414 Dr. J. R. Ash worth on the Theory of 



of a ferro-magnetic substance is that it consists of an 

 assemblage o£ molecules held in definite positions by 

 molecular forces and subject to a molecular intrinsic field ; 

 that these molecules are magnets subject to a magnetic 

 intrinsic field ; that the molecular field imposes no rotational 

 constraints on the molecular magnets ; that a magnetic 

 field does not appreciably affect displacements and trans- 

 latory movements ; that the translatory movements due 

 to thermal action can develop corresponding rotational 

 movements in the molecules in virtue of the mutual magnetic 

 forces existing between them, but that rotational movements 

 cannot produce translations ; lastly, that the molecular and 

 magnetic intrinsic fields become small and, in effect, vanish at 

 the critical temperature. 



7. We now apply these views to the ferro-magnetic 

 equation, which, written in terms of susceptibility, is 



(H + a'P)([-i)=R'T, . . . . ■ (1) 



H being the applied field, I the intensity o£ magnetization, 

 I its maximum value, T the absolute temperature, and R' 

 the reciprocal of Curie's constant and such that I 2 R/ = fl the 

 gas constant. 



Instead of the large intrinsic field a'T 2 of the order 10 7 

 gausses, we substitute for it a small magnetic field hi 2 , where 

 b is a small constant, and add a term A' to the left side which 

 is a function of the molecular field and the temperature, 



thus /i i \ 



A' + (H+M 2 )(j-^) = R'T (2) 



Consider first the effect of the temperature varying while 

 the applied field remains constant; then, since thermal 

 agitation induces rotational vibrations, which are a copy of 

 the molecular movements, both may be treated as the same 

 function of the temperature, and the equation may be put 





(H + bV) 



(K> 



=-B/T. 



-A 



' = -R'T, 



n 





• (3) 



where n to 

 numerically 



a close 

 of the oi 



approximation 

 der 10 7 . 



is 



equal to 



a'/b 



and is 



From this 



. we get 















dl 



1 

 n 



R' 







R' 





• (4) 



d? 



H ,* 



r^*A\ 



nH 



X.n'1 1 _9 



I\ 



