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:

• The ‘bar magnet’ created in the sample lines up so that the magnetic field inside the material is in-creased. The energy of the atoms in the material is lowered by applying a field and so the sample tries to move into regions of high magnetic field. Such materials are called paramagnets.
• The ‘bar magnet’ created in the sample lines up so that the field inside the material is decreased. The energy of the atoms in the material is increased by applying a field and so the sample tries to move away from regions of high magnetic field. Such materials are called diamagnets.

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:

• We begin by discussing the data on the magnetic properties of the elements. However, in order to do this we need to acquaint ourselves with some of the terminology of magnetism and immediately we come face to face to face with the vexed issue of magnetic units. All I wish to say on this matter is that things are better now than they used to be, but any scientist using non-SI units to describe their results should be ashamed of themselves.
• We then take a look at the data on the elements and move on to try to understand the basic phenomena which we observe. We need to consider many features of the electronic structure of solids in order to ar-rive at even a semi-quantitative understanding of the data. We will see that the magnetic response of any particular substance can be considered as the sum of many different contributions.
• Finally, we mention briefly the truly astounding phenomenon of ferromagnetism. In my experience as a physicist I have seen nothing which continually fascinates and astounds me more than the strange repul-sion and attraction of ferromagnets.