544 K. Honda, S. Shimizu, and S. Kusakabe on Change of 



These numbers are very close to the corresponding ones in 

 soft iron, except in weak fields, 



7. Cobalt. — As in the case of steel, the effect of magneti- 

 zation on the rigidity of a cobalt bar is very small. The 

 rigidity always increases by magnetization, as shown in 

 figs. 7 and 4, and the following table :— 



-^xlO 2 . 



100. 

 0-05 



200. 400. 



0-12 0-21 



Figr. 7. 



600. 



0-28 



800. 

 0-31 



0-5 



S0 

















- — (61 



















(4) 



- (2) 

















H 



200 



400 



600 



800 



Thus the course of the curves is less steep in cobalt than 

 in iron or steel ; the inflexion point is not so marked in the 

 former metal as in the latter. The change of rigidity is also 

 independent of the applied couple. 



So far as we are aware the effect of torsion on the mag- 

 netization of cobalt has not yet been studied; but if the 

 reciprocal relation holds in the case of cobalt the above results 

 show that the effect of torsion on the magnetization of cobalt 

 is the same as in iron. We have seen from the experiment of 

 Prof. Nagaoka and one of us that the behaviour of the cast 

 cobalt as regards magnetostriction is remarkably different 

 from that of annealed cobalt. The present specimen was 

 well annealed, so that the above inference is to be restricted 

 to an annealed cobalt. 



8. Nickel. — The change of rigidity of a nickel bar is so 

 large that it was necessary to reduce the sensibility of the 

 apparatus by using a rotating cylinder of thicker diameter. 

 As in the case of the change of elasticity, we again observe 

 in the metal a singular phenomenon that the change of 

 torsion by magnetization alters its sign as the magnetizing 

 force is increased. The results are graphically drawn in 

 fig. 8. In weak fields the deflexion shows a further twisting 

 of nickel, that is a decrease of rigidity. This decrease 

 reaches a maximum as the field becomes stronger ; it then 

 begins to decrease, and in a field of about 100 C.G.s. units 

 the rigidity returns to its original value. When the field is 

 further increased the rigidity rapidly increases, and then its 

 rate of increase becomes gradually less. Thus the character 

 of the change is quite analogous to that of the change of 

 elasticity. 



