THERMAL AND ELECTRICAL CONDUCTIVITY. 40! 



It is very remarkable that for all pure metals the value of a is 

 very near 0*0038, and is therefore sensibly equal to the coefficient 

 of the expansion of gases. 



The resistance of pure metals is then sensibly proportional to the 

 absolute temperature. Nevertheless, the resistance of pure mercury 

 in the liquid state offers far less variation (924) ; but MM. Cail- 

 letet and Bouty* have found that the conductivity of this body 

 becomes 4-08 times as great as it solidifies, and that when solid, 

 the coefficient of variation approaches that of other pure metals. 



The coefficient a is still smaller in the case of alloys. With 

 argentan, which is an alloy of copper and nickel, numbers are 

 found for this coefficient varying from 0*00027 to 0*00044. The 

 smaller values from 0*00022 to 0*00031 have been obtained with 

 the alloy of platinum silver. 



According to Sir W. Siemens,! parabolic formulae no longer 

 suffice when we get beyond the temperature of 100. The re- 

 sistances of platinum, iron, and copper are represented with 

 sufficient accuracy by an expression of the form 



in which a and b are constants and T is the absolute temperature 

 calculated from 273. 



The resistance of the metals is in general greater in the liquid 

 than in the solid state. Bismuth forms an exception, as probably do 

 all those metals which diminish in volume in changing to the liquid 

 state. 



999. THERMAL AND ELECTRICAL CONDUCTIVITY. Forbes | was 

 the first to observe that the order of the metals as regards the 

 conduction of heat is the same as for the electrical conductivity. 

 Wiedemann and Franz, going still further, have ascertained that 

 the two kinds of conductivity are sensibly proportional. It remained 

 to be seen whether, as regards temperature, the thermal conductivity 

 follows the same laws as the electrical conductivity, and, like it, 

 diminishes as the temperature increases. 



* CAILLETET and BOUTY. Comptes rendus, Vol. c., p. 1188. 1885. 



t W. SIEMENS. Journal of Soc. Tel. Engineers) Vol. I., p. 123, 1872; 

 Vol. in., p. 296, 1874. 



J FORBES. Phil. Mag., Vol. iv., p. 15. 1834. 



WIEDEMANN and FRANZ. Pogg. Annalen, Vol. LXXXIX, p. 530. 1853. 



VOL. II. D D 



