of magnetic matter." It has already been mentioned that it is physically impossible to isolate north from south magnetism as completely as we can isolate positive from negative electric charges. Magnetism always appears as an attribute of a magnetic material, such as steel; and when one end of a steel bar shows north magnetisation, the other shows south magnetisation. Thus a perfect isolation of magnetic matter of one kind is not possible. The isolation can only be partial, but this need not deter us from assuming, merely for the purpose of a definition, that at a particular point, say the end of a long wire, a definite amount of north magnetic matter is accumulated, whilst the corresponding south end of the wire is so far removed that it does not interfere with any test we may make. Imagine, then, that we have in two points of space, D cm. apart, the magnetic masses M and m concentrated. The force acting between them is, by the general law discussed in Chapter I, given by the expression
Let M be fixed in space and move m round it on the surface of a sphere, then the same
