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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 ''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 of 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 (CGPM) 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). ==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. *The conventional volt, ''V'', is the electromotive force (or electric potential difference) measured against a Josephson effect standard using the defined value of the Josephson constant, ''K''. ''See Josephson voltage standard.'' *The conventional ohm, ''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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Conventional electrical unit」の詳細全文を読む スポンサード リンク
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