For most core materials, μ μ r ≈ ≈ 2000 – 6000 . So in equation (2) above, the second term dominates. Therefore, in magnetic circuits with an air gap, B depends strongly on the length of the air gap, and the length of the flux path in the core does not matter much. Given an air gap of 1mm, a magnetomotive force of about 796 Ampere-turns is required to produce a magnetic field of 1T.