140 PROFESSOR W. THOMSON ON THE 



known facts which show remarkably different laws of electro-motive force in ther- 

 mo-electric pairs of different metals. I therefore inferred, that besides the Peltier 

 effect there must be other reversible thermal effects ; and I showed that these can 

 be due to no other cause than the inequalities of temperature in single metals in 

 the circuit. A convective effect of electricity in an unequally heated conductor of 

 one metal was thus first demonstrated by theoretical reasoning ; but only the differ- 

 ence of the amount of this effect produced by currents of equal strength in differ- 

 ent metals, not its quality or its absolute value in any one metal, could be inferred 

 from the data of thermo-electric force alone. The case of a thermo-electric circuit 

 of copper and iron, being that which first forced on me the conclusion that an 

 electric current must produce different effects according as it passes from hot to 

 cold, or from cold to hot, in an unequally heated metal, was taken as an example 

 in my first communication of the Theory to this Soci§ty;* and the two metals, 

 copper and iron, were made the subjects of a consequent experimental investiga- 

 tion, to ascertain the quality of the anticipated property in each of them sepa- 

 rately. The application of the general reasoning to this particular case, and the 

 answers which I have derived by experiment to the question which it raises, are 

 described in the following extract of a Report communicated to the Royal Society 

 of London, March 31, and published in the Proceedings, May, of the present year : — 

 122. " Becquerel discovered that if one junction of copper and iron, in a circuit 

 of the two metals, be kept at an ordinary atmospheric temperature, while the other 

 is raised gradually, a current first sets from copper to iron through the hot junc- 

 tion, increasing in strength only as long as the temperature is below about 300° 

 Cent. ; and becoming feebler with farther elevation of temperature until it ceases, 

 and a current actually sets in the contrary direction when a high red heat is at- 

 tained.f 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 unexpected as it appeared), but as a 

 phenomenon to be looked for between any two metals, when tried through a suf- 

 ficient range of temperatures. M. Regnault has verified M. Becquerel's con- 

 clusion so far, in finding that the strength of a 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 ; 



* See Proceedings, R. S. E., Dec. 15, 1851. 



j" Since this was written, I have found that thermo-electric inversions between copper and an 

 alloy of antimony and bismuth, and between silver and the same alloy, precisely analogous to that 

 between copper and iron more recently discovered by M. Becquerel, were discovered as early as 

 1823, by Professor Cumming of Cambridge, shortly after the thermo-electricity of metals was first 

 brought to light by Seebeck. These, with other experiments, leading to important results, especially 

 as to the order of metals and metallic compounds in the thermo-electric series, are described in the 

 Cambridge Transactions for 1823, and in Professor Cumming's Treatise on Electro-dynamics. 



