Introduction

We know that there are certain metallic bodies which attract one another and which, if suitably suspended will tend to point in the north-south direction. These materials are magnetic. We also know that if a current is passed through a coil of wire, the coil itself behaves like a magnet and will attract a bar magnet or another similar coil. If we insert certain ‘magnetic’ materials into the coils the interaction between the coils will be changed. The coil also possesses a certain self-inductance and if we insert similar ‘magnetic’ materials into the coil its self-inductance will be increased.

These are some of the basic observations involved in magnetism and the purpose of this chapter is to discuss the magnetic properties of matter in terms of atomic and electronic mechanisms. There are many resemblances to dielectric effects the main differences being that

1. (a) there is no such thing as an isolated magnetic pole;

2. (b) there is no magnetic equivalent of a condenser;

3. (c) there is no dielectric analogue of a solenoid.

The most fundamental approach to magnetism is due to Einstein. He considered the electric field generated by a moving charge and the force it would exert on another charge moving parallel to it with the same velocity. Because of relativity effects the space ahead of the moving charge is crowded and this compression of space in which the electric field operates produces an additional force on the other charge over and above that calculated from the classical coulombic law.