517 



and four on cooling), we deduced a formula by the method of least 

 squares for the correction of the conducting power for temperature. 

 It was found that the conducting power or resistance of a metal does 

 not decrease or increase in direct ratio to the temperature, as stated 

 by Becquerel*, Arndsten-f, and Siemens J, who assume that the 

 formula for the correction of resistance for temperature between 

 0-100 may be expressed by 



but that, on the contrary, the formula must be 



\=x+yt+yt\ 



where \ is the resistance at t degrees, x the resistance at 0, and y 

 and y constants. One fact seems to have escaped the observation 

 of former experimenters, namely, that when a wire of a metal is 

 heated for the first time to 100 and again cooled, an alteration in the 

 conducting power takes place ; with most metals it is necessary to 

 heat them for several days before their conducting power becomes 

 constant. In the third part we have deduced from the results ob- 

 tained, the law that all pure metals in a solid state vary in con- 

 ducting power to the same extent between and 100 C. In cases 

 where very great accuracy is required, it is absolutely necessary to 

 experiment on the conductor itself ; for we have found almost the 

 same differences between formulae obtained for wires of the same 

 metal as between the mean of those deduced for the different metals. 

 This behaviour may be attributed to the fact that the molecular 

 arrangement is not the same even in wires of the same metal ; for we 

 find that copper wires, when kept at 100 for several days, behave very 

 differently from each other : thus, in the case of the three copper wires 

 experimented with, wire 1 increased in conducting power almost to the 

 same extent as if it had been annealed, wire 2 partially so, and wire 

 3 hardly at all. With bismuth, wire 1 increased its conducting power 

 16 per cent.; wire 2, 19 per cent.; and wire 3, 12 per cent. Again, in 

 the case of cadmium, which becomes quite brittle and crystalline at 80 

 (for cadmium may be powdered in a hot mortar), we found the 

 formula for each wire very different. On the other hand, the formulae? 

 of the wires of those metals which, after being kept at 100 for some 

 time, show a very slight or no alteration in the conducting power 



* Ann. de Chim. et de Phys. (3) xvii. 242. f Fogg. Ann. civ. 1. 



J Fogg. Ann. cxiii. 91. 



