On the Production of Transient Electric Currents. 129 



parallel to the lines of force its magnetism is increased, provided that 

 the magnetising force does not exceed a certain value, but decreased if 

 the magnetising force does exceed that limit. Experiments of my 

 own, not yet published, have shown that the Yillari reversal of the 

 effect of pull occurs at a particular value of the magnetisation rather 

 than at a particular value of the magnetising force. They have also 

 shown that, apart from its influence on magnetic susceptibility, pull 

 increases residual magnetism when that is not too strong (this 

 Villari had observed), and further, that the changes of either induced 

 or residual magnetism produced by changes of stress always exhibit 

 the action I have called hysteresis, with the characteristic described 

 in § 11, viz., diminution of range in successive operations, with an 

 approach to a cyclic condition when a cyclic change of stress is 

 repeated. The lagging of magnetic change is just such as might, on 

 Weber's theory of magnetic induction, be ascribed to a frictionai 

 resistance opposing the rotation of the magnetic molecules ; further, 

 it is capable of being, in great part, destroyed by mechanical 

 vibration. 



The experiments now under review exhibit the same actions 

 occurring when we deal with the combined pull and push stress set 

 up by torsion. 



§ 21- In view of the discovery of Villari and Thomson I expected 

 that with a high magnetising force the signs of the transient currents 

 would be reversed, so that they would then indicate decrease of 

 magnetism by pull. This did not occur (§ 18) apparently for the 

 following reason. My own direct experiments show that moderate* 

 pull increases the magnetism of iron only so long as that is less than 

 (very roughly) about three-fourths of its limiting value. Now, when 

 a longitudinally magnetised rod is twisted, the pull and push of which 

 the stress is made up act not on the full magnetisation but on com- 

 ponents of it inclined at 45° to the axis. The intensity of magnetism 

 on which the stress acts is therefore rather less than three-fourths of the 

 whole, and hence, even when we increase the magnetising" force 

 indefinitely, the action still occurs below, though very near to, the 

 Villari critical point. Under these circumstances the transient 

 currents become much diminished in amount, but they preserve the 

 same signs as they have when the magnetising force is small, the 

 signs, namely, which correspond to increase of magnetism by pull.f 



* The precise point of reversal depends, amongst other things, on the amount of 

 pull applied. 



f Sir W. Thomson (loc. cit., § 229) has applied his discovery of the effects of 

 stress on the magnetic inductive susceptibility of iron to explain G. Wiedemann's 

 observation that an iron rod traversed longitudinally by an electric current becomes 

 a magnet when twisted, and expresses his surprise at finding no reversal of the poles 

 when a strong current was used ; but if the above explanation of the non-reversal 

 VOL. XXXVI. K 



