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In an atom, most of the various orbital and spin magnetic moments of the electrons add up to zero. For some metals the atom has a net magnetic moment of the order ofWhen the metal is placed in a magnetic fieldthe field exerts a torqueon each magnetic moment given bywhich tend to align the magnetic moments with the field so as to minimise the potential energy. The net effect is to add to the external magnetic field.

The additional fieldproduced by microscopic electric current loops is proportional to the total magnetic momentper unit volume in the material. This quantity is called the magnetization of the material and is denoted by

The additional magnetic field due to magnetization of the material turns out to be equal towhereWhen such a material completely surrounds a current carrying conductor the total magnetic fieldin the material iswhereis the field cause by the current in the conductor.

A material displaying the behaviour described is called paramagnetic .The magnetic field at any point in the material is greater by a factorcalled the relative permeability of the material. The relative permeability of a vacuum is 1.

The amount by which the relative permeability differs from 1 is called the magnetic susceptibility, denoted bySome values ofm for different material are contained in the following table.


Iron Ammonium Alum










Oxygen Gas


Some material reduce the electric field in the material to levels below any externally applied filed. Such materials are called diamagnetic.