44 Effects of Pressure on the Magnetisation of Cobalt. [Dec. 19, 



the break of the current, it was only in the residual magnetisation 

 left after the weakest fields that the maximum answered to pressure 

 " on." When the strength of the pre-existing field was raised, the 

 effect passed through the value zero and changed sign. In the 

 absence of all pressure during the flow of the current this critical 

 field was only about 18 C.Gr.S. units. The application of pressure 

 cycles during the flow of the current raised the critical field to about 

 30 C.Gr.S. units, but in fields over 60 C.Gr.S. units seemed to have 

 extremely little effect. The intensity of the residual magnetisation 

 when the cyclic effect of pressure vanished was of course in either 

 case only a small fraction of the intensity of the induced magnetisa- 

 tion in the Villari critical field. 



In the absence of all pressure the residual magnetisation left after 

 the break of weak fields was very small, but in the weakest experi- 

 mental fields its amount was increased in the ratio of 4 or 5 to 1 by 

 the application of pressure cycles during the flow of the current. In 

 fields over 30 C.Gr.S. units this effect of pressure cycles became 

 extremely small. 



The application of the first pressure subsequent to the break of 

 the magnetising current and the removal of a pressure existing 

 during the break of the current were found to shake out a consider- 

 able amount of the residual magnetisation. The percentage shaken 

 out by the application of the pressure was decidedly the higher in 

 strong fields, but in weak fields the reverse was the case when pressure 

 cycles had been applied during the flow of the current. 



Various other kindred phenomena were observed, and the laws of 

 their variation examined. Most of the results obtained are given in 

 tables, and curves are drawn showing the dependence of the more 

 important effects on the strength of the field. 



All the observations were made on a single specimen of cobalt, and 

 by repeating certain of the observations at intervals it was proved 

 that the condition of the specimen must have remained essentially 

 unaltered while the different phenomena were being examined. Thus 

 the several effects can be directly compared, and their mutual relation- 

 ships are not masked by those differences of condition which are sure 

 to exist between different specimens. An attempt has thus been 

 made from an analysis of the phenomena to attain some knowledge 

 of the true character of the magnetic changes effected by the applica- 

 tion of pressure. 



The experiments were all conducted in the Cavendish Laboratory, 

 and the author is much indebted to Professor Thomson for the facilities 

 afforded him, and for suggestions as to the form of the apparatus and 

 the methods employed. 



