1897.] on Contact Electricity of Metals. 537 



proper method, of course, would he to use the metal-sheathed Volta- 

 condenser and compensation (§ 9 above), and with it measure the 

 Volta-dilFerences between copper and ir -n at different tfraperatures, 

 the same for the two metals in each ctis(\ Tiie slieith and everything 

 in it should, in each experiment, be kept at one and the same constant 

 temperature. But it would probably be very diffioult to get a decisive 

 answer, because of the uncertainties and time-lags of changes in the 

 Volta-potential of metallic surfaces with change of temperature, which, 

 if we may judge from Pellat's and Murray's experiments on effects of 

 temperature when the two metals are unequally heated, would probably 

 also be found when the temperatures of the two metals, kept exactly 

 equal, are raised or lowered at the same time. 



§ 21. The thermoelectric difference between bismuth and antimony 

 is about ten times that between copper and iron for temperature diff- 

 erences of ten or twenty degrees on the two sides of 20° C, and their 

 Volta-contact difference is exceedingly small (according to Pellat, just 

 one one-hundredth of a volt when both their surfaces are strongly 

 scratched by rubbing with emery). It would be very interesting, 

 and probably instructive, to find how much their Volta-contact differ- 

 ence varies with temperature by the method at present suggested. 

 The great variations of Yolta-surface potentials, found by Pellat and 

 Murray, when one of the two metals is heated, may have been due 

 to difference of temperatures between the two opposed plates with 

 air between them ; and it is possible that no such large variation, or 

 that large variation only due to changes of cohering gases, may bo 

 found when the two metals are kept at equal temperatures, and these 

 temperatures are varied as in the experiment I am now suggesting. 



§ 22. Peltier's admirable discovery (18^4) of cold produced where 

 an electric current crosses from bismuth to antimony, and heat where 

 it crosses from antimony to bismuth, in a circuit of the two metals, 

 with a current maintained through it by an independent electromotive 

 force, is highly important in theory, or in attempts for theory, of the 

 contact electricity of metals. 



From an unsatisfactory * hypothetical application of Carnot's 

 principle to the thermodynamics of thermoelectric currents I long 

 ago inferred f that probably electricity crossing a contact between 

 copper and iron in the direction from copper to iron would pro- 

 duce cold, and in the c(mtrary direction heat when the tempera- 

 ture is below 280° C. (the thermoelectric neutral temperature of 

 copper and iron),| and I verified this conclusion by experiment.! 



* * Mathematical and Phj^ical Papers,' vol. i. art. xlviii. § 106, reprinted from 

 * Transactions of the Koyal Society of Edinburgh,' May 1854. 



t Ibid. § 116 (19). 



J In a thermoelectric circuit of copper and iron the current is from copper to 

 iron through hot when both junctions are below 280° C. It is from iron to 

 copper through hot when both junctions are above 280° C. 



§ 'Experimental Researches in Tliermoeleetricity,' Proc. R. S. May 1854; 

 republished as art. li. in * Mathematical and Physical Papers,' vol. i. (seo pp. 

 464-465). 



