Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/257

 mutual action of the molecular magnets themselves. In the unmagnetized condition the molecules "arrange themselves so as to satisfy their mutual attraction by the shortest path, and thus form a complete closed circuit of attraction," as D. E. Hughes wrote in 1883; when an external magnetizing force is applied, these small circuits are broken up; and at any stage of the process a molecular magnet is in equilibrium under the joint influence of the external force and the forces due to the other molecules.

This hypothesis was suggested by Maxwell, and has been since developed by J. A. Ewing: its consequences may be illustrated by the following simple examples :—

Consider two magnetic molecules, each of magnetic moment m, whose centres are fixed at a distance c apart. When undisturbed, they dispose themselves in the position of stable equilibrium, in which they point in the same direction along the line c. Now let an increasing magnetic force H be made to act on them in a direction at right angles to the line c. The magnets tum towards the direction of H; and when H attains the value 3m/c3, they become perpendicular to the line c, after which they remain in this position, when H is increased further. Thus they display the phenomena of induction initially proportional to the magnetizing force, and of saturation. If the magnetizing force H be supposed to act parallel to the line c, in the direction in which the axes originally pointed, the magnets will remain at rest. But if H acts in the opposite direction, the equilibrium will be stable only so long as H is less than mics; when H increases beyond this limit, the equilibrium becomes unstable, and the magnets turn over so as to point in the direction of H; when H is gradually decreased to zero, they remain in their new positions, thus illustrating the phenomenon of residual magnetism.