THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



SEPTEMBER 1890. 



XXV. Contributions to the Molecular Theory of Induced 

 Magnetism. By J. A. Ewing, F.R.S., Professor of 

 Engineering in University College, Dundee*. 



AS the facts of induced magnetism become better known, 

 increasing interest attaches to molecular theories and 

 increasing difficulty attends the theories that are current. 

 Weber's fundamental conception that the molecules of iron 

 or nickel or cobalt are always magnets, and that the process 

 of magnetizing consists in turning them from many directions 

 towards one direction, has been strongly confirmed by the 

 now well-established fact that there is a true saturation 

 value, a finite limit to the intensity of magnetism, which may 

 be reached or very closely approached by using a strong 

 magnetic force f. Without going further back, to enquire 

 (with Ampere) how the molecules come to be magnets, we 

 may take this conception as the natural starting-point of a 

 theory. But when we go on to examine the conditions of 

 constraint on the part of the rotatable molecules which have 

 been suggested to make the theory square with what is known 

 about permeability, about residual magnetism and other 

 effects of magnetic hysteresis, about the effects of stress, of 

 temperature, of mechanical vibration, and so forth, we find a 

 mass of arbitrary assumptions which still leave the subject 



* Communicated by the Author, having been read before the Royal 

 Society, June 19, 1890. 



t Ewing and Low, Phil. Trans., 1889, A, p. 221 ; see also H. E. J. G. 

 du Bois, Phil. Mag. April 1890. 



Phil. Mag. S. 5. Vol. 30. No. 184. Sept. 1890. Q 



