A system of units, now obsolete, in which the fundamental units of mass, length, and time are the kilogram, metre, and second and the fourth fundamental quantity necessary to define completely electric and magnetic quantities is the permeability of free space, μ0. The permeability was originally defined as having the value 10−7 henry/metre (Compare CGS system).
It was later shown that if μ0 were given the value 4π ×10−7 henry/metre then electric and magnetic equations would appear more rational: equations concerned with spherical systems would contain the factor 4π, which characterizes any concept of sphericity; those concerned with circular or cylindrical systems, such as a coil of wire, would contain the characteristic factor 2π; those concerned with a linear system, such as a straight wire, would not contain the factor π. Adoption of the value 4π ×10−7 H/m for μ0 leads to the rationalized MKS system, which is the basis of the system of SI units.