52 ART. 7. K. HOXDA AND T. TEEADA. 



relation c = r7, wheie >■ is the radius of the tliin tube. As in the 

 former case, we have 



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



or, very nearly '"(w)z=K4^L- 



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

 the wire of radius H gives 



2^'\ÜH )l n\öz J Jii n \ Uz )„ ' 



where Ira is the mean intensity of magnetization. Hence 



«-"y (wl= i^ (-^),; ^^) 



From a thermodynamical consideration, A. Heydweiller ob- 

 tained two relations, neglecting small quantities, 



9e _ 9/ , 7(1 -2a) . 



aiy- a^' "^ E ' *^^ 



J__ô^_ 0-/ 1-2(7 97 



Ä^ Th -~ l\T' ~ E OT 



rrr , (^0 



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

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

 obtained by diti'erentiating equation (8), considering <t and E to 

 be constant. But in magnetic fields, both n and 7'.' vary con- 

 siderably wûth tension, as is shown by our previous experiment, 

 so that if we retain the second term in the right-hand side of 



