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The unit of electric current in SI, one of the base units, 1946 – present. 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 It is named for André Marie Ampère (1775–1836).

engraving of Ampère

At first the ampere was realized with delicate balances able to compare electrical and mechanical forces (see, for example, Weber). Today in practice better results are obtained by realizing it 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. 

The definition to come

Recommendation 1 of the 94th meeting of the CIPM (2005) anticipated a redefinition of the ampere at the 24th CGPM in 2011. At the 24th meeting (Paris, October 2011), the CGPM again only declared its intention, but gave fuller details. The value of the elementary charge e will be made a matter of definition, rather than something to be determined experimentally. The new value will be exactly 1.602 17X × 10−19 coulombs, where X stands for one or more yet to be determined digits.

The value of the elementary charge e, in SI base units, is second amperes. The value of the second has already been fixed, by specifying by definition the frequency of light emitted by certain cesium atoms. Fixing the numerical value of the elementary charge thus determines the size of the ampere.

The new definition will not be adopted before 2014.

See history of the ampere.

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


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