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Becquerel discovered that if one junction of copper and iron, in a 

 circuit of the two metals, be kept at an ordinary atmospheric tem- 

 perature, while the other is raised gradually to a red or white heat, 

 a current first sets from copper to iron through the hot junction, in- 

 creasing in strength only as long as the temperature is below about 

 300 Cent. ; and becoming feebler with farther elevations of tempera- 

 ture until it ceases, and a current actually sets in the contrary direc- 

 tion when a high red heat is attained. Many experimenters have 

 professed themselves unable to verify this extraordinary discovery, 

 but the description which M. Becquerel gives of his experiments 

 leaves no room for the doubts which some have thrown upon his 

 conclusion, and establishes the thermo-electric inversion between 

 iron and copper, not as a singular case (extraordinary and unex- 

 pected as it appeared), but as a phenomenon to be looked for be- 

 tween any two metals, when tried through a sufficient range of tem- 

 perature, especially any two which lie near one another in the 

 thermo-electric series for ordinary temperatures. M. Regnault has 

 verified M. Becquerel's conclusion so far, in finding that the strength 

 of the current in a circuit of copper and iron wire did not increase 

 sensibly for elevations of temperature above 240 Cent., and began to 

 diminish when the temperature considerably exceeded this limit ; 

 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 electromotive force, and not to the increased resist- 

 ance known to be produced by an elevation of temperature. 



From Becquerel's discovery it follows that, for temperatures be- 

 low a certain limit, which, for particular specimens of copper and 

 iron wire, I have ascertained, by a mode of experimenting described 

 below, to be 280 Cent., copper is on the negative side of iron in the 

 thermo-electric series, and on the positive side for higher tempera- 

 tures ; and at the limiting temperature copper and iron are thermo- 

 electrically neutral to one another. It follows, according to the 

 general mechanical theory of thermo-electric currents referred to 

 above, that electricity passing from copper to iron causes the absorp- 

 tion or the evolution of heat according as the temperature of the 

 metals is below or above the neutral point ; but neither evolution 

 nor absorption of heat, if the temperature be precisely that of neu- 

 trality (a conclusion which I have already partially verified by 



