All substances respond to an applied magnetic field by becoming magnetised i.e. they behave as if they were a bar magnet themselves. The quantitative measure of the extent to which they behave like a bar magnet is called the magnetisation of the substance. The responses of different substances to applied magnetic fields normally falls in one of three broad categories:

By far the majority of substances fall into one or other of these two categories. In either category, when the sample is removed from the magnetic field, the magnetisation returns to zero. In the third category the sam-ple can possess a non-zero magnetisation in the absence of any applied magnetic field. Such materials are called ferromagnets. Other more unusual forms of magnetic response do exist, but their phenomenology is rather complex and it is not appropriate to discuss them here. The response of diamagnetic and paramagnetic substances to an applied magnetic field is shown schematically in Figure W2.1. The key feature of ferromag-netic behaviour is illustrated in Figure W2.2

This short supplementary chapter is structured as follows: