The ampere (A)

Quantity, unit, symbol and definition  

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 

Realisation and use at The National Physical Laboratory

 The ampere is realised, via the watt, to about 0.08 µA using NPL's current-weighing and induced-emf method. The ohm is realised at NPL via a Thompson-Lampard calculable capacitor to about 0.05 µΩ and maintained via the quantised Hall resistance to about 0.01 µΩ. The volt is maintained to 0.01 µV using the Josephson effects of superconductivity.