Unit of electric current: ampere (A)

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.

It follows that the magnetic constant, µ0, also known as the permeability of free space, is exactly 4 x 10–7 henries per metre, µ0 = 4 x 10–7 H/m.

Unit of electric potential: volt (V)

The volt is the potential difference between two points of a conducting wire carrying a constant current of 1 ampere,
when the power dissipated between these points is equal to 1 watt.

Unit of electric resistance: ohm (Ω)

The ohm is the electric resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force. 

Practical realization of the ampere, volt and ohm 

Because the realization to high accuracy of the ampere (a base unit of the SI), the ohm and the volt (derived units of the SI) directly in terms of their definitions is difficult and time consuming, a practrical system of units was introduced in 1990. In this so-called convention  system, all electrical units are derived from the electrical quantum standards for the electrical voltage based on the Josephson Effect and the electrical resistance, based on the Quantum Hall Effect.Josephson.png

Practical representation of the ampere relies on the conventional units of the volt and the ohm, based on the agreed values of the Josephson constant (483 597.9 GHz/V) and von Klitzing constants (25 812.807 Ω).

Practical representation of the volt: the Josephson effect together with the Josephson constant can be used to establish a reference standard of electromotive force.​

Practical representation of the ohm: the quantum Hall effect together with the von Klitzing constant can be used to establish a reference standard of resistance.

Unit of capacitance: farad (F)

The farad is the capacitance of a capacitor between the plates of which there appears a potential difference of 1 volt when it is charged by a quantity of electricity of 1 coulomb

Unit of electric inductance: henry (H) 

The henry is the inductance of a closed circuit in which an electromotive force of 1 volt is produced when the electric current in the circuit varies uniformly at the rate of 1 ampere per second.

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