physics. Symbol ɛ0. SI The two elements electric permittivity (dimensionally farads per metre) and magnetic permeability (henrys per metre) of a vacuum are immeasurable, but their product equals the reciprocal of the square of c, the speed of light in vacuum. Within this constraint, the values can be chosen arbitrarily. With the c.g.s. system two basic competing choices were made; the e.m.u. system had permeability set at 1, the e.s.u. system had permittivity set at 1. The practical units derived from the former and which became those of the pioneer m.k.s. system, effectively had permeability set at 10-7.
The equations of electromagnetic theory routinely involve these two elements and the irrational number pi, with 4π as a multiplier of permittivity, and a divider of permeability. To avoid unnecessary proliferation of irrational numbers, for the SI an offsetting 4π was built into the adopted definitions of the elements (making it a ‘rationalized’ system), giving
permittivity ɛ0 =(4π·c2)-1 × 107 F·m-1 = 8.854 188~ × 10-12 C2·m-1·J-1
permeability μ0 = 4π × 10-7 H·m-1 = 1.256 637~ × 10-6 N·A-2now called, respectively, the electric constant and the magnetic constant.
