716 Dr. J. R. Ash worth on the Application of 



The maximum intensity of magnetization I has been the 

 subject of experiments by Ewing, Du Bois, and others, so 

 that it is known with considerable certainty for the ferro- 

 magnetic metals. We may put 



For iron I = 1685 

 For nickel I = 510 

 For cobalt I =1300 



The value of a' cannot be directly determined, but it may 

 be estimated from a knowledge of the other constants. If 

 T c =785° + 273° for iron, then from the equation to the 

 critical temperature we have 



,_27B/T C 27 3-56x1058 



a ~ 8 I ~ 8 1685 ~ 7,b - 



Another estimate of the value of this constant can be made 

 as follows. By differentiation of the ferromagnetic equation 

 with respect to the temperature we have 



cR_ R' 



dt ~~ 



gw(l-2*) 



The temperature coefficient a t of a magnet is -.- — , 

 so that x>/ 



omitting ™ > which may be made negligibly small. 

 If I=ml this becomes 



r' 



at a'ml (l-2m)' 



Now a t for iron when m is chosen to be 0"92 is — 0*00037 *, 

 and therefore 



R' 3-56 =7#3 



-a t ml (l-2m) ~ 0'00037 x 0'92 x 1685 X 0*84 



Calculations made in the same way for nickel give 



a' = 92 from the critical temperature, 

 a' = 90 from the temperature coefficient, 



when m = 0'92 and a t = 0*00059 *. 



* Ashworth, Phil. Mag. xxm. p. 36, Jan. 1912. 



