Prof, Thomson on the Dynamical Theory of Heat. 289 



.(c) shows that its absolute vahie is rather more than one-third 

 of a thermal unit per second per unit strength of current. 



120. If the specific heats of current electricity either vanished 

 or were equal in the different metals, we should have by (15) 

 and (16), 



— = constant (20), 



and 



F=J^(T-T') (21), 



or the Peltier thermal effect at a junction of two metals would 

 be propoi'tional to the absolute temperature at which it takes 

 place, and the electromotive force in a circuit of any two metals 

 would vary in the simple ratio of the difference of temperature 

 on the new absolute scale between their junctions*. Whatever 

 thermometric system be followed, the second of these conclusions 

 would require the same law of variation of electromotive force 

 with the temperatures of the junctions in every pair of metals 

 used as a thermo-electric element. 



121. Before the existence of a convective effect of electricity 

 in an unequally heated metal had even been conjectured, I 

 arrived at the preceding conclusions by a theory in which the 

 Peltier effect w^as taken as the only thermal effect reversible with 

 the current in a thermo-electric circuit, and found them at va- 

 riance with known facts which show remarkably different laws 

 of electromotive force in thermo-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 de- 

 monstrated by theoretical reasoning ; but only the difference of 

 the amount of this effect produced by currents of equal strength 

 in different 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 



• When the theory was first corainunicated to the Royal Society of Edin- 

 burgh, I stated these conclusions with reference to temj)eratiu-e by the air 

 thermometer, and therefore in terms of Carnot's absolute function of the 

 temperature, not simply, as now, in terms of absolute temperature. At 

 the same time I gave, as consequences of Mayer's hypothesis, the same 

 statement in terms of air-therraomcter temperatures, as is now made abso- 

 lutely. Sec 'Proceedings,' Dec. 15,1851; or Philosophical Magazine, 

 vol. iii. p. 5.32. 



