MEASUREMENT OF THE THOMSON EFFECT. 441 



If it be assumed that the variation of temperature from one bath 

 to the other is uniform, the effect observed in the centre of the bar 

 is proportional to the specific heat of the metal for the mean 

 temperature. Le Roux found first that this effect is proportional 

 to the intensity of the current. It follows also from his experiments 

 that the specific heat of electricity is zero for lead ; positive for brass, 

 copper, silver, zinc, cadmium, antimony, and an alloy of bismuth 

 with a tenth of antimony; negative for tin, aluminum, platinum, 

 argentan, and pure bismuth. In the two lists the metals have 

 been arranged in the order of increasing values of the specific heat 

 of electricity. 



1035. THERMOELECTRIC ELECTROMOTIVE FORCES. In a thermo- 

 electric couple, the solderings of which are at different temperatures, 

 the electromotive force E is the sum of the Peltier and the Thomson 

 effects, of which it is the seat. We have seen how Sir W. Thomson* 

 established, between the electromotive force, the Peltier effects, and 

 the specific heat of electricity, the equation 



= h CT (7. 



If we compare the couple to a reversible thermal machine, we 

 deduce 



W H = T 



dT 



so that the change of potential at the soldering of the two metals 

 may be deduced from the variations of the electromotive force of the 

 couple which they form. 



We have nothing to add to what has been said above on the 

 measurement of thermoelectric electromotive force. Gaugain's ex- 

 periments have shown that the electromotive force of a couple at 

 work, as a function of the difference of temperatures, may be repre- 

 sented by a parabola (373). We shall only mention here two 

 experiments, made with a view of verifying the formula (i), by 

 M. Bellati for the iron-zinc couple, and by M. Bouty for the couple 

 copper-copper sulphate. 



* THOMSON. Transactions of the Roy. Soc., Vol. xxi., p. 751. 1854.-^- 

 Math. and Phys. Papers, Vol. I., p. 232. 



