352 Prof. J. A. Ewing. Contributions to the [June 19 r 



changes of magnetic force by reducing their stability during the 

 intervals when they ^ recede from each other, but tends indirectly 

 towards the same result by setting them swinging. 



The effects of temperature which are common to the three magnetic 

 metals may be stated thus : Let any moderate magnetising force be 

 applied not strong enough to produce anything like an approach to 

 magnetic saturation, and let the temperature be raised. Then the 

 permeability increases until the temperature reaches a certain (high) 

 critical value, at which, almost suddenly, there is an almost complete 

 disappearance of magnetic equality. As regards the first effect, it is- 

 clear that an increase of permeability is to be expected from the 

 theory ; expansion with rise of temperature involves a separation of 

 the molecular centres, and therefore a reduction of stability. As 

 regards the almost sudden loss of susceptibility which occurs at a- 

 high temperature, it may do no harm to hazard a rather wild con- 

 jecture. We may suppose the molecular magnets to be swinging 

 more or less, the violence of the swings increasing as the temperature 

 rises, until finally it develops into rotation. Should this happen, all 

 trace of polarity would of course disappear. The conjecture that the 

 molecular magnets oscillate more and more as the temperature 

 rises, is at least supported by the fact (carefully investigated by 

 Hopkinson* in iron and nickel ; data for cobalt also have lately been 

 supplied by du Boisf) that under strong magnetic forces rise 

 of temperature reduces magnetism ; for with strong forces the mole- 

 cular magnets are already ranged so that their mean direction is nearly 

 parallel to ; hence the earlier effect of heat (to diminish stability and 

 facilitate alignment) does not tell, and the increased swinging simply 

 results in reducing the mean value for each molecule of its moment 

 resolved parallel the magnetising force. 



Before referring to effects of stress we may consider shortly the 

 stability of a pair or line of magnets, treating each as a pair of poles 

 subject to the law of inverse squares. Take first a single pair of 

 equal magnets with centres at C and C'. The poles P, P' would lie 

 in the line CC', but for the imposed force <, which produces a deflec- 

 tion CC'P' or C'CP = 0. 



Let a. be the angle which < makes with the line of centres, in the 

 pole strength, and r the half length of the magnetic axis of each 

 magnet. The deflecting moment is 



2<mr sin (a 0), 

 and the restoring moment is 



m 2 CN 



pp/~2> 



* c Phil. Trans.,' 1889, A, p. 443 ; 'Roy. Soc. Proc.,' June, 1888. 

 t ' Phil. Mag.,' April, 1890. 



