CHANGE OF DIMENSIONS BY -MAGNETIZATION. Do 



Concluding Remarks. 



17. From the experiment on the relation between magnetism 

 and twist, Knott" concluded that the pure strain effects on a 

 ferromagnetic ^vire caused by tension and longitudinal current 

 through it are of an opposite character, and also, on the ground 

 of Maxwell's explanation for Wiedemann's effect, that in an 

 iron or nickel wire carrying an electric current, the change of 

 length by magnetization must be greater than when there is no 

 longitudinal current. Since the change of length for cobalt is 

 scarcely affected by tension, the same must also be the case for 

 longitudinal current. The consideration is partially verified by 

 the experiment of Bidwell and also by the present one. 



The same phenomenon may also be more concisely explained 

 in the following manner. Suppose our samples to be isotropic 

 and to have no residual effect. Let / and ^36 two magnetic 

 forces acting longitudinally and circularly in two perpendicular 

 directions. When these two forces act simultaneously, we have 

 a resultant force H\ this force occasions the change of dimen- 

 sions in our ferromao-netics. The dilatation in the direction of 

 the resultant force, as well as that in the direction perpendicular 

 to it, can be expressed by f{H) and F{H) respectively, which 

 are even functions of H. To obtain the dihitation in the longi- 

 tudinal direction, we have simply to cpnstruct a strain ellipsoid 

 at any point of the ferromagnetics and to find the change of 

 length of the radius vector in this direction. The simple cal- 

 culation gives 



^=/(//)^ + /-(//)Jr. 



1) Knott, Trans. Ftoy. Soc. Edinb. 36, pt. IT., 485. 



