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The unit of electric current in SI, one of the base units. Symbol A. After 20 May 2019,

It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10⁻¹⁹ when expressed in the unit C, which is equal to A s, where the second is defined in terms of Δν_{cs}.

Resolutions Adopted.

Resolution 1, appendix 3.

26 CGPM, Versailles 13-16 November 2018.

BIPM.

Here C is the symbol for the coulomb, the SI unit of charge.

The previous definition was abrogated. The redefinition of the ampere was part of a revolutionary revision of SI as a whole, with the aim of basing the system solely on fundamental constants given exact values by definition. The ampere's definition is based on two such constants: *e*, the elementary charge on the electron, given the value quoted above, and the unperturbed ground state hyperfine transition frequency of the cesium 133 atom (which is the Δν_{cs} in the definition), set at exactly 9 192 631 770 hertz. Defining a frequency in hertz defines the second.

Recommendation 1 of the 94th meeting of the CIPM (2005) anticipated the ampere would be redefined at the 24^{th} CGPM in 2011. At that meeting (Paris, October 2011), the CGPM again only declared its intention, but gave fuller details. The value of the elementary charge *e* was to be made a matter of definition, rather than something to be determined experimentally. The new value was to be *exactly* 1.602 17X × 10⁻¹⁹ coulombs, where X stood for one or more yet to be determined digits. More years of measuring *e* enabled them to add 4 more decimal places.

For a number of years experimentalists had found the previous definition of the ampere was not precise enough for their needs, and they stopped using the SI unit. Instead, they realized the ampere from the relationship “amperes equals voltage divided by resistance,” using quantum voltage and resistance standards: the voltage determined by a Josephson voltage standard and the resistance by a quantized Hall resistance. In the literature, such work is often said to have been done in “conventional” units, an indication that SI units are not meant.

The ampere is named for André Marie Ampère (1775–1836).

It had been proposed by the CIPM in 1946 (resolution 2) and adopted by the CGPM in 1948 (9th meeting) One 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 a vacuum, would produce between these conductors a force equal to 2 × 10⁻⁷ newtons per meter of length.¹

So far as is known, no lab has a pair of infinite conductors, not even in ambient air. Calculations on results from an actual setup must be used to determine the amperes numerical value in the ideal state described.

Previous to the 1948 definition the ampere had been realized in a variety of ways: with delicate balances able to compare electrical and mechanical forces (see, for example, Weber), by electroplating defined masses of metal, and so on. See history of the ampere.

1. CIPM 1946 Resolution 2, approved by the Ninth CGPM in 1948.

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Last revised: 27 November 2018.