[ 189 .1 
VIII. On the Relation between Magnetic Stress and Magnetic Deformation in Nickel. 
Big E. Taylor Jones, D.Sc. 
Communicated by Professor Andrew Gray, F.R.S. 
Received January 13,—Read February 25, 1897. 
The object of the experiments described below was to determine how much of the 
contraction, which occurs in a nickel wire when magnetised, is due to stresses brought 
into play by magnetisation. This has been attempted by several experimenters, but 
it has been assumed by some that the magnetic stress concerned is a contracting stress 
of magnitude W'/Stt, an assumption which has only recently been shown to be 
unjustihable.'*' 
That the system of stresses in a magnetic held, described by Maxwell (“ Elec- 
tricity and Magnetism,” Art. 642) as explaining the observed forces between 
magnetic bodies, is not suthcient to account for the observed deformation of bodies in 
the held, is clear from the fact that these stresses increase continually with the 
magnetisation and held-strength, while the deformation in iron and cobalt becomes 
reversed when the field reaches a certain value. It can also be shown that 
Maxwell’s stresses would produce no change in the dimensions of a ring magnetised 
by a uniform circumferential field, whereas BidwellI has found considerable change 
both in the diameter and the volume of rings of iron. 
ITe theory of magnetic stress has, however, been extended, chiefly by v. Helm¬ 
holtz,! Kirchhoff,§ Professor J. J. Thomson, || and Hertz, ^ who have shown that, 
in addition to Maxwell’s distribution of stress, there are other stresses in a mag¬ 
netised body due to the fact that the magnetisation depends upon the strain in 
the bodv. 
A/ 
Hitherto no direct experiments have been made with the object of ascertaining 
whether these stresses, in addition to those of Maxwell, are sufficient to account 
for the observed deformation of bodies placed in a magnetic field. The facts that 
“ Villari reversals ” of opposite kinds exist in iron and cobalt, and also opposite 
reversals of magnetic deformation, and that in nickel (at any rate at moderate field- 
* C. Chree, ‘ Nature,’ January 23, 1896 ; H. Nagaoka and E. T. Jones, ‘ Phil. Mag.,’ May, 1896. 
t S. Bidwell, ‘ Proc. Roy. Soc.,’ vol. 55, p. 228 ; vol. 56, p. 94. 
X ‘ Wied. Ann.,’ vol. 13, p. 400, 1881. 
§ ‘Wied. Ann.,’ vol. 24, p. 52, 1885. 
II ‘Applications of Dynamics to Physics and Chemistry,’ p. 48, 1888. 
^ ‘ Ausbreitung der Elektrischen Kraft,’ p. 275, 1892. 
10.6.97 
