Ampere
The ampere (A) 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 2·10-7 newton per metre of length.
The definition of the ampere in use today was proposed in 1946 by the International Committee for Weights and Measures (CIPM), and - along with the other electrical units, then called "absolute units" - adopted internationally in 1948. They were denominated "absolute" since, following purely theoretical considerations, only the magnitude of the electrical units was fixed, but not their practical realization. As a result, the definition of the ampere is not suitable for the practical realization of the unit of current; it fixes only the value of the magnetic field constant µ0. If indeed the force between the two parallel conductors placed one metre apart is calculated according to Ampere's law, and the values given in the definition are introduced into the equation, it follows
F/l: force per metre of conductor length; I: current strength; d: separation of the conductors
The definition of the ampere serves, as does the definition of the metre, only for the determination of a fundamental constant. Through the definition of µ0 and the speed of light c (definition of the metre) the electrical field constant ε0 is also defined. With knowledge of these values and the known laws of physics, many possibilities present themselves for the realization of absolute values of electrical quantities for calibration purposes. In many national metrology institutes this is done with help of quantum effects.