IX.—On Models of Ferromagnetic Induction

Abstract

In 1890 I published a theory of ferromagnetic induction in which it was suggested that the equilibrium of Weber's elementary magnetic particles was due only to magnetic forces. It was shown that when the elementary magnets are made to turn by applying a magnetising force which is progressively increased, the conditions of equilibrium must be such that there is first a small amount of stable (reversible) deflection, then a break away with irreversible deflection into a new position of stability, and finally a reversible approach to the position of complete parallelism which corresponds to saturation. A model was constructed showing that these conditions could be satisfied by a purely magnetic control. It was made up of little magnets, pivoted on fixed centres which were uniformly spaced. The magnets were all free to turn, but controlled one another by their mutual magnetic forces. They tended to form rows, and when an external field was applied the rows broke up and fresh rows were formed more nearly in the direction of the field. With this simple model the known characteristics of the magnetising process were, qualitatively, well reproduced. A recent study of the stability of such rows of magnets has, however, led me to abandon this model, and to design instead a model in which each atom forms a magnetic system comprising a Weber element capable of turning, but controlled by the magnetic forces exerted on it by other parts of the atom which are taken as fixed. As in the old model, the control is wholly magnetic. Various forms of the new model will be described in a later part of this paper, but in the first place it may be useful to give some account of the investigations of stability which have convinced me that the old model fails, quantitatively, to represent the process.