66 Royal Society, 



metal through which the current goes from cold to hot above the 

 specific heat of the same electricity in the other metal, at the tem- 

 perature t ; F the thermo-electric force in the circuit when the two 

 junctions are kept at tlie temperatures S and T respectively, of which 

 the former is the higher ; and 9g the amount of heat absorbed per 

 unit of electricity crossing the hot junction. The following relation 

 (similarly simplified in form) was also established : — 



^ Q de 



These relations show how important it is towards the special ob- 

 ject of determining the 8j)ecific heats of electricity in metals, to in- 

 vestigate the law of electronjotive force in various cases, and to de- 

 termine the thermal effect of electricity in passing from one metal to 

 another at various temperatures. Both of these objects of research 

 are therefore included in the general investigation of the subject. 



The only progress I have as yet made in the last-mentioned 

 branch of the inquiry, has been to demonstrate experimentally that 

 there is a cooling or heating effect produced by a current between 

 copper and iron at an ordinary atmospheric temperature according 

 as it passes from copper to iron or from iron to copper, in verifica- 

 tion of a theoretical conclusion mentioned above : but I intend 

 shortly to extend the verification of theory to a demonstration that 

 reverse effects take place between those metals at a temperature 

 above their neutral point of about 280° Cent. ; and I hope also to 

 be able to make determinations in absolute measure of the amount 

 of the Peltier eflfect for a given strength of current between various 

 pairs of metals. 



With reference to laws of electromotive force in various cases, I 

 have commenced by determining the order of several specimens of 

 metals in the thermo-electric series, and have ascertained some very 

 curious facts regarding varieties in this series which exist at different 

 temperatures. In this I have only followed Becquerel's remarkable 

 discovery, from which I had been led to the reasoning and experimental 

 investigation regarding copper and iron described above. My way 

 of experimenting has been to raise the temperature first of one 

 junction as far as the circumstances admit, keeping the other cold, 

 and then to raise the temperature of the other gradually, and watch 

 the indications of a galvanometer during the whole process. When 

 an inversion of the current is noticed, the changing temperature is 

 brought back till the galvanometer shows no current ; and then (by 

 a process quite analogous to that followed by Mr. Joule and Dr. 

 Lyon Playfair in ascertaining the temperature at which water is of 

 maximum density) the temperatures of the two junctions are ap- 

 proximated, the galvanometer always being kept as near zero as 

 possible. When the difference between any two temperatures on 

 each side of the neutral })oint which give no current is not very great, 

 their arithmetical mean will be the neutral temperature. A regular 

 deviation of the mean temperature from the true neutral tempera- 

 ture is to be looked for with wide ranges, and a determination of it 



