Conventional electrical unit | Wikipedia audio article
Channel: wikipedia tts
Date: 2019-06-28

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Conventional_electrical_unit


00:02:44 1 Definition
00:03:45 2 Conversion to SI units
00:05:08 3 Comparison with natural units
00:05:48 4 See also
00:06:07 5 References
00:06:40 6 External links



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SUMMARY
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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 the universal constants e, h, and ΔνCs.  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 the physical constants used 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:

In 1967, the thirteenth General Conference on Weights and Measures (CGPM) defined the second of atomic time in the International System of Units as the duration of 9192631770 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 299792458 m/s.
In 1988, the CIPM recommended adoption of conventional values for the Josephson constant as exactly KJ-90 = 483597.9×109 Hz/V and for the von Klitzing constant as exactly RK-90 = 25812.807 Ω as of 1 January 1990.
In 1991, the eighteenth CGPM noted the conventional values for the Josephson constant and the von Klitzing constant.
In 2000, the CIPM approved the use of the quantum Hall effect, with the value of RK-90 to be used to establish a reference standard of resistance.
In 2018, the twenty-sixth CGPM resolved to abrogate the conventional values of the Josephson and von Klitzing constants with the 2019 redefinition of SI base units.