It is used to express the magnitudes of various quantities in electricity and magnetism. Three systems of such units, all based on Metric system, are commonly used. One of these, the mksa-practical system, is defined in terms of the units of the mks system and has the Ampere of electric current as its basic unit. The units of this system – the Volt, Ohm, Watt and farad – are those commonly used by scientists and engineers to make practical measurements.

The two other systems, now being gradually abandoned, are both based on the cgs system.

Electrostatic units (cgs-esu) are defined in a way that simplifies the description of interactions between static electric charges. There are no corresponding magnetic units in this system.

Electromagnetic units, on the other hand, are defined especially for the description of phenomena associated with moving electric charges, i.e., electric currents and magnetic poles.