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A conventional electrical unit (or conventional unit where there is no risk of ambiguity) is a unit of measurement in the field of electricity which is based on the so-called "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 V90 – 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 universal constants. They represent a significant step towards using "natural" fundamental physics for practical measurement purposes. However, the conventional electrical units are unlike other systems of natural units in that some physical constants are not set to unity but rather set to fixed numerical values that are very close to (but not precisely the same as) those in the SI system of units.
Several significant steps have been taken in the last half century to increase the precision and utility of measurement units:
Conventional electrical units are based on defined values of the Josephson constant and the von Klitzing constant, which allow practical measurements of electromotive force and electrical resistance respectively. 
|Constant||Conventional (defined) value
|Empirical value (in SI units)|
|Josephson constant||KJ-90 = 483597.9GHz/V||KJ = 483597.8525(30)GHz/V|
|von Klitzing constant||RK-90 = 25812.807Ω||RK = 25812.8074555(59)Ω|
|Unit||Definition||SI equivalent (CODATA 2014)|
|conventional volt||see above||V90 = (KJ-90/KJ) V = (1 + 9.83(61)×10−8) V|
|conventional ohm||see above||Ω90 = (RK/RK-90) Ω = (1 + 1.765(23)×10−8) Ω|
|conventional ampere||A90 = V90/Ω90||A90 = (1 + 8.06(61)×10−8) A|
|conventional coulomb||C90 = A90⋅s = s⋅V90/Ω90||C90 = (1 + 8.06(61)×10−8) C|
|conventional watt||W90 = A90V90 = V902/Ω90||W90 = (1 + 17.9(1.2)×10−8) W|
|conventional joule||J90 = C90⋅V90 = s⋅V902/Ω90||J90 = (1 + 17.9(1.2)×10−8) J|
|conventional farad||F90 = C90/V90 = s/Ω90||F90 = (1 − 1.765(23)×10−8) F|
|conventional henry||H90 = Ω90⋅s||H90 = (1 + 1.765(23)×10−8) H|
The 2019 redefinition of SI base units defines all these units in a way that fixes the numeric values of KJ and RK exactly, albeit with values that differ slightly from the conventional values, as well as leaving the definition of the second unchanged. Consequently, these conventional units all have known exact values in terms of the redefined SI units. Because of this, there is be no accuracy benefit from maintaining the conventional values.
Conventional electrical units can be thought of as a scaled version of a system of natural units defined as
The following table provides a comparison of conventional electrical units with other natural unit systems:
|Quantity||Other Systems||Conventional electrical units|
|Speed of light in vacuum|
|Reduced Planck's constant|
|von Klitzing constant|
|Characteristic impedance of vacuum|
|Electric constant (vacuum permittivity)|
|Magnetic constant (vacuum permeability)|
|Newtonian constant of gravitation|
|Cesium ground state hyperfine transition frequency|