652 PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 
Heat*, I found a relation between the quantities of heat absorbed or evolved by 
currents crossing metallic junctions at different temperatures; which led imme- 
diately to a general expression for the electrical condition of a circuit of two metals 
with their junctions kept at any stated temperatures. 
6. From this it appeared that the electro-motive force should follow the same law 
of variation in every case, being expressed by a constant, (representing the thermo- 
electric difference between the two metals,) multiplied into an absolute function of 
the temperatures of their junctions, namely, the difference of their temperature on 
the absolute thermometric scale since proposed by Mr. Joule and myself, and demon- 
strated by our experiments'!' to agree very approximately with their difference of 
temperature as indicated by an air-thermometer. Finding this conclusion contra- 
dictory to the statements made by experimenters, that the electro-motive force does 
not vary with the temperature of the junctions according to the same law in circuits 
composed of different metals, I perceived that Peltier’s discovery did not afford a 
sufficient explanation of the source whence a thermo-electric current derives its 
energy, but that electric currents must possess the previously undiscovered property 
of producing different thermal effects in passing from cold to hot and from hot to 
cold in the same metal, and must possess this property to different amounts in 
different metals. 
7 . Taking this new property of electric currents into account along with that 
discovered by Peltier, and introducing an application of Carnot’s principle, I arrived 
at expressions for the relations between the heat absorbed and evolved in various 
parts of a circuit of any different metals, and between the electro-motive force and 
the temperatures of the junctions, which appear to be in complete accordance with 
the facts. These investigations were communicated in December 1851, to the Royal 
Society of Edinburgh :|:. 
8. Still simpler theoretical considerations (§§ 10 to 18 below) regarding the source 
of energy drawn upon in a thermo-electric current, make it certain that the pheno- 
mena of inversion discovered by Gumming could not exist, unless the metals present- 
ing them had the property of experiencing, when unequally heated, unequal thermal 
effects from electric currents passing through them from hot to cold, and from cold 
to hot. Having satisfied myself, both by an examination of the evidence afforded by 
Becquerel’s experiments (the original investigation on the subject by Gumming being 
at that time unknown to me), and by actual observation, in experiments of my own 
* This, the true form of Carnot’s principle, was first published by Clausius in May 1850 (Poggendorff’s 
‘ Annalen’). It had occurred to myself, and I had used it in discovering the true expression for the duty of a 
perfect thermo-dynamic engine shortly before that time. It was not, however, until the beginning of the year 
1851 that I thought on a demonstration which would probably be admitted as conclusive in establishing the 
principle, and my investigation on the subject was only communicated in March 1851 to the Royal Society of 
Edinburgh. See Trans. Roy. Soc. Ed. of that date, “ Dynamical Theory of Heat,” § 14. 
t “ On the Thermal Effects of Fluids in Motion,” Transactions, June 1854. 
t See Proceedings of that date, and Philosophical Magazine, June 1852. 
§ See below. Part II. §§ 79, 80, 81, 83, 84, &c. 
