Conventional electrical unit explained

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, the von Klitzing constant agreed by the International Committee for Weights and Measures (CIPM) in 1988, as well as ΔνCs used to define the second. These units are very similar in scale to their corresponding SI units, but are not identical because of the different values used for the constants. 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, and are exactly defined in terms of the universal constants e and h. The conventional electrical units represent a significant step towards using "natural" fundamental physics for practical measurement purposes. They achieved acceptance as an international standard in parallel to the SI system of units and are commonly used outside of the physics community in both engineering and industry. Addition of the constant c would be needed to define units for all dimensions used in physics, as in the SI.

The SI system made the transition to equivalent definitions 29 years later but with values of the constants defined to match the old SI units more precisely. Consequently, the conventional electrical units differ slightly from the corresponding SI units, now with exactly defined ratios.

Historical development

Several significant steps have been taken in the last half century to increase the precision and utility of measurement units:

Definition

Conventional electrical units are based on defined values of the caesium-133 hyperfine transition frequency, Josephson constant and the von Klitzing constant, the first two which allow a very precise practical measurement of time and electromotive force, and the last which allows a very precise practical measurement of electrical resistance.[8]

ConstantConventional exact value(CIPM, 1988; until 2018)Empirical value (in SI units)(CODATA, 2014)Exact valu(SI units, 2019)
133Cs hyperfine transition frequency
Josephson constantK = K = K =
von Klitzing constantR = R = R =

Conversion to SI units

UnitSymbolDefinitionRelated to SISI value (CODATA 2014)SI value (2019)
conventional voltVsee above V
conventional ohmΩ see above Ω
conventional ampereA V/Ω⋅ A
conventional coulombCsA = sV/Ω⋅ C
conventional wattWAV = V2/Ω⋅ W
conventional faradFC/V = s/Ω F
conventional henryHsΩ H

The 2019 redefinition of SI base units defines all these units in a way that fixes the numeric values of K, R and ΔνCs exactly, albeit with values of the first two that differ slightly from the conventional values. Consequently, these conventional units all have known exact values in terms of the redefined SI units. Because of this, there is no accuracy benefit from maintaining the conventional values.

See also

External links

Notes and References

  1. Web site: Resolution 1 of the 13th CGPM (1967) – SI unit of time (second) . 18 February 2019 . 11 April 2021 . https://web.archive.org/web/20210411132806/https://www.bipm.org/en/CGPM/db/13/1/ . dead .
  2. Web site: Resolution 1 of the 17th CGPM (1983) – Definition of the metre . 18 February 2019 . 29 March 2019 . https://web.archive.org/web/20190329222730/https://www.bipm.org/en/CGPM/db/17/1/ . dead .
  3. Web site: CIPM, 1988: Recommendation 1 – Representation of the volt by means of the Josephson effect . 18 February 2019 . 21 January 2021 . https://web.archive.org/web/20210121214055/https://www.bipm.org/en/CIPM/db/1988/1/ . dead .
  4. Web site: CIPM, 1988: Recommendation 2 – Representation of the ohm by means of the quantum Hall effect . 18 February 2019 . 21 January 2021 . https://web.archive.org/web/20210121221140/https://www.bipm.org/en/CIPM/db/1988/2/ . dead .
  5. Web site: Resolution 2 of the 19th CGPM (1991) – The Josephson and quantum-Hall effects . 18 February 2019 . 26 January 2021 . https://web.archive.org/web/20210126100105/https://www.bipm.org/en/CGPM/db/19/2/ . dead .
  6. Web site: CIPM, 2000 – use of the von Klitzing constant to express the value of a reference standard of resistance as a function of the quantum Hall effect . 18 February 2019 . 21 January 2021 . https://web.archive.org/web/20210121212712/https://www.bipm.org/en/committees/cipm/cipm-2000.html . dead .
  7. Web site: 26th CGPM Resolutions . BIPM . 18 February 2019.
  8. Mohr . Peter J. . Newell . David B. . Taylor . Barry N. . 2015 . CODATA recommended values of the fundamental physical constants: 2014 . Zenodo . 1507.07956 . 10.5281/zenodo.22826.