4G2 
DR, J. HOPKINSON OR THE MAGNETISATION OF IRON. 
the rate of increase of resistance was nearly uniform. The specific resistance at 0° C. 
was inferred to be 0'000102, and the rate of increase was 0 - 00083 per degree 
centigrade.—J. H.] 
Another very striking feature is the way in which any substantial proportion of 
manganese annihilates the magnetic property of iron; the sample with 12 per 
cent, of manganese is practically non-magnetic. The induction noted in the table 
= 310 corresponds to a magnetising force of 244. If all the substances in this 
sample other than the iron were mechanically mixed with the iron, and arranged in 
such wise as to have the greatest effect upon its magnetic property, no such annihila¬ 
tion of magnetic property would ensue. This question of mixture will be considered 
somewhat more closely below. 
The permeability and susceptibility are given in the following table for some of the 
samples containing much manganese :— 
No. 
Permeability. 
Susceptibility. 
X. 
1-27 
•0215 
XIV. 
3-59 
•206 
XVI. 
3-57 
•2046 
XXXV. 
1-84 
•0668 
It is therefore clear that the small quantity of manganese present enters into that 
which must be regarded for magnetic purposes as the molecule of iron, and completely 
changes its properties. The fact is one which must have great significance in any 
theory as to what is the molecular nature of magnetisation. Another clearly marked 
fact is the exceptionally great effect which hardening has both upon the magnetic 
properties and the electrical resistance of chrome steel. 
Note also that in those cases where the maximum induction is low, the residual 
magnetism is proportionately lower still, but that the coercive force is not uniformly 
lower. This is in accordance with the supposition that these samples are to be 
regarded as mechanical mixtures of a strongly magnetic substance, such as ordinary 
iron or steel, and a non-magnetic substance, such as manganese steel with 12 per 
cent, of manganese. A feature present in all the curves may some day have a 
bearing on the molecular theory of magnetism. It is this : the ascending curve twice 
crosses the continuation of the descending curve; in other words, the fact that a 
sample has been strongly magnetised in a reverse direction, renders it for small forces, 
or for large forces, more difficult to magnetise than a virgin sample, but distinctly 
easier for intermediate forces. This is best seen in the case of the hardest steel, 
No. IX., fig. 9, in which the two curves cut in the points marked M, N. A similar 
phenomenon has been observed and investigated by G. Wiedemann (vide ‘Die Lehre 
vom Ga.lvanisrnus,’ first edition, vol. ii., p. 340, et seq.). 
