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