478 PROFESSOR KNOTT ON THE STRAINS PRODUCED IN IRON, 



theory. Dr Taylor Jones has proved* that only a small part of the longitudinal con 

 tractions of magnetized nickel wire can be accounted for by means of Kirchhoff's or 

 Thomsons theory. Nagaoka and Jones t have shown that Kirchhoff's theory, when 

 applied to an anchor ring uniformly magnetized, leads to the conclusion that the cubical 

 dilatation should be three times the linear dilatation — a result not borne out by 

 Bidwell's experiments. J 



Under the influence of strong magnetic forces, iron, nickel, and cobalt are no doubt 

 brought into an aeolotropic state, very different from the state before the magnetic 

 forces were applied. The molecules or molecular groups are thrown into new con- 

 figurations, which, by their mutual action, give rise to the accompanying strains. To get 

 some idea of the nature of these strains under conditions favourable for fairly accurate 

 measurement has been the object of this paper. The significance of many of the facts 

 described is by no means clear, and no recognised theory of magnetic stress seems able 

 to elucidate them. The introduction of terms representing the rotation of molecules § 

 adds greatly to the complexity of the equations, and it is difficult to see how these 

 could be experimentally tested. It is not, however, so much the rotation of the 

 individual molecule that we have to consider, as the resultant effect of new configura- 

 tions of molecular groups. 



Appendix — September 1898. — As this paper was passing through the press, I 

 received from my former pupil and colleague, Professor Nagaoka, of the Imperial 

 University, Japan, an important and masterly discussion || of the applicability of 

 Kirchhoff's theory of magneto-striction to the co-ordination of the inter-relations of 

 magnetism and strain. In this paper, Nagaoka and Honda give measurements of the 

 volume changes due to magnetization in iron and nickel, which are fairly concordant 

 with the values given here. The nickel rod they used was much smaller in section 

 than my nickel B tube, which may, perhaps, account for the greater values of the 

 cubical dilatation obtained by them. They compare their results with my measurements 

 of the changes of volume of bore as given in Part I. ; but such a comparison cannot 

 really be made. It is not merely, as they suggest, that their "measurements of the 

 volumes were external" while mine " were made on the changes in the internal capacity 

 of a nickel tube." As already pointed out (see above, p. 473), the boring out of a 

 nickel bar completely changes its behaviour in a magnetic field, the outer surface 

 becoming subject to a displacement much greater than what was found with the bar 

 solid throughout. For the same reason their remark that a certain inconvenience 

 inseparable from my earlier form of experiment " will disappear if the change of volume 



* Phil. Trans., Series A., vol. 189, pp. 189-200, 1895. 

 t Phil. Mag., May 1896, p. 454. 

 I Proc. Roy. Soc, 1890. 



.^ See Duhem, American Journal of Mathematics, xvii. p. 117, 1895. 



|| "Researches on Magneto-striction." By Professor Nagaoka and Mr Honda. Journal of the College of Science, 

 Imperial University, Japan, vol. ix. p. 353 ; also in Phil. Mag. for September 1898. 



