

V 



Messrs. H. Nagaoka and K. Honda on Magnetostriction. 277 



The magnetization of iron in the stretched state increases 

 with the magnetizing force till it reaches a maximum in 

 H = 14 nearly; it then goes on slowly diminishing, and 

 ultimately becomes less than in the free state. In nickel 

 there is decrease of magnetization in the stretched state, 

 except in weak fields, where a slight increase was observed. 

 Corresponding to a critical field in iron, for which the change 

 of magnetization is maximum, there is also a critical field for 

 which the diminution of magnetization in nickel is maximum. 

 It will be seen from the figures that the change of magnetiza- 



r* o ft 



Hon is not exactly proportional to the amount of longitudinal 

 stress. 



It appears from Prof. E wing's * experiment that the 

 increase of magnetization in iron in weak fields becomes more 

 pronounced with greater loading, but the field at which the 

 magnetization becomes smaller than in the unloaded state 

 recedes towards the weaker side. Although Prof. Ewing 

 did not observe these points in fields greater than 8, with 

 loading which is far greater than that in the present experi- 

 ment, we can see from the course of curves of magnetization 

 that if the loading be greatly diminished, the above-mentioned 

 field will become correspondingly large. In the present 

 experiment, it occurs in H = 48 for iron with longitudinal 

 stress 0"38 kg. sq. mm. Thus the general feature of the present 

 investigation agrees with that of Prof. Ewing. 



In nickel, there was a slight increase of magnetization in 

 a weak field when the rod was loaded ; whether this has any 

 connexion with the Villari effect observed by Heydweiller f is 

 a question which, without special examination, cannot be 

 easily decided. 



§3. Calculation of the Coefficients k' and k". Comparison 

 between Theory and Experiment. 



According to Kirchhoff's % theory of magnetostriction, the 

 coefficients k, k\ k" are defined by the equations : 



l x ={k-k'(\ x + \ + \ z )-k ! \}H x , 



. i,={*-*'C\+\+x.)-*tyfl, 



L= {k-kW + ^+K) -k"\ z \~H; ; 



where 1^, I ? , I, are the components of the magnetization, 

 H x , H , H 2 those of the magnetizing force, and \, X , X 2 are 



* Ewing, Phil. Trans, clxxvi. (ii.) p. 608 (1885). 

 t Heydweiller, Wied. Ann. lii. p. 462 (1894). 



t Kirchhoff, luc. cit.) see also Pockels, Arch. d. Math. u. Phys. (2) xii 

 p. 57 (1893). 



Phil. Mag. S. 5. Vol. 46. No. 280. Sept. 1898. X 



