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Electricity and Magnetism: Units for electrical quantities
Units for electrical quantities
SI Brochure, Section

    Electric units, called "international units", for current and resistance, were introduced by the International Electrical Congress held in Chicago in 1893, and definitions of the "international ampere" and "international ohm" were confirmed by the International Conference in London in 1908.

    Although it was already obvious on the occasion of the 8th CGPM (1933) that there was a unanimous desire to replace those "international units" by so-called "absolute units", the official decision to abolish them was only taken by the 9th CGPM (1948), which adopted the ampere for the unit of electric current, following a definition proposed by the CIPM (1946, Resolution 2):

      The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.

    It follows that the magnetic constant, mu0, also known as the permeability of free space, is exactly 4 x 10–7 henries per metre, mu0 = 4 x 10–7 H/m.

    The expression "MKS unit of force" which occurs in the original text of 1946 has been replaced here by "newton", a name adopted for this unit by the 9th CGPM (1948, Resolution 7).

Decisions relating to the base units of the SI
SI Brochure, from Appendix 1

    Electrical units:

    CIPM, 1946 definition of mechanical and electrical units in the SI
    14th CGPM, 1971 adopts the name siemens, synmbol S, for electrical conductance
    18th CGPM, 1987 forthcoming adjustment to the representation of the volt and of the ohm
    CIPM, 1988 Josephson effect
    CIPM, 1988 quantum Hall effect
    CIPM, 2000 realization of the ohm using the value of the von Klitzing constant
    23rd CGPM, 2007 on the possible redefinition of certain base units of the International System of Units (SI)
    24th CGPM, 2011 on the possible future revision of the International System of Units, the SI
    25th CGPM, 2014 on the future revision of the International System of Units, the SI
Electricity and Magnetism
Units for electrical quantities
Committee structure
Strategic plan
International equivalence
Calibration and measurement capabilities (CMCs)
International comparisons
At the BIPM
BIPM laboratory work
Ongoing BIPM comparisons
BIPM calibrations of electrical standards
Recent publications in the scientific literature
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