The force between two wires 1 metre apart and each carrying 1 amp is 2 x 10^-7 Newtons when in a vacuum (or in practice, air).However, it is found experimentally that the force is different if the wires are embedded in other materialsConsequently it is necessary to introduce a further parameter, known as the 'relative permeability', µ_r, so that the force on a wire of length l is:

Where µ_o = 4µ x 10^-7 Henrys/metre and µris a constant for a given material known as the relative permeability. It takes account of the effect that the medium has on the forceNote that the relative permeability is a dimensionless quantity that can be greater or less than unity.