Effect of Stress on Magnetization. 105 



whore e m and r m are the magnetic strains. By differentiating 

 the above equations with respect to H, we have 



3l 



=| T ( H S> ^ 



Cantone tested the second relation by experiment and found 

 a satisfactory agreement in iron and nickel. For the first 

 relation, he also made a comparison between theory and 

 experiment, but the data he used were taken from experi- 

 ments by different physicists, so that they do not refer to 

 the same specimen. Though the comparison shows a satis- 

 factory agreement, it is not certain whether it was by chance 

 or not. 



By a direct method, Dr. S. Sano obtained the relation 



•be _5I , I(l-2<r) . 27r 3Q 2 H 2 ) 

 3H"BT + E "*" E BH ' * ' l DJ 



where k is a term in the expression of susceptibility, which 

 is independent of the strain. Since I=a: H, Sano's equation 

 practically coincides with Gans's. For the change of elasticity, 

 Dr. Sano obtained 



I» a .W (16) 



which is practically the same as Heydweiller's equation, but 

 different from Gans's by a term not negligibly small in weak 

 fields. The above equation was obtained independently of 



the relation for -^-rr. As to the effect of twist, Dr. Sano 

 On 



obtained an equation which can be transformed into (12). 



Thus far, the relations for ^t given by Heydweiller, Gans, 



Kolacek, and Sano all agree with one another in the first 



important term ^ # Relation (1) given by J. J.Thomson 



does not differ in reality from others. Relation (13) given 

 by Cantone also coincides with others in the first term, 

 provided k is independent of H. The se3ond term 

 1(1 — 2<r)/E in (8) and (10) maybe neglected for the first 



