In the SI system, the base units used for electrodynamics are the meter, the kilogram, the second, and the ampere; hence this system is sometimes called MKSA. The ampere (A) is defined as the amount of current that produces a specified magnetic force per unit length between two long, straight parallel wires separated by a specified distance. The unit of electric charge is the coulomb (C), which is regarded as a derived unit:

The equations of electrodynamics in the CGS electrostatic, or Gaussian, system and the SI system are different because the latter system treats the vacuum as though it were a material medium of permittivity ? ₀ and permeability ? ₀ Hence, factors of ? ₀ and ? ₀ appear in many equations in the SI system. Besides, sundry extra factors of 4tt are inserted into some equations for the sake of simplifying a few formulas heavily used by engineers.

Because of these differences, the CGS and SI units for electric and magnetic quantities do not carry the same dimensions, and they cannot be regarded as multiples or submultiples of each other. However, for any given amount of a physical quantity say the electric charge of one electron there is a correspondence between the CGS numerical value of this charge and the SI numerical value of this charge.

Table A.1 lists these correspondences between the numerical values of physical quantities expressed in CGS and SI units. For instance, according to this table, 1 coulomb of charge corresponds...