68 Prof. Nagaoka and K. Honda on Magnetostriction 

 npon by the forces above specified are given by 



Y f — .(^+i+|)*'orf«+J(i'+*-I?)(«'+V). 



Z I= X Z = (-5,+ *+ f ) *$ sin e, 



X y =Y x = {± +k+ |J A 2 sin^ cos 0. 

 The moment about the axis of the wire is given by 

 N=JJ(Z r ,-Z*)<My, 



= -JJ(&+*+lf')*^^> 



The moment just outside the wire amounts to 



Thus the effective couple is equal to 



-^C£R 2 =-^^x Cross section. . . . (C) 



Since the amount of torsion of a cylindrical wire by a given 

 couple is inversely proportional to the fourth power of its 

 radius, it is evident that for given longitudinal current and 

 field the angle of twist is inversely proportional to the square 

 of the radius. This inference was approximately verified in 

 the present experiments. 



