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Prof. J. A. Ewing. 



by rapidly interrupting the current through the wire. This (which I 

 have verified with a ballistic galvanometer) is a direct consequence 

 of Wiedemann's discovery alluded to in a footnote to § 21 above. 



§ 26. Apart altogether from the magnetic origin to which the 

 transient currents described in this paper have been assigned, another 

 cause probably contributes in some small degree to the production of 

 the observed results. From Sir William Thomson's discovery of the 

 effects of stress on electrical conductivity it follows that an originally 

 isotropic conductor will, when under torsion, possess a helical quality 

 with respect to electrical conductivity, and it has in fact been shown 

 experimentally by Professor W. Gr. Adams and Mr. J. T. Bottomley 

 that a brass tube, conveying a current longitudinally, becomes when 

 twisted equivalent to a conducting helix. Hence when the longitu- 

 dinal magnetism of a twisted iron rod (forming part of a circuit) is 

 altered, a transient current will be induced in the helical lines of 

 greatest conductivity by the magnetic change of the interior portion 

 of the rod. The direction of greatest resistance in iron is (probably) 

 that of pull, and the lines of greatest conductivity will therefore be 

 helices opposite in sign to the twist of the rod. Magnetisation will 

 therefore give a current from S. to N". when the twist is that of a 

 common screw. The actual transient current, however, flows from 

 N. to S. Moreover it is clear that the development of a helical 

 quality with regard to conductivity will not explain the fact that the 

 transient currents pass a maximum when the current in the solenoid 

 is strengthened, nor apply at all to the gas-pipe experiment of § 4. 

 It has been suggested to me that another more recondite partial 

 origin of the transient currents given when a magnetised iron rod is 

 twisted may be looked for in the fact that the diminution of longitu- 

 dinal magnetism brought about by torsion induces a current in the 

 magnetising solenoid, which again reacts on the helical lines of con- 

 ductivity in the rod itself, — an idea which was perhaps present w r ith 

 Professor Hughes when he spoke of the transient currents as 

 " tertiary." It is safe to say that these and perhaps other influences 

 enter into the production of the effects which have formed the subject 

 of this paper ; at the same time it may be affirmed with confidence 

 that the phenomena, as they are observed, find a perfectly satisfactory 

 and | sufficient explanation in the setting up of a state of circular 

 magnetisation by the influence of torsional stress on the existing 

 longitudinal magnetism and on the susceptibility to magnetic 

 induction. 



