electromagnetics Metric-c.g.s. One of two widely used schemes of electrical units developed in the 1860s and 1870s within the defunct c.g.s. system, the other being the e.s.u. system. The unitary base of the e.m.u. system is defined by a theoretical magnetic pole that exerts a force of 1 dyne on an identical pole 1 centimetre distant in a vacuum, with a magnetic permeability value set at 1. Thus all the electrical units are based on purely mechanical units; none are base units, such as the ampere is in the SI. All units are derived by established relational equations, with the adoption of value 1 for magnetic permeability; most are labelled with familiar metric names prefixed by ‘ab’ (from absolute); all can be referred to as ‘e.m. unit of …’ or ‘electromagnetic unit of …’.
The e.s.u. units have the same definitions and relationships but with dielectric permittivity set at 1. Since the product of dielectric permittivity and magnetic permeability inherently equals the square of c, the speed of light, the corresponding units of the respective systems are related by various powers of c (established during the c.g.s. era as 2.997 930~ × 1010cm·s-1). See e.s.u. system for the cross-system numeric factors.
Reference should be made to electromagnetic unit or to particular unit names for relevant definitions and SI equivalences.
These absolute units were mostly far from the amounts occurring in everyday work, so were augmented by decimal-power multipliers to become practical units, introducing the now-familiar unqualified names:
| 1 ampere | = 10-1 abampere |
| 1 volt | = 108 abvolt |
| 1 ohm | = 109 abohm |
| 1 farad | = 109 abfarad |
| 1 henry | = 109 abhenry |
The basis of definition was changed to an ohm expressed in terms of the resistance of a specified column of mercury (Hg ohm), with an equivalent ampere (Ag ampere) expressed as a depositional rate of silver. In 1901 it was also noted that, were permeability set at 10-7 rather than 1, the whole array of terms fitted an m.k.s. scheme without those decimal multipliers. However, at the same time, the new laboratory definitions were found to be at minor variance with the previous ones. The consequence was agreement in 1908 on distinctively labelled international units, with
| 1 international ohm | = 1.000 49~ abohm |
| 1 international volt | = 1.000 34~ abvolt |
