On the Production of Transient Electric Currents. 133 



Supplementary Note to the Original Paper. Received 

 August 30, 1882. 



In the original paper under this title, communicated to the Royal 

 Society on September 7th, 1881, it was suggested that the phenomena 

 were due to the production of a state of circular magnetisation by the 

 action of twisting stress on longitudinal magnetisation, the effect of 

 twist being to produce a difference of magnetic susceptibility in the 

 two directions of pull and push, inclined at 45° to the axis. Recent 

 experiments of my own on the effect which pull has on the magnetic 

 susceptibility and residual magnetism of iron (an account of which 

 will be given separately) confirmed this conjecture ; and. guided by 

 the light which they threw on tbe subject, I have now made a short 

 supplementary examination of the effects of torsion, which has shown 

 that the conjectural explanation is perfectly satisfactory. 



To show that the transient currents produced by twisting a magnet 

 are due to the development of circular magnetisation, I substituted 

 for the iron or steel wire used in former experiments an iron gas-pipe, 

 itself insulated but carrying along its interior an insulated copper 

 wire which was in circuit with a ballistic galvanometer. The gas- 

 pipe was longitudinally magnetised by a surrounding solenoid. When 

 the pipe was twisted, a transient current passed along the wire in its 

 interior. Another transient current was given when the longitudinal 

 magnetisation was reversed, the state of twist remaining constant. 

 By this arrangement, in brief, all those phenomena could be repro- 

 duced which were described in the paper as exhibited when the two 

 functions of inducing magnet and conductor were both discharged by 

 a solid iron wire. The transient currents given by the tube were 

 much more powerful, partly because the position of the conductor 

 was more advantageous than when conduction was taking place 

 throughout the twisted metal itself, but chiefly because of the rela- 

 tively large size of the tube. By winding the insulated wire in the 

 centre so that it passed several times through the tube, the effects 

 were proportionately increased. 



In the paper it was shown that when the longitudinal magnetising 

 force was increased, the transient currents given by reversing that 

 force while the wire was kept in a constant state of twist passed a 

 maximum when (with one specimen) the value of the force was 

 15 c.g.s. units, after which the effects diminished slowly as the force 

 was raised to 24 c.g.s. units, that being the highest value then used. 

 The effects were such as to correspond to increase of magnetism along 

 the lines of pull. From the discovery of Villari and Thomson that 

 with a certain value of the magnetising force the effect of pull on 

 magnetisation becomes reversed, we might expect the signs of the 



