurement based only on universal physical constants. For example the elementary charge e is a natural unit of electric charge, or the speed of light c is a natural unit of speed...
but that no unit mass is fixed. Like n.u. or QCD units, with conventional electrical units any observed variation over space or time in the value of the fine-structure constant, α, is attributed to variation in the Coulomb constant or vacuum permittivity or, since the speed of light, c, is fixed, as a variation in the vacuum permeability
Vacuum permeability
The physical constant μ0, commonly called the vacuum permeability, permeability of free space, or magnetic constant is an ideal, physical constant, which is the value of magnetic permeability in a classical vacuum...
.
The following table provides a comparison of conventional electrical units with other natural unit systems:
| Quantity / Symbol | Planck | Stoney | Schrödinger | Atomic | Electronic | Conventional Electrical Units |
|---|---|---|---|---|---|---|
speed of light in vacuum  |
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Planck's constant  |
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reduced Planck's constant  |
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| elementary charge Elementary charge The elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...  |
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Josephson constant  |
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von Klitzing constant  |
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characteristic impedance of vacuum  |
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| electric constant Electric constant The physical constant ε0, commonly called the vacuum permittivity, permittivity of free space or electric constant is an ideal, physical constant, which is the value of the absolute dielectric permittivity of classical vacuum... (vacuum permittivity)  |
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magnetic constant (vacuum permeability)  |
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| Newtonian constant of gravitation Gravitational constant The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...  |
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| electron mass Electron The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...  |
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| Hartree energy Hartree energy The hartree , also known as the Hartree energy, is the atomic unit of energy, named after the British physicist Douglas Hartree. It is defined as...  |
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| Rydberg constant Rydberg constant The Rydberg constant, symbol R∞, named after the Swedish physicist Johannes Rydberg, is a physical constant relating to atomic spectra in the science of spectroscopy. Rydberg initially determined its value empirically from spectroscopy, but Niels Bohr later showed that its value could be calculated...  |
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| caesium Caesium Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature... ground state Ground state The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state... hyperfine Hyperfine structure The term hyperfine structure refers to a collection of different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with electron... transition frequency Second The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock.... |
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