The equivalent equations for the magnetic B -field are the same except for a multiplicative factor of μ 0 = 4 π × 10 H / m, where μ 0 is known as the vacuum permeability. For example: B ( r ) = μ μ 0 4 π π 3 r ^ ^ ( r ^ ^ ⋅ ⋅ m ) − − m | r | 3 .