1889.] 



Pressure on the Magnetisation of Cobalt. 



43 



neighbourhood of the " wende-punkt," or point where the coefficient 

 of magnetic induction is a maximum. Relatively, however, to the 

 intensity of the pre-existing induced magnetisation both the effects 

 continually diminished in importance, as the strength of the field was 

 raised from zero. In the weakest experimental field the first pressure 

 increased the induced magnetisation by over 50 per cent., and fully 

 4 per cent, of the magnetisation took part in the cyclic change accom- 

 panying the pressure cycles. In some respects these results present 

 a close resemblance to those observed by Professor Ewing in iron. 



It was found that the existence of pressure previous to and during 

 the introduction of the rod into a coil traversed by a current had an 

 effect of the same general character, though not exactly of the same 

 magnitude, as the first application of pressure when the rod on its 

 introduction into the coil was free from pressure. Also on the break 

 of a current during which pressure cycles had been applied, the rod 

 manifested a polar character, in that, when exposed a second time 

 without intermediate demagnetisation to the same strength of current, 

 it possessed a greater intensity of magnetisation when the current 

 passed in one direction than when it passed in the other. Both these 

 effects had critical fields where they vanished and changed sign, and 

 these fields were close to, if not identical with, the field in which the 

 effect of the first pressure vanished. In fields below the critical the 

 magnetisation of the rod when exposed a second time, without inter- 

 mediate demagnetisation, to a current of the same strength as one in 

 which pressure cycles had just been applied was greatest when the 

 direction of the current was unchanged ; but in fields above the 

 critical the reverse was the case. 



Both Villari and Professor Ewing observed that after the break of 

 the magnetising current cyclic changes of tension produced eventually 

 in iron wires cyclic changes of the residual magnetisation. In these 

 the maximum magnetisation answered, as in the induced magnetisa- 

 tion in fields below the Villari point, to tension " on." Professor Ewing 

 apparently examined the effect only in weak fields, but he does not 

 seem to have anticipated that it would change its character in stronger 

 fields. 



In the present investigation the existence of a cyclic change in the 

 residual magnetisation of cobalt accompanying cyclic changes of 

 pressure has been established, and the magnitude of the effect 

 examined in a large number of fields, extending from to 400 C.G.S. 

 units. It was found that not only the magnitude but the sign even 

 of the effect depended largely on the condition of the rod during 

 the break of the current. When the rod was under pressure during 

 the break, the residual magnetisation in the cyclic state showed a 

 maximum under pressure, whatever was the strength of the pre- 

 existing field. "When, however, the rod was free from pressure during 



