514 



Mr. H. Tomlinson. The Influence of 



the thermo-electric properties of the metal. The heating was, how- 

 ever, in this case accomplished by means of an air-chamber, consist- 

 ing of two concentric brass cylinders with a layer of water between 

 them. The strips were placed, clamped together at their centres, in 

 the axis of the chamber, the two extremities of each projecting about 

 8 inches from either end of the chamber ; a thermometer was also 

 placed with its bulb in the centre of the latter, and the temperature 

 in the first instance raised to 100° C. When the needle of the 

 galvanometer was at rest, a load of 8 kilos, was put on the lever, and 

 after a space of 2 minutes had elapsed a deflection of three divisions 

 of the scale resulted. This deflection again showed the stressed metal 

 to he positive to the unstretched, and on the removal of the load the 

 needle came back again to nearly its old position. Trials were made 

 with several loads from 2 to 8 kilos., and always with the same result 

 as regards the nature of the effect; the mean value of the deflection 

 per kilo, being as nearly as could be judged 0*375 division of the 

 scale. The electromotive force developed by the highest load was 

 found to be 0'429 microvolt, and the electromotive force which would 

 be produced by a stress of 1 gram per square centimetre would be 

 659 X 10 -9 microvolts, the temperature of one junction being at 100° C. 

 and of the other about 16° C. The chamber was next permitted to 

 cool down, first to 60° C, and afterwards to 42° C, and at both these 

 temperatures the effect of the stress was found to be in the same 

 direction as before, and though no attempt was made to measure the 

 actual deflection, this was evidently diminished with the temperature. 



Finally, it should be added that both in this experiment and in the 

 two previous ones, the extremities of the strips and the clamps at the 

 ends of the stressed strip were all well shielded from the source of 

 heat. It would seem then that we have good reason for concluding 

 that for any temperature between 16° C. and 100° C, cobalt, whether 

 in the annealed or unannealed condition, is rendered by traction 

 thermo-electrically positive to unstretched cobalt.* 



The Effect of Excessive Loading on the Electrical Resistance of Hard 



Piano-steel. 



Experiment VII. 



I have already shownf that for moderate amounts of stress the 

 effect on the electrical resistance of piano-steel is of the same nature 

 as the effect on iron, but, as according to Mr. H. Johnson, J the 



* As I thought that want of purity might influence the result, Mr. J. M. Thom- 

 son of King's College kindly made an analysis of two small pieces of the metal, and 

 informed me that they contained more than 98 per cent, of cobalt, no trace of nickel, 

 and barely a trace of iron. 



f Loc. cit. 



X " The Electrician," May 17th, 1884. 



