456 
DR, J. HOPKINSON ON THE MAGNETISATION OF IRON. 
material of suitable given form will retain after removal of external magnetising force, 
but, as will be shown, the descending curves which express the relation of magnetisa¬ 
tion and force, where these are diminishing, can be at once used for this purpose. 
Such curves can also be used, as has been shown by Warburg and by Ewing 
(Report Brit. Assn., 1883), to determine the energy dissipated when the magnetisation 
of iron is reversed between given limits. That such dissipation must occur is clear, 
but some knowledge of its amount is important for some of the recent practical 
applications of electromagnetism. Probably Professor Ewing has made a more 
complete experimental study of magnetisation of iron than any one else. The 
researches of Professor Hughes should be mentioned here, as, although his results are 
not given in any absolute measure, his method of experiment is remarkable beyond all 
others for its beautiful simplicity. I have had great doubts whether it was desirable 
that I should publish my own experiments at all. My reason for deciding to offer 
them to the Royal Society is that a considerable variety of samples have been 
examined, that in nearly all cases I am able to give the composition of the samples, 
that the samples are substantial rods forged or cast and not drawn into wire, and that 
determinations of specific electric resistance have been made on these rods which have 
some interest from a practical point of view. 
Method of experiment. 
Let $ be the magnetic force at any point, S3 the magnetic induction, and 3 the 
magnetisation (vide Thomson, reprint. Maxwell, vol. ii., ‘ Electricity and Magnetism’), 
then 33 = <§ + 47 t 3- We may therefore express any results obtained as a relation between 
any two of these three vectors; the most natural to select are the induction and the 
magnetic force, as it is these which are directly observed. S3 is subject to the sole- 
noidal condition, and consequently it is often possible to infer approximately its value 
at all points, from a knowledge of its value at one, by guessing the form of the tubes 
of induction. .§ is a force having a potential, and its line integral around any closed 
curve must be zero if no electric currents pass through such closed curve, but is equal 
to 47rc if c be the total current passing through the closed curve. In arranging the 
apparatus for my experiments, I had other objects in view than attaining to a very 
small probable error in individual results. I wished to apply with ordinary means 
very considerable magnetising forces; also to use samples in a form easily obtained ; 
but above all to be able to measure not only changes of induction but the actual 
induction at any time. The general arrangement of the experiments is shown in 
fig. 1, and the apparatus in which the samples are placed in fig. 2. In the latter, 
fig. A A is a block of annealed wrought Ron 457 millims. long, 165 wide, and 51 deep. 
A rectangular space is cut out for the magnetising coils BB. The test samples consist 
of two bars CCf, 12'65 millims. in diameter; these are carefully turned, and slide in 
holes bored in the block, an accurate but loose fit; the ends which come in contact 
