Definition:CGS Unrationalised Electromagnetic System

Definition

The CGS unrationalised electromagnetic system of units is a system of measurement in the field of electromagnetism.

This is obtained by taking as the basis the equation for the Force between Infinite Parallel Straight Conductors carrying Steady Current:

$\mathbf F \propto \dfrac {2 I_1 I_2} r$

where:

$\mathbf F$ denotes the force between two parallel infinitely long conductors in a vacuum carrying steady currents $I_1$ and $I_2$

$r$ denotes the distance between $s_1$ and $s_2$.

Let us arrange it such that:

$\mathbf F$ is expressed in dynes

$r$ is expressed in centimetres

the constant of proportion is taken to be $1$

the current is the same in both conductors: $I_1 = I_2 = I$

The base unit of electric current is defined to be the electromagnetic unit:

the electric current $I$ such as to produce a force of $1$ dyne between the two conductors when positioned $1$ centimetre apart.

The constant of proportion plays the same role in this equation as the vacuum permeability of the SI system.

As a consequence of this, the vacuum permittivity works out as being $\dfrac 1 {c^2}$.

Sources

• 1964: Milton Abramowitz and Irene A. Stegun: Handbook of Mathematical Functions ... (previous) ... (next): $2$. Physical Constants and Conversion Factors