Effect of Stress on Magnetization. 103 



in which L is the twisting conple. Equation (6) then 

 becomes 



&(]fL\ -(*£-\ _ 1 /si\ 



^J\bEk~\^Rh~n\de ) K ' 



or, very nearly, 



n\OT/B. 



m 



The last equation, if it be integrated over the cross-section of 

 the wire of radius R gives 



B '(j£H(*f-o.=4m- 



where I m is the mean intensity of magnetization. Hence 

 finally 



(§h) l = nW\^) s (?) 



From a thermodynamical consideration, A. Heydweiller 

 obtained two relations, neglecting small quantities, 



> _ 31 , I(l-2<r) ,. 



BH~3T + E ' W 



19E: 3 2 I 1-2(7 31 



E 2 3H~ 3T 2 E 5T' 



(9) 



where E is the modulus of elasticity, and <r the Poisson ratio. 

 In his original paper, a was put equal to \. Equation (9) 

 was obtained by differentiating equation (8), considering a 

 and E to be constant. But in magnetic fields, both a and E 

 vary considerably with tension, as is shown by our previous 

 experiment, so that if we retain the second term in the right- 

 hand side of equation (9), the term I^tji( — ^ — ) must be 



subtracted from it. But these terms being small compared 

 with the first term, they may be neglected without causing 

 any considerable error. The second term in equation (8) is 

 also very small. 



On another occasion*, Heydweiller gave relations which 

 are very nearly equal to the last two with the second terms 

 suppressed, and remarked that they are correct. Heyd- 

 weiller's equation (8) differs from that given by J. J. 

 Thomson by a term of second importance. 



* Rensing, loc. cit. p. 377. 



