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DR. J. HOPKINSON ON THE MAGNETISATION OF IRON. 
Weber’s Theory of Magnetism. 
In Weber’s theory it is, in effect, assumed that the magnetic force tending to 
deflect a molecule is that which it would experience if it were placed in a long- 
cylindrical cavity, the axis of the cylinder being in the direction of magnetisation. 
This seems a rather unnatural supposition. If instead of this we assume that the 
deflecting force is that which it would experience in a spherical cavity, and draw a 
curve connecting either the induction or magnetisation with the deflecting force on a 
molecule within a spherical cavity, we shall find that the curve differs very little from 
a straight line. In the curves already given we have taken $3 and <§ as the variables, 
where 
33 = £ + 47t3- 
Suppose we take 03 and A where 
33 = £ + 4tt3. 
and 
Jt=$+f3. 
The curves would then be hardly distinguishable from straight lines, the same 
scales being used for ordinates and abscissa; ; it requires no great stretch of 
imagination to suppose that if this curve were continued far enough it would differ 
but little from that given by Maxwell, vol. ii., p. 79. 
Now, in dealing with Weber’s theory it would seem more suitable to take $, 
the magnetic force in a spherical cavity, as the independent variable. If we assume 
Weber’s theory with this modification we arrive at the following conclusions :— 
1. All observations yet made upon the magnetisation of iron are upon the straight 
part of Weber’s curve. 
2. The particular features of curves of magnetisation as ordinarily observed arise 
from a slight irregularity in Weber’s curve, magnified by the near approach of iron 
to a state in which a random distribution of the magnetic axes of the molecules is 
unstable. 
I do not put these remarks forward as indicating more than the fact that we are a 
very long way from obtaining a range of facts sufficiently extended for testing a 
molecular theory of magnetism. The broad fact which strikes the mind most forcibly 
is the specific difference which exists between magnetic and non-magnetic bodies. 
Most bodies are either very slightly ferro-magnetic or very slightly diamagnetic. On 
the other hand iron, nickel, and cobalt are enormously magnetic. 
Iron with 12 per cent, of manganese, and some small quantities of carbon and 
other substances, is so little magnetic that its magnetism would be accounted for by 
supposing that in its mass were distributed a few little bits of pure iron. There 
