TRANSACTIONS OF SECTION A. 403 
magnetic susceptibility and thermo-electric quality. Preliminary notices of some of 
his earlier results have appeared in the ‘ Proceedings of the Royal Society’ (Nos. 214 
and 216, 1881, and No. 220, 1882), but a detailed account of the work has still to 
be given. Meanwhile the following points, not previously noticed, are perhaps of 
sufficient interest to warrant their separate publication. 
In the experiments on magnetisation iron and steel wires were used, either 
welded into rings or in the form of straight pieces whose length was great enough 
to make the influence of the ends negligible. Curves were obtained, in some 
cases by the ballistic method and in some by the direct magnetometric method, 
showing the changes of magnetisation which occurred when magnetising force was 
gradually applied, withdrawn, reapplied, reversed, and so on. 
The results of many experiments with several specimens of carefully annealed 
soft iron wires have shown that they possess, in very high degree, a property not 
generally credited to soft iron—the property of remaining strongly magnetic when 
the magnetising force is removed. 
As an example, the case may be cited of an annealed iron wire which was 
subjected to a magnetising force of 22-4 c.g.s. units. This gave it a magnetic 
induction amounting to 16,000 ¢.g.s. units, corresponding to a magnetisation of 
1,270. When the magnetising force was removed (gradually and completely) the 
induction fell only to 15,000. In other words, the intensity of residual magnetisa- 
tion was equal to nearly 1,200 ¢.g.s. units. 
Here more than 93 per cent. of the whole induced magnetisation survived the 
removal of the magnetising force ; and in many other cases the residual magnetism 
amounted to nearly 90 per cent. The somewhat extraordinary spectacle was thus 
presented of a piece of soft iron, entirely free from magnetic influence, and never- 
theless holding an amount of magnetism far in excess of what is ever held by a 
permanent magnet of the best tempered steel. 
In this condition, however, the magnetic character of the iron is highly unstable. 
The application of reverse magnetising force quickly causes demagnetisation, and 
the slightest mechanical disturbance has a similar effect. Gentle tapping removes 
nearly all the residual magnetism. Variations of temperature reduce it greatly, 
and so does any application of stress. On the other hand, if the iron be carefully 
protected from disturbance it seems that the residual magnetism disappears only 
very slowly, if at all, with the mere lapse of time. 
If, after magnetisation, the magnetising force be suddenly removed, the 
residual magnetism is (as might be expected) considerably less than when the 
force is removed gradually. 
The ratio of residual to total magnetisation is always small when the intensity 
of raagnetisation is small, and passes a maximum when the intensity is increased. 
This maximum is particularly distinct in wires which have been hardened by 
stretching, but it occurs also in soft annealed wires. In one instance, where the 
wire had been hardened by stretching, the maximum ratio of residual to total 
magnetism was 0:60, which was given by applying and removing a magnetising 
force of about 10 ¢.g.s. units, but on the application of a force of 90 units the ratio 
fell to 0°33. With steel the maximum in this ratio is less sharp, but still distinct. 
Neither in hard iron nor in steel is the ratio, even at its maximum, so great as in 
soft iron, where (as has been said above) it frequently reaches 0-9. 
During the first magnetisation of soft iron wires the ratio («) of intensity of 
magnetisation (I) to magnetising force was generally about 200, sometimes nearly 
300, And by gently tapping the wire during the application of magnetising force 
this coefficient was on one occasion raised to the enormous value of 1,590. In the 
case alluded to the magnetisation went on so rapidly, as the magnetising force was 
increased, that a force of 1 c.g.s. unit gave an induction of 10,000. 
In this and other particulars the experiments have been strongly confirmatory 
of the idea that there is, in soft iron, a static frictional resistance to the rotation of 
the magnetic molecules, which is the principal cause of the remarkable retentive- 
ness described above, and which is overcome by mechanical agitation. 
Numerous measurements have been made of the energy expended in taking 
iron and steel through cyclic changes of magnetisation. For example, in chanzing 
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