Stress or Permanent Strain to a Bright-red Heat. 259 



brass block, B, resting on a wooden support fitting on to the 

 top of the tube, and provided with a terminal for making 

 connexion with one pole of a battery of thirty Grove's cells, 

 arranged ten in series and three in parallel arc. The current 

 from the battery passed through a set of resistance-coils 

 arranged so that the resistance could be altered by small 

 amounts at a time ; the current was also conducted through 

 an amperemeter and through the wire, passing in or out of 

 the latter through the intermediation of a mercury-cup, C. 

 The lower extremity of the wire was clamped into a second 

 brass block, 0, to which was secured a mirror, M, reflecting 

 the light of a lamp on to a scale placed at a distance of one 

 metre. This block was provided with a circular aperture, Q, 

 into which, if necessary, a bar could be introduced, and at its 

 lower extremity terminated in a brass cylinder, D, having a 

 piece of rather stout brass wire, K, projecting from its centre, 

 vertically downwards, and dipping into the mercury-cup C. 

 When it was required to subject the wire to torsional stress, 

 two fine silk threads were wrapped in opposite directions round 

 D, and passed, as in the figure, over two fixed pulleys, P, to 

 two small cardboard scale-pans, S, on which weights could be 

 placed. When the wire was not required to be torsionally 

 stressed the fine silk threads and the pulleys were dispensed 

 with. 



Experiment I. — A piece of very soft and carefully annealed 

 iron wire*, 30 centim. long and 1 millim. in diameter, was 

 subjected to torsional stress in the manner described above. 

 The pans S each weighed 10 grms., and in each of them was 

 placed a load of 20 grms., so that the torsional couple 

 amounted to 30 x 1*6 in gramme-centimetre-units f. A cur- 

 rent of gradually increased amount was passed through the 

 wire, and when the temperature approached a bright red the 

 wire began to twist rapidly and permanently under the influ- 

 ence of the stress J. The current was shortly afterwards 

 stopped ; and when a temperature between bright red and 

 dull red had been reached the wire began suddenly to twist 

 further, the amount of the sudden twist being about 90 degrees. 

 When the wire had cooled down to the temperature of the 

 room there was left a considerable permanent twist. In this 

 experiment there was no perceptible temporary untwist on 

 heating, but merely a slight check in the rate at which the 

 wire was permanently twisting as soon as the critical tempe- 



* This wire was specially prepared for me by Messrs. Johnson and 

 Nephew, and is capable of suffering a permanent elongation of 25 per 

 cent, before breaking. 



t The diameter of D was 1*6 centimetre. 



% The first appearance, however, of sensible permanent twist occurred 

 rather suddenly at a dull red heat. 



S2 



