54 



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

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

 the former is the higher ; and 6 S the amount of heat absorbed per 

 unit of electricity crossing the hot junction. The following relation 

 (similarly simplified in form) was also established : 



dQ 

 * = 7~~dt' 



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

 ject of determining the specific heats of electricity in metals, to in- 

 vestigate the law of electromotive 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 effect 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 



