The constant of proportionality μ0, which appears in a number of the formulae of electromagnetism when they are written in the International System of Units (SI). Thus the induction B of a magnetic field (the magnetic induction) and its field strength H are related in vacuum by B=μ0H, where μ0 = 4π*10-7 H/m = 1.256637*10-6 H/m.