100 Lord Kelvin on 



iron surfaces) if the temperature at J and throughout the 

 system is uniform at about 15° C. Keeping now the tem- 

 perature of C C, P I exactly at 15°, let the temperature of J 

 he raised to 25°. The difference of potentials between C C 

 and P I would be increased to '16148 volt, supposing -16000 

 to have been exactly the difference of potentials when the 

 temperature of J was 15°. This difference of differences of 

 potentials would be just perceptible on the most delicate 

 quadrant electrometer connected as indicated in the diagram. 

 Lastly, raise the temperature of C C and P I to exactly 25°, 

 J being still kept at this temperature : the spot of light of the 

 electrometer will return exactly to its metallic zero. But, 

 would the Yolta-difFerence of potentials between the surfaces 

 C C, I' 1 remain unchanged, or would it return exactly to its 

 previous value of "16000, or would it come to some other 

 value ? We cannot answer this question without experiment. 

 The proper method, of course, would be to use the metal- 

 sheathed Volta-condenser and compensation (§ 9 above), and 

 with it measure the Yolta-differences between copper and iron 

 at different temperatures, the same for the two metals in each 

 case. The sheath and everything in it should, in each experi- 

 ment, be kept at one and the same constant temperature. 

 But it would probably be very difficult to get a decisive 

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

 in the Yolta-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 differences of ten or twenty degrees on the two 

 sides of 20° (J., and their Volta-contact difference is exceed- 

 ingly 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 difference 

 varies with temperature by the method at present suggested. 

 The great variations of Volta-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 be found when the two 

 metals are kept at equal temperatures, and these temperatures 

 are varied as in the experiment I am now suggesting. 



