DYNAMICAL THEORY OF HEAT. 141 



but the actual inversion observed by M. Becquerel is required to show that the 

 diminution of strength in the current is due to a real falling off in the electro- 

 motive force, and not to the increased resistance known to be produced by an 

 elevation of temperature. 



123. "From Becquerel's discovery it follows that, for temperatures below a 

 certain limit, which, for particular specimens of copper and iron wire, I have ascer- 

 tained, by a mode of experimenting described below, to be 280° Cent., copper is on 

 the negative side of iron in the thermo-electric series ; on the positive side for higher 

 temperatures ; and at the limiting temperature these two metals are thermo- 

 electrically neutral to one another. It follows, according to the general mecha- 

 nical theory* of thermo-electric currents, referred to above, that electricity pass- 

 ing from copper to iron causes the absorption or the evolution of heat according 

 as the temperature of the metals is below or above the neutral point ; but neither 

 absorption nor evolution of heat, if the temperature be precisely that of neutrality ; 

 (a conclusion which I have already partially verified by experiment). Hence, if in 

 a circuit of copper and iron, one junction be kept about 280°, that is, at the neutral 

 temperature, and the other at any lower temperature, a thermo-electric current 

 will set from copper to iron through the hot, and from iron to copper through the 

 cold, junction ; causing the evolution of heat in the latter, and the raising of 

 weights, too, if it be employed to work an electro-magnetic engine, but not causing 

 the absorption of any heat at the hot junction. Hence there must be an absorp- 

 tion of heat at some part or parts of the circuit consisting solely of one metal or 

 of the other, to an amount equivalent to the heat evolved at the cold junction, 

 together with the thermal value of any mechanical effects produced on other 

 parts of the circuit. The locality of this absorption can only be where the tem- 

 peratures of the single metals are non-uniform, since the thermal effect of a cur- 

 rent in any homogeneous uniformly-heated conductor is always an evolution of 



* This is the only part of the theoretical reasoning as first given, which depended on the appli- 

 cation of Carnot's principle, and consequently, is the only part capable of being objected to as un- 

 certain. All doubt would be removed by an experimental verification of the stated Peltier effects for 

 copper and iron, at the different temperatures, such as I hope very soon to have completed. In the 

 meantime, instead of the theoretical reasoning, we may, if it is preferred, use an ample foundation of 

 analogy to conclude that heat is absorbed at the hotter junction, and evolved at the colder, by the 

 actual thermo-electric current in every case of a circuit of two metals, with their junctions differing 

 but little in temperature. For it was found by Peltier himself, that currents from bismuth to copper, 

 from copper to antimony, from zinc to iron, from copper to iron, and from platinum to iron, cause 

 absorption, and the reverse current in each case, evolution of heat ; experimental conclusions, with 

 which I was not acquainted when I first published the Theory. Very soon after I found, myself, 

 by experiment, that copper and iron at ordinary atmospheric temperatures, exhibit the anticipated 

 thermal phenomenon ; and corresponding experimental results have been obtained still more recently 

 in the cases of bismuth and copper, copper and antimony, copper and iron, German silver and iron, by 

 Frankenheim. (Poggendorff's Annalen, Feb. 1854); in every case, the current which would be 

 produced by heating one junction a little, being that which in the same junction causes an absorption 

 of heat. If we consider the induction sufficient to establish this as a universal law in thermo-elec- 

 tricity, the reasoning in the text becomes independent of any hypothesis to which objections can pos- 

 sibly be raised. 



