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 Sociéty;* 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. « BrcqueREL 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.+ Many experimenters have professed themselves unable to verify this 
extraordinary discovery ; but the description which M. BrcquEre 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. Recnavir 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., Dee. 15, 1851. 
+ Since this was written, I have found that thermo-electrie 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. Bucauerex, were discovered as early as 
1823, by Professor Cummine of Cambridge, shortly after the thermo-clectricity of metals was first 
brought to light by Seeseck. 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 
Camb¥idge Transactions for 1823, and in Professor Cummine’s Treatise on Hlectro-dynamics, 
