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Yin. Magnetisation of Iron. 
By John Hopkinson, M.A., D.Sc., F.R.S. 
Received March 30,—Read April 23, 1885. 
[Plates 46-52.J 
Preliminary. 
The experimental determination of the relation between magnetisation and magnetising 
force, would be a simple matter if the expression of such relation were not complicated 
by the fact that the magnetisation depends not alone on the magnetising force at the 
instant, but also upon previous magnetising forces ; in fact, if it were not complicated 
by the phenomena of residual magnetism. In the absence of any satisfactory theory, 
we can only experimentally attack particular cases, and the results obtained have 
only a limited application ; for example, we may secure that the sample examined has 
never been submitted to greater magnetising force than that then operating, and we 
may determine a curve showing the relation of magnetisation to magnetising force 
when the latter is always increasing; we may also determine the residual magnetism 
when after each experiment the magnetising force has been removed. Such curves 
have been determined by Rowland (Phil. Mag., Aug., 1873) and others. For many 
purposes a more useful curve is one expressing the relation of the magnetising force 
and magnetisation when the former is first raised to a maximum and then let down to 
a defined point; such curves have been called descending curves. One or two 
descending curves are given in a paper by Mr. Shida (Proc. R. S., 1883, p. 404). 
It has been shown by Sir W. Thomson and others that the magnetisation of non 
depends greatly upon the mechanical force to which the iron is at the time submitted. 
In the following experiments the samples were not intentionally submitted to any 
externally applied force. Clerk Maxwell gives in his ‘ Electricity and Magnetism,’ 
chap. 6, vol. ii., a modification of Weber’s theory of induced magnetism, and from 
this he deduces, amongst other things, what had been already observed, that non may 
he strongly magnetised and then completely demagnetised by a reversed force, but 
that it will not even then be in the condition of iron which has never been magnetised, 
but will be more easily affected by forces in one direction than in the other. This I 
have verified in several cases. The ordinary determinations of residual magnetisation 
are not applicable to determine the permanent magnetism which a piece of the 
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