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IV. On the Effects of Pressure on the Magnetisation of Cohalt. 
By C. Chree, M.A., Fellow of King's College, Camhridge. 
CornmunicaMd by Professor J. J. Thomson, F.R.S. 
Received Novembei’ 22,—Read December 19, 1889. 
[Plates 15, 16.] 
§ 1. In August, 1888, I eoiumenced an investigation into the effects of pressure on the 
magnetic properties of cobalt, at the suggestion of Professor J. J. Thomson. In his 
Applications of Dynamics to Physics and Chemistry,” he has arrived at certain 
conclusions as to the relations of magnetisations and mechanical strains, and the 
primary object of the following investigations was to obtain experiinental data, 
whereby these conclusions might be tested. 
During the process of the experiments, numerous points presented themselves 
whose elucidation seemed of importance for a satisfactory comparison of theory and 
experiment. The experiments have thus included a wider range than was originally 
intended, and have occupied a considerable amount of time. 
Phenomena resembling very closely in some points those to be presently discussed 
have been noticed by previous observers in iron and nickel. As frequent references to 
these will be found essential for an adequate discussion of the phenomena presented 
by cobalt, a brief outline of the more prominent characteristics is here interposed. 
§ 2. If a bar of some magnetic metal, subjected to a constant longitudinal stress, 
be exposed to the action of a magnetising current in a surrounding spiral it becomes 
magnetised, and in general simultaneously alters its length. If the current cease to 
flow the bar retains in general part of its magnetisation, and it may not return to its 
original length. On the other hand, if the bar while under the action of a constant 
current in the magnetising spiral, be subjected to a longitudinal stress, it in general 
simultaneously alters its magnetisation. Also, if the stress cease to act the magneti¬ 
sation may not return to its original value. Phenomena similar to the last are also 
shown by the magnetisation which is residual after the break of the current. 
These phenomena all depend on the intensity of magnetisation in the bar. The 
mode of this dependence will be discussed in considerable detail presently, but 
erroneous ideas are not unlikely to arise unless the following practical considerations 
be kept clearly in view. 
MDCCCXC.— A. 2 17 
7.7.9U. 
