Relative increase or decrease in the magnetic field inside a material compared with the magnetic field in which the material is located. In empty space the magnetic permeability is 1, because there is no matter to modify the field. Materials may be classified by the value of their magnetic permeability. Diamagnetic materials (see diamagnetism) have constant relative permeabilities of slightly less than 1. Paramagnetic materials (see paramagnetism) have constant relative permeabilities of slightly more than 1. The relative permeability of ferromagnetic materials (see ferromagnetism) increases as the magnetizing field increases, reaches a maximum, and then decreases. Pure iron and some alloys have relative permeabilities of 100,000 or more.
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