VIII. On the relation between magnetic stress and magnetic deformation in nickel

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 B2 /8π, an assumption which has only recently been shown to be unjustifiable. That the system of stresses in a magnetic field, described by Maxwell (“Electricity and Magnetism,” Art. 642) as explaining the observed forces between magnetic bodies, is not sufficient to account for the observed deformation of bodies in the field, is clear from the fact that these stresses increase continually with the magnetisation and field-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 Bidwell has found considerable change both in the diameter and the volume of rings of iron.