394 M, G. Wiedemann on the Transmission of Heat in Metals. 



observed; under t are found the corresponding deflections of 

 the galvanometer when the thermo-electric element used in mea- 

 suring the temperatures was applied to those parts of the bar ; 

 under q the quotients are given which are obtained by dividing 

 the t standing near the corresponding q, into the sum of the 

 temperatures standing above and below it. 



If the highest temperature observed in each series be assumed 

 equal to 100, the other temperatures will be as follows : — 



From the mean value of the quotients 5^ = 2-321, the relative 

 couductibility of the zinc bar may be estimated at 20'3, if the 

 conductibility of an equally thick silver bar be placed at 100. If 

 the deflections of the galvanometer given in the first table be 

 reduced to actual excess of temperature of the several points of 

 the bar above the surrounding air, the conductibility of zine 

 against that of silver at 100 will be — 

 190. 



Becquerel found that the couductibility of zinc for electricity 

 was 24 (silver =100). Zinc takes, therefore, pretty much the 

 same place in the list of metals, whether considered as a con- 

 ductor of heat or electricity. 



2. Passage of Heat from one metal to another. 



In considering the relative conductibilities of the different 

 metals for heat, the question suggests itself, what takes place on 

 the passage of heat from one metal to another. 



Besides the old experiments of Fourier*, made with the con- 

 tact thermometer, from which, however, no certain conclusions 

 can be drawn, this question has been ti'eated by M. Despretzf, 

 and not long since, briefly, by M. Angstrom J. 



M. Despretz pressed together the sections of a copper and a 

 tin bar, and heated the free end of the copper bar. The tempe- 

 ratures were observed by means of thermometers sunk in dift'er- 

 ent parts of the bars, and from these Despretz calculated the 

 temperatures at the point of contact, and found that the tempe- 

 rature of the copper bar there was l'^-47 C. higher than that of 



* Poggendorff 's Annalm, vol. xiii. p. 327- 



t Ibid. vol. xlvi. p. 484. J Ibid. vol. Ixxxviii. p. 165. 



