Properties of magnetic materials In an electrical coil in vaccum (number of turns n, current I, length l) a magnetic field H is produced and also a magnetic flux density or inductance B. H = nI/l B = μoH μo is the magnetic permeability in vaccum. If for example an iron core is placed in the coil B is greatly increased and we obtain B = μH μ > μo μr = μ/μo where μr is the relative permeability of the material.

Going back to the coil in vacuum; by what amount M must we increase the magnetic field in order to obtain the same inductance B as observed with the iron core? The answer is B = μH = μo(H+M) ≈ μoM since M >> H with the iron core. M is the magnetisation. The magnetic suceptibility χ = M/H is another parameter used to characterize magnetism. It is related to μr in the following way: μr = μ/μo = (H+M)/H = 1 + (M/H) = 1+ χ. A material that enhances the magnetism has μr >1, χ > 0 and a material that weakens the magnetism has μr