e.s.u. system

electrostatic units

electromagnetics Metric-c.g.s. One of two widely used schemes of electrical units within the defunct c.g.s. system, the other and more used being the e.m.u. system. The unitary base of the e.s.u. system is defined as the charge that exerts a force of 1 dyne on an identical charge 1 cm distant in a vacuum. 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 dielectric permittivity; most are labelled with familiar metric names prefixed by ‘stat’ (from electrostatic), though such a prefix may be omitted within a clear context; all can be referred to as ‘e.s. unit of …’ or ‘electrostatic unit of …’.

The e.m.u. system adopts the same basic definitions and relational equations, but has magnetic permeability set at 1. Since the product of magnetic permeability and dielectric permittivity 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~ × 10¹⁰ cm·s^-1). Thus for:

volt	ab- = c-1 stat-	stat- = c+1 ab-
so	ab- = 33.356 35~ × 10-12 stat-	stat- = 29.979 30~ × 109 ab-

ampere,

coulomb	ab- = c+1 stat-	stat- = c-1 ab-
so	ab- = 29.979 30~ × 109 stat-	stat- = 33.356 35~ × 10-12 ab-

farad,

mho	ab- = c+2 stat-	stat- = c-2 ab-
so	ab- = 898.758 4~ × 1018 stat-	stat- = 1.112 646~ × 10-21 ab-

henry,

ohm	ab- = c-2 stat-	stat- = c+2 ab-
so	ab- = 1.112 646~ × 10-21 stat-	stat- = 898.758 4~ × 1018 ab-

Reference should be made to electrostatic unit or to particular unit names for relevant definitions and SI equivalences, and to e.m.u. system for comparative notes.