The ampere is the unit of electric current in the International System of Units (SI). It is one of the seven SI base units. Electric units, called "international," for current and resistance were introduced by the International Electrical Congress held in Chicago in 1893, and the definitions of the "international" ampere and the "international" ohm were confirmed by the International Conference of London in 1908. The ampere is named for André Marie Ampère (1775-1836) the self-educated French physicist and mathematician who as a key figure in the development of the fields of electricity and magnetism.
Although it was already obvious on the occasion of the 8th General Conference on Weights and Measures (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 International Committee for Weights and Measures (CIPM) in 1946:
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 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length.
The expression "MKS [meter-kilogram-second] unit of force" which occurs in the original text has been replaced here by "newton," the name adopted for this unit by the 9th CGPM (1948). Note that the effect of this definition is to fix the magnetic constant (permeability of vacuum) at exactly 4 x 10-7 H · m-1.
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