355 



III. " On the Thermo-electricity of Ferruginous Metals, and on 

 the Thermal Effects of stretching Solid Bodies." By J. P. 

 JOULE, F.R.S. Received January 29, 1857. 



The experiments on the above subjects were made with a thermo- 

 multiplier placed in the vacuum of an air-pump. Its sensibility was 

 such that with the junction antimony and bismuth, a thermometric 

 effect not greater than -g-oVo f a degree Centigrade could be esti- 

 mated. In determining the thermo-electric position of the metals, it 

 - was necessary to increase the resistance of the instrument a hundred- 

 fold, by placing in the circuit a coil of fine wire. In thermo-electric 

 arrangement steel was found to be nearer copper than iron was. By 

 hardening, steel was raised almost to the place of copper. Cast iron 

 was found to surpass copper ; so that the junction cast iron and 

 copper is reverse to that of wrought iron and copper, and the 

 arrangement cast iron and wrought iron is much more powerful than 

 copper and wrought iron. A new test of the quality and purity of 

 ferruginous metals is thus indicated, which will probably be found of 

 value to the arts. 



The experiments on the stretching of solids showed, in the case of 

 the metals, a decrease of temperature when the stretching weight was 

 applied, and a heating effect when the weight was removed. An iron 

 wire of an inch in diameter was cooled - of a degree Centigrade 

 when stretched by a weight of 775 Ibs. Similar results were ob- 

 tained with cast iron, hard steel, copper, and lead. The thermal 

 effects were in all these cases found to be almost identical with those 

 deduced from Professor Thomson's theoretical investigation, the par- 

 ticular formula applicable to the case in question being H= - x Pe, 



J 



where H is the heat absorbed in a wire one foot long, t the absolute 

 temperature, J the mechanical equivalent of the thermal unit, P the 

 weight applied, and e the coefficient of expansion per 1. With 

 gutta-percha also a cooling effect on extension was observed ; but a 

 reverse action was discovered in the case of vulcanized india- 

 rubber, which became heated when the weight is laid on, and cooled 

 when the weight was removed. On learning this curious result, 

 Professor Thomson, who had already intimated the probability of 



VOL. VIII. 2 E 



