42 



Mr. C. Chree. On the Effects of [Dec. 19, 



Now Professor J. J. Thomson has shown that on dynamical prin- 

 ciples the effect of changes of magnetisation on the length of a rod of 

 magnetic metal, and the effect of changes in the length of the rod 

 on the magnetisation, must be fundamentally connected. In his 

 " Applications of Dynamics to Physics and Chemistry, 1 ' he has 

 arrived at mathematical equations connecting the two phenomena, 

 such that from a knowledge of the one set of phenomena the cha- 

 racter of the other set can be deduced. 



The conclusions derived from the theory are in the case of iron in 

 accordance with the results of experiment, at least in their general 

 character. In cobalt there is also an agreement between theory and 

 experiment, so far as Sir W. Thomson's experiments go. In the 

 absence of further experiments it would, however, be impossible to 

 tell whether or not this agreement extended to the strong fields in 

 which occurred the important phenomena observed by Mr. Shelford 

 Bid well. The application of Professor J. J. Thomson's formulae to 

 Mr. Shelford Bidwell's results led him to the conclusion that under 

 cyclic applications of pressure a cobalt rod should experience cyclic 

 change of magnetisation, and that the maximum magnetisation should 

 answer to pressure "on," or to pressure "off," according as the 

 magnetic field was weaker or stronger than a critical field, corre- 

 sponding to the Villari field in iron. It was for the purpose of deter- 

 mining whether such a critical field did actually exist that the present 

 investigation was commenced at Professor J. J. Thomson's sugges- 

 tion. 



Employing the magnetometric method, it was found that the agree- 

 ment between theory and experiment was at deast as satisfactory in 

 cobalt as in iron. The application of pressure cycles in a magnetic 

 field led to a cyclic change of magnetisation in a cobalt rod, in which 

 the maximum magnetisation occurred when pressure was " on," or 

 when it was " off," according as the strength of the field was below 

 or above 120 C.Gr.S. units. This accordingly was the Villari critical 

 field foreshadowed by theory. 



Various phenomena which promised to throw light on the true 

 character of the relations of strain and magnetisation having been 

 noticed at an early period of the investigation, it was decided to make 

 an attempt to isolate the phenomena, and examine them exhaustively. 



In weak fields the first pressure applied after the introduction of 

 the cobalt rod into the magnetising coil caused a large increase in the 

 induced magnetisation. As the strength of the field was raised this 

 change in the magnetisation attained a maximum, then diminishing 

 vanished in a field considerably stronger than the Villari field for the 

 cyclic effect, and in all stronger fields consisted in a diminution of 

 magnetisation. The fields in which the cyclic effect of pressure and 

 the effect of the first pressure were absolutely greatest occurred in the 



