Degrees of Heat in Absolute Measure, 65 



From what has been said, this increase of temperature is 



216xl0 5 of a Centigrade degree; 



and in accordance with the definition given, 

 1 degree Centigrade =0005075 A = 216 x 10 5 absolute units. 



Besides the connexion between heat and electricity which is 

 expressed by Dulong and Petit's law, and by Faraday's electro- 

 lytical law, and which we have used in establishing a definition 

 of the absolute degree of heat, there is another connexion, which 

 found its first expression in Wiedemann and Franz's law, that 

 the conductivity of the various metals for heat and for electricity 

 are in the same ratio. Subsequent investigations have shown, 

 however, that this ratio changes with the temperature, and 

 therefore that the law in its original form cannot be completely 

 valid, but requires a modification. 



The influence of heat upon electrical conductivity has been in- 

 vestigated by several physicists, as Lenz, Becquerel, Arendtsen, 

 and especially by Matthiessen and von Bose*. These physicists 

 investigated the conductivity of ten different pure metals, namely 

 silver, copper, gold, zinc, cadmium, tin, lead, arsenic, antimony, 

 and bismuth. From their investigations the remarkable result 

 was obtained, that the diminution of electrical conductivity for 

 an increase from 0° to 100° C. is the same for all these metals, 

 namely 29'307 per cent. The resistance increases, therefore, 

 for the same increase of temperature by 41*46 per cent. — that 

 is, in a somewhat greater ratio than the increase of temperature 

 (36*6 per cent.) if we calculate the temperature from the abso- 

 lute zero ( — 273° C). Matthiessen and Vogtf found subse- 

 quently that among the pure metals iron forms an exception, 

 inasmuch as its conductivity may decrease by more than 38 

 per cent. 



But few experiments have been made as to the influence of 

 temperature on conductivity for heat. It must, however, be 

 remarked that all the older experiments on thermal conduction 

 agree with the former assumption, that the conductivity is inde- 

 pendent of the temperature. Angstrom J found with reference 

 to two bars of copper, which, however, were probably not quite 

 pure, that between 0° and 100° there was a decrease of con- 

 ducting-power of from 15 to 21 per cent., and for iron of 28*7 

 per cent. ; while Forbes§ found for bar iron a decrease of 15*7 

 and 23'3 per cent. 



* Phil. Mag. S. 4. vol. xxiv. p. 405. 

 t Phil. Mag. S. 4. vol. xxvi. p. 542. 



% (Efvers. a/K. Vetensk. Forhandl. 1862. Pogg. Ann. vol. cxviii. p. 423. 

 § Trans. Edinb. Royal Soc. 1862-64. 

 Phil. Mag. S. 4. Vol. 46. No. 303. July 1873. F 



