ON THE MAGNETISATION OF COBALT. 
343 
the residual magnetisation were here observed when tlie treatment of the rod during 
the flow and break of the current was varied. 
When pressure cycles were applied during the flow of the current, and jDressure was 
“on” during its break, the cyclic effect had invariably the same sign while the strength 
of the field was raised from 11 to 400 C.G.S. units. The eftect in this case was in the 
same direction as in the induced magnetisation below the Villapa point, i.e., the 
magnetisation was greatest when pressure was “ on.” 
When, however, the rod was free from pressure during the break of the current, a 
critical fleld was found to exist. In the magnetisation residual after fields above the 
critical, the magnetisation was least when pressure was “on.” This critical field was 
raised by the application of pressure cycles during the flow of the current, but it was 
in any case much lower than the critical field for the induced magnetisation. It 
follows, of course, that the intensity of the critical residual magnetisation was very 
much less than that of the critical induced. 
The amounts of the cyclic effects under the various conditions are shown in the 
fifth columns of Tables II. and VIII., and in the sixth columns of Tables VI. and VII. 
These results show conclusively that for cobalt, and so in all probability for iron, the 
relations between stress and residual magnetisation must be largely dependent on 
circumstances other than the so-called intensity of magnetisation, This, as pointed 
out in § 2, was d priori far from improbable, and should act as a warning against 
extending to residual magnetisation laws proved only for the induced. 
The fact that in fields over 60 or 70 C.G.S. units both the intensity of the residual 
magnetisation and the character of the cyclic effect of pressure are very little affected 
by the application of pressure cycles during the flow of the current, while markedly 
influenced by the existence of pressure during the break of the current, seems worthy 
of special notice. 
All the phenomena observed in the residual magnetisation apparently altered but 
little in magnitude, as the strength of the field was raised from 100 to 400 C.G.S. 
units. This further emphasises the difference between them and the phenomena 
observed in the induced magnetisation. 
The Apparatus. 
§ 19. The rod experimented on was supplied by Messrs. Johnson and Matthey. 
The mean of a series of measurements gave 16’98 cms. for its length and '546 sq, cm. 
for the area of its cross section. During the experiments the rod was contained in a 
brass tube, the closed end of which was sufficiently thick to afford an unyielding 
resistance to the rod when under pressure. The rod could be slid in or out of the 
tube, but the fit was tight enough to prevent any lateral movement. On the outside 
of the brass tube, at its open end, a screw of small pitch was cut, answering to the 
screw cut on the inside of a brass cap. The cap carried a projecting arm, forming a 
