metallic Conductors and their Resistance to Electric Currents. 1 1 



replaced by one ^^ diameter. The counterpoise was changed till, after several 

 trials, it was found tliat a tension of four ounces at zero of the pyrometer, was 

 sufficient. I was surprised to find that a given battery produced nearly the 

 same ignition in this as in the smaller ^vire, the increased cm-rent compensating 

 the greater mass ; but, from the greater quantity of heat evolved, the upper 

 part of tlie pyrometer became very hot, so that the clip which holds the top of 

 the wire was blued. This apparently made the resistance greater than the 

 truth ; but is not likely to have affected its changes, as the experiments were 

 made in the inverse order of the table, so that the highest temperature was ob- 

 tained first. After five results the wire was broken by an accident close to the 

 cjdinder ; and as I had no more of that diameter, I rejoined it by a loop, which, 

 being beyond the part that is ignited, might be expected not to interfere with 

 the temperature produced. 



Each result is a mean of five. 



The increase is evident here also, although the discordances are greater 

 than in the last series. 



Lastly, a wire of Jg was used. At No. 40 the pyrometer was immersed in 

 diluted alcohol, to ascertain whether the resistance increased with the cur- 

 rent when the wire was kept cool. The fluid boiled round it with a sharp 



= - 4.6 ; P' = 140.0 ; zero after, + 24.0. On the same supposition, P = 148.1 ; T = 2803.0 ; and 

 the resistance = 953.0. In the third trial it gave way ; the ends, however, were not fused, though 

 a very little more would certainly have melted it. The temperature of the centre of the wire in 

 the first of these experiments must have exceeded 3200°. 



c2 



