SECTIONAL COMMUNICATIONS 



Ordered By the General Committkk 'lo he Printed in extenso. 



DISCUSSION ON THE ORIGIN OF MAGNETISM. 



Opening reinarhfi by Professor P. Weiss. 



I REGRET that the absence of Professor Langevin deprives us of hear- 

 ing from him an account of his remarkable work on magnetism. 



M. Langevin assumes that the rotation of the molecules, each of 

 which has a magnetic moment, is completely free, and, by investigating 

 according to the methods of statistical dynamics the problem of the 

 orientation of these molecules under the combined influence of an 

 external field and their own thermal agitation, he obtains the law of 

 magnetisation of paramagnetic substances in terms of the field and the 

 temperature. In the region of fields experimentally attainable it reduces 

 to the proportionality of magnetisation and field, and to the variation of 

 susceptibility in inverse proportion to the absolute temperature, 



This law was found several years ago by Curie in his experiments on 

 gaseous oxygen. It is only possible to get outside the region of pro- 

 portionality between magnetisation and field when T reaches a very 

 low value. Recently Kammerlingh Onnes, in studying the magnetisa- 

 tion of gadolinium sulphate, has observed, at 1.5° absolute, unmis- 

 takable indications of the approach of saturation. 



Langevin "s theory may perhaps be compared with the kinetic theory, 

 which deals with the compressibility and dilatation of fluids of low 

 density and leads to the laws of Boyle and Gay-Lussac, p'!; = ET. We 

 know that van der Waals has extended this theory to fluids of great 

 density by adding to the external pressure p an internal pressure which 

 expresses the mutual action of the molecules. I have tried to proceed 

 in an analogous manner with regard to magnetism by adding to the 

 external field a molecular field, H„j=NI, proportional to the intensity 

 of magnetisation, and having the same direction. This molecular field 

 expresses the turning action which the assemblage of magnetic mole- 

 cules constituting a body exerts on any one among them. 



The consideration of the mutual action of magnetic molecules is 

 not new. We all know with what success Sir J. A. Ewing has made 

 use of it to explain a whole series of phenomena which, before his work, 

 were obscure; the shape of the curve of magnetisation, hysteresis, 

 magnetostriction, etc. I shall show in the course of this exposition 

 that the forces represented by the molecular field are of an order of 

 magnitude quite different from those invoked in Ewing's theory, and 

 that they explain other phenomena. Moreover, whilst Ewing's forces 

 are mutual magnetic actions, the molecular field is only a magnetic- 

 notation for forces which are, in reality, non-magnetic. 



