The formal definition of the ampere is that it is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between those conductors a force equal to 2 x 10-7 newtons per metre of length.
However, the ampere is difficult to realise in practise with sufficient accuracy, so it is realised via the watt (the SI unit for power). The electrical power generated in a controlled experiment is compared to mechanical power, and using an accurate measurement of resistance the current can be calculated via: Power = (Current)2 x Resistance.
At the NPL, the volt is realised from the AC Josephson effect. Due to this effect, the potential difference between two superconductors separated by a narrow gap and exposed to electromagnetic radiation, takes discrete values dependent on the Josephson constant (483597.9 gigahertz per volt) and the frequency of radiation. This gives the volt to an accuracy of 1 hundred millionth of a volt (0.000 000 01 volt).
From May 2019, it is expected that the realisation of the ampere will be based on counting the single electrons flowing in a sophisticated semiconductor circuit. In a device under investigation at NPL, electrons are pushed through a narrow channel using tiny oscillating barriers. The current is easily compared with theory as it is simply the charge on an electron times the number of electrons transported per cycle of the barrier (frequency).