DYNAMICAL THEORY OF HEAT. 141 
but the actual inversion observed by M. BecquEREL is required to show that the 
i 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 BrcQueREL’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, ifthe 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 Carnor’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 Penriur 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 Pexrrer 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 
Franxennerm. (Poccenporrr’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. 
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