For those brought up on the SI system of units, perhaps I should say a few words about the cgs units used in almost all the papers referred to. There are essentially three differences between the cgs and SI systems:

1. Mechanical units of the cgs system were based on the Centimetre, Gram, Second, rather than on the Metre, Kilogram, Second of the (mks) Systéme International. Unit force is that needed to give unit acceleration to unit mass, so that the (cgs) dyne = 10⁻⁵ (mks) newton.
2. Electrical equations were not rationalized; in the un-rationalized (cgs) unit system the factor 4p appears in situations involving plane geometry, while in the rationalized (mks = SI) system it appears in situations involving spherical geometry. (Note that the gravitational constant G is defined by an un-rationalized equation in all systems!)
3. In the cgs system there were two sub-systems used for electrical situations:
   1. Those interested mostly in ElectroStatic (Unit) situations used cgsesu, in which e₀, the permittivity of vacuum, was defined to be unity, and unit charges were subject to unit force when separated by 1 cm.
   2. Those interested mostly in ElectroMagnetic (Unit) situations used cgsemu, in which m₀, the permeability of vacuum, was defined to be unity, and unit magnetic poles were subject to unit force when separated by 1 cm. (Note that in those days the magnetostatic field was thought of as produced by magnetic "poles"; in fact in much of the 20^th century the teaching of "electricity and magnetism" started with the (more easily demonstrated) forces and fields from (di)pole magnets, leading on to electrostatics. The teaching also included quite a bit about measurement of the geomagnetic field!)

Those working in mixed situations had to remember to put the conversion factor of c in the right place!