EFFECT OF STRESS ON MAGNETIZATIO:^I. 03 



oquiition (*)), [hv term /^yr( — T^) "lust be subtracted from it. 

 lint the'tJL' terms l)eiiig small comjiared 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 sup- 

 pressed, and remarked that they are correct. Heydweiller's 

 equation (8) diiVers from that given l)v »1. .1. Thomson by a term 

 of second importance. 



Reusing experimentally tested relation (i>) in the case of iron 

 and nickel, and showed a fair agreement between the theory and 

 the experiment. 



11. Gans criticized Heydweiller's equations and proposed his 

 own, i.e., 



bH ~ 47Z II T ^ 47:J'J ^^ \ fi„ ö// J' 



It' the medium surrounding the magnet be air, we may put 

 /^=1 ; hence 



dH ~ Qf '^ E ^ K dH ^ ^ 



Thus, Gans's equation differs from that of Heydweiller by the 



term -^ t^ , which generally is not very small, but in weak 



fields, it sometimes overweighs the first term. As in the case of 

 Heydweiller, differentiating the above relation with respect to T, 

 Gans obtained an expression for the change of elasticity, which 

 differs from that of Heydweiller bv the term — ^^7- ...... rr • Here 



again, it was assumed that n and £ are inde])endent of 7', a 

 supposition not admissible in a magnetized wire. 



*i Rensing, loc. cit. \>. ;'.77. 



