Elasticity of Ferromagnetic Substances hy Magnetization. 81 



by magnetizing the wire traversed by an electric current is 

 generally greater than the twist caused by passing the current 

 through the magnetized wire. The difference is remarkable ; 

 in some cases * the former is several times greater than the 

 latter. In nickel-steels * the difference is, however, very 

 small. Again take the case of magnetizing a wire traversed 

 by an electric current *j\ The change of magnetization by 

 the longitudinal current is considerably greater, in the case 

 of magnetizing the wire traversed by the current, than in the 

 case of passing the current through the magnetized wire. All 

 these phenomena may perhaps arise from the hysteresis effect 

 of magnetization ; L e. the final states attained by a magnetic 

 substance differ according to the order of applying the field 

 and the stress, or the field and the longitudinal current. 



In conclusion, it maybe remarked that, since the change of 

 the elastic constants by magnetization is not so small as is 

 generallv believed, in any theory of magnetostriction aiming 

 at the quantitative agreement between the theory and the 

 experiment, these changes must necessarily be taken into 

 account. In addition to this, an equality such as 



in which Q is a quantity which depends upon two apparently 

 independent variables x and y, cannot be used without 

 experimental verification. These facts make the development 

 of the theory very difficult. 



In passing, the following remarks may be added. If wo 

 consider, in the above equation, Q as the length of a 

 specimen, x the temperature and y the magnetic force, 

 we have 



-dR\btj" : dt\di±r 



provided I is independent of the order of applying the field 

 and the temperature. Integrating, we get 



xbX Vai/o" 



~dt 



11 a be the coefficient of thermal expansion, 



B(/.i-U 



* K. Honda and S. Shimizu, Jour. Sc. Coll. xvi. Art. 14. 

 t K. Honda, ibid. xi. p. 284, 1899. 



Phil. Mag. S. 6. Vol. 13. No. I'd. Jan. 1907. G 



