190 
DR. E. TAYLOR JONES ON THE RELATION BETWEEN' 
strengths) there is neither Villari reversal nor reversal of deformation, suggest that 
there is a close connexion between the two phenomena, and this general similarity 
between the effects of stress on magnetisation, on the one band, and magnetic 
deformation on the other, has led some experimenters to assume that magnetic 
deformation is completely accounted for by the above stresses, and to calculate from 
the observed deformation the general effect of stress on magnetisation. ” 
Before this assumption can be legitimately made, however, it is necessary to make 
direct experiments to test its truth, and it was with this object that the following 
experiments were instituted. 
The experiments were made in the Physical Laboratory of the University College 
of North Wales, and, before proceeding to describe them, I wish here to express my 
great indebtedness to Professor A. Gray for kindly placing at my disposal all the 
necessary apparatus, and for many valuable suggestions. 
Theory. 
One method of experimenting, suggested by Mr. NAGAOKAand the present writer,t 
would be to measure both the effect of hydrostatic pressure on magnetisation and the 
magnetic deformation of a ring of soft magnetic material. From these measurements 
a direct comparison could be made of the observed deformation and the value 
calculated from the theory of Kirchhoff. 
There is however another method which, though theoretically not so simple, is 
probably easier to carry out experimentally. It has been shown by Cantone^ that 
the elongation M of aii ellipsoid of revolution of great eccentricity and soft magnetic 
material, when placed in a uniform longitudinal field H is, on Kirchhoff’s theory, 
given by 
U 47rPa + 6'\ IH K'm 
l '~ 3E \1 + 2el 2E(1 + 20) 2E(l + 2^) 2E •••()’ 
where 
I = length of ellipsoid, 
I = magnetisation, 
K, k" are the coefficients of change of magnetic susceptibility with change of 
density and of elongation § respectively, 
E = Young’s Modulus for the material, and 6 is defined by 
^ = ngid'ty = n. 
* Cantone, ‘Mem. R. Acc. Line.,' ser. 4, vol. 6, 1890; Winkelmann, ‘ Handbuch der Physik,’ Bd. 3, 
Part II., p. 250, 1895. 
t Loc. cit., p. 461. f Loc. cit. 
§ The fundamental equation defining k and k" is 1 = (e + / + S^) ~ a." e) H, where e,/, g are 
the dilatations parallel to the direction of magnetisation, and to two axes at right angles to it. 
