A conventional electrical unit (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" – eg, the conventional volt has the symbol V₉₀ – as they came into international use on 1 January 1990.

[edit] Definition

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 (CIPM, 1988)	Empirical value (in SI units) (CODATA, 2006)
Josephson constant	KJ–90 = 483 597.9 GHz/V90	KJ = 483 597.891(12) GHz/V
von Klitzing constant	RK–90 = 25 812.807 Ω90	RK = 25 812.807 557(18) Ω

• The conventional volt, V₉₀, is the electromotive force (or electric potential difference) measured against a Josephson effect standard using the defined value of the Josephson constant, K_J–90.
• The conventional ohm, Ω₉₀, is the electrical resistance measured against a quantum Hall effect standard using the defined value of the von Klitzing constant, R_K–90.
• Other conventional electrical units are defined by the normal physical relationships, as in the conversion table below.

[edit] Conversion to SI units

Unit	Definition	SI equivalent (CODATA 2006)
conventional volt	see above	V90 = (KJ–90/KJ) V = [1 + 1.9(2.5) × 10–8] V
conventional ohm	see above	Ω90 = (RK/RK–90) Ω = [1 + 2.159(68) × 10–8] Ω
conventional ampere	A90 = V90/Ω90	A90 = [1 – 0.3(2.5) × 10–8] A
conventional coulomb	C90 = A90 s = s V90/Ω90	C90 = [1 – 0.3(2.5) × 10–8] C
conventional watt	W90 = A90V90 = V902/Ω90	W90 = [1 + 1.6(5.0) × 10–8] W
conventional farad	F90 = C90/V90 = s/Ω90	F90 = [1 – 2.159(68) × 10–8] F
conventional henry	H90 = Ω90 s	H90 = [1 + 2.159(68) × 10–8] H

[edit] References

• Mohr, Peter J.; Taylor, Barry N.; Newell, David B. (2008). "CODATA Recommended Values of the Fundamental Physical Constants: 2006". Rev. Mod. Phys. 80: 633–730. doi:10.1103/RevModPhys.80.633. http://physics.nist.gov/cuu/Constants/codata.pdf.