. M AGNETISATION AND M AGNETIC I NTENSITY The earth abounds with a bewildering variety of elements and compounds. In addition, we have been synthesising new alloys, compounds and even elements. One would like to classify the magnetic properties of these substances.
In the present section, we define and explain certain terms which will help us to carry out this exercise. We have seen that a circulating electron in an atom has a magnetic moment. In a bulk material, these moments add up vectorially and they can give a net magnetic moment which is non-zero. We define magnetisation M of a sample to be equal to its net magnetic moment per unit volume: net = m M ( .
) M is a vector with dimensions L – A and is measured in a units of A m – . Consider a long solenoid of n turns per unit length and carrying a current I . The magnetic field in the interior of the solenoid was shown to be given by