22 MB. H. TOMLINSON ON THE INFLUENCE OF STRESS AND 



from the elevation of temperature produced by the current. When, however, the 

 experiments were concluded which have just been recorded, and, moreover, no appreci- 

 able effect had been found to be produced on the torsional elasticity by the passage of 

 a current, the author was led to endeavour to ascertain how far WERTHEIM might be 

 justified in the above-mentioned conclusion as regards the effect of a current on the 

 longitudinal elasticity. 



First, in order to avoid heating as much as possible, currents from '2 to '3 ampere 

 were employed currents which, though comparatively weak, are, as has been shown, 

 capable of producing very sensible circular magnetisation. The same length of wire 

 and the same apparatus as that described in the author's former papers on Elasticity* 

 were used, with this difference, however, that now the wire to be examined and the 

 comparison-wire were secured at their upper extremities to separate clamps which 

 were insulated from each other and from the bracket on which they rested. Near 

 their lower extremities the wire and the comparison-wire were united by a short piece 

 of copper wire, so that the current from the battery might, after passing through a 

 tangent-galvanometer and a commutator, continue its course down one wire and up 

 the other to the other pole of the battery. By this arrangement it may be seen that the 

 heating effect of the current will not cause any error except that due to slight differ- 

 ence in the thermal expansibility of the wire and the comparison- wire, due to the differ- 

 ence in the load on the two wires.t Any change of elasticity wrought by the current, 

 amounting to *1 per cent., could have readily been detected, but after some five or six 

 hours had been spent, and loads of very different amounts, almost up to the breaking- 

 load of the wire, used, the attempt was abandoned, as it seemed certain that for these 

 comparatively small currents there was no appreciable change in the elasticity resulting 

 from the passage of the current when the latter was maintained constant. 



Next the effect of much more powerful currents was tried, the same arrangements 

 being employed as in Experiment XVIII., with the exception that the magnetising 

 solenoid was removed, and in its place two terminal screws substituted, one near each 

 end of the wire, but, of course, beyond the points where the wires are clamped. 

 These terminal screws served to connect the wire with a battery of 10 GROVE'S cells 

 and a box of resistance-coils, together with a tangent-galvanometer and commutator. 

 The wire was rubbed along its length with a resined glove, and the pitch of the note 

 determined by means of the syren first when the current was not passing through 

 the wire, next when it was, and finally, a second time, with no current. The following 

 were the results obtained with annealed iron wire and unannealed piano-steel wire : 



* ' Phil. Trans.,' 1883 (vol. 174, Part I.), pp. 2-4. 



t Even this source of error can be eliminated by testing the wire, in the first place, with the permanent 

 load on the comparison-wire greater, and in the second place less, by an eqnal amount than that on the 

 wire to be examined. 



