On the Production of Transient Electric Currents. 119 



§ 4. The existence of these transient currents is due to the produc- 

 tion of a state of circular magnetisation as the result of torsional stress 

 and longitudinal magnetism acting jointly. To prove this, I substi- 

 tuted for the wire a long piece of iron gas-pipe, itself insulated, but 

 carrying in its interior an independent copper wire, which was in 

 circuit with the ballistic galvanometer. The gas-pipe was longi- 

 tudinally magnetised by a surrounding solenoid. The making and 

 reversing of a current in the solenoid whilst the pipe was held twisted, 

 or the sudden twisting of the pipe during or after the operation of the 

 magnetising force, gave transient currents along the insulated wire 

 inside precisely similar to those which had been observed when the 

 two functions of inducing magnet and conductor were both discharged 

 by a solid iron wire. The pipe gave much greater currents, chiefly 

 because of its greater size, but partly also because the metal in it was 

 more advantageously disposed than in a solid rod, both as regards the 

 stress and the subsequent inductive effect on the conductor inside. 



§ 5. That longitudinal magnetisation combined with torsion should 

 give rise to circular magnetism follows from Sir William Thomson's 

 discovery that aeolotropic stress developes an asolotropic difference of 

 magnetic susceptibility in iron. When the wire or tube is twisted the 

 stress is equivalent to pull and push along lines perpendicular to the 

 radius and inclined at 45° to the direcbion of the length. Along one 

 of these the magnetic susceptibility is increased ; along the other, at 

 right angles to that, it is diminished. The effect is to change the 

 lines of induction, originally straight and parallel to the axis, into 

 screws, whose circular components produce the transient currents now 

 described. The direction of the currents is that corresponding to 

 increase of magnetism by pull. 



I shall now give a brief account of the experimental results, and 

 afterwards point out their relation to the discoveries of Villari and 

 Thomson on the effects of stress on magnetism, as well as to certain 

 recent experiments of my own on the same subject. 



§ 6. The wire described in § 2 was twisted oppositely to a common 

 screw by turning the pointer through an angle of 60° to the side 

 marked positive (see fig. 1). As the wire was initially free from any 

 considerable magnetisation, this operation gave no transient current. 

 The battery current A was then made, the wire being held twisted. 

 This gave a positive transient current along the wire (from a to h), 

 which produced a deflection of 39 scale divisions of the ballistic 

 galvanometer. The throw of the galvanometer will be taken as giving, 

 on an arbitrary scale, a measure of the circular magnetisation of the 

 wire. 



Then keeping the wire still twisted at +60°, the current A was 

 broken. This gave almost no transient current : in other words, the 

 circular magnetism was scarcely at all affected by the removal of the 



