Prof. Thomson on the Dynamical Theory of Heat. 285 



n(T)-n(T') = ^T, 



\^,{'^\-<^2)dt={<T^-(T^)T ; 

 and equation (13) becomes 



If we make use of (16) in thisj we have 



F = JyT (19). 



The fii'st of these expressions for the electromotive force involves 

 no hypothesis, but only the general principle of equivalence of 

 heat and work. Its agreement with any experimental results is 

 only to be looked on as a verification of the accuracy of the ex- 

 periments, and can add nothing to the certainty of the part of 

 the theory from which it is deduced. On the other hand, it 

 would be extremely important to test the second expression (19) 

 by direct experiment, and so confirm or correct the only doubtful 

 part of the theory. The way to do so would be to determine in 

 absolute measure the electromotive force, F, due to a small dif- 

 ference of temperature, r, in any thermo-electric pair, and to 

 determine, in known thermal units, the amount of the Peltier 

 effect at a junction of the two metals with a current of strength 

 measured in electro-dynamic units, as we should then, by these 

 determinations, be able to evaluate from direct experiments the 

 values of the two members separately which appear equated in (19). 

 As yet no observations have been made which lead directly or 

 indirectly to the evaluation of the second member of (19) in any 

 case, but I hope before long to succeed in carrying out a plan I 

 have formed for this object. Neither have any observations been 

 made yet which give in any case a determination of the first 

 member ; but they may easily be accomplished by any person 

 who possesses a conductor of which the resistance has been de- 

 termined in absolute measure. Mr. Joule having kindly put me 

 in possession of the silver wire on which his observations of the 

 electrical generation of heat, in 1845, were made with currents 

 measured by a tangent galvanometer used by him about the 

 same time in experimenting on the electrolysis of sulphate of 

 copper and sulphate of zinc, I hope to be able to complete the test 

 of the theoretical result without difficulty, in any case in which 

 I may succeed in determining the amount of the Peltier thermal 

 effect. 



117. In the mean time it is interesting to form an estimate. 



