76 RISEARCH COMMITTEES. 
of an ice calorimeter the heat evolved by the chemical actions 
taking place in a large number of cells, whose e. m. f.’s he also 
determined ; the temperature coefficients were then deduced for 
the different cells, and found to be in good agreement with the 
values determined directly, in some cases by Gockel, in others by 
Jahn himself. 
5. Electromotive Force and Peltier Effect. 
It is now necessary to account for the energy dissipated in the 
cell, other than that dissipated in accordance with Joule’s Law. 
This dissipation of energy, denoted by —-T is obviously due to 
oF 
or 
a reversible generation of heat; this Dene so, we see that if the 
current through the cell be revers —the 
energy which the rev ersing dynamo needs to expend ‘will be less 
or greater than the mechanical equivalent of the heat absorbed in 
the reversed chemical actions, according as the cell absorbs or 
evolves heat reversibly in its direct action. 
Now we know one such class of reversible heat actions in the 
circuit, viz. :—thermoelectric actions at the various junctions of 
dissimilar substances ; and we note at once that the mathematical 
form taken by a thermoelectric e. m. f. is identical with that of the 
ter For if «x denote the thermoelectric e. m.f. in a circuit 
consisting of two substances whose junctions are maintained at 
different temperatures, we know that II, the coefticient of the 
Peltier effect at either junction, i 
the temperature of the junction considered. 
Now there are several such junctions in any voltaic cireuit ; if 
then we denote by & (II) the sum of their Peltier effects we know 
a priort that 
5k < 
ff eee SS (iM 
To assume off hand that —T ss is equal to } (If) amounts to 
asserting the absence of any source of reversible generation of 
heat other than the Peltier effects; an assumption we have no 
right to make—though it often was made—without experimental 
ce This bee been supplied by Jahn* and Gill, who 
independently measured the Peltier effects at the bounding surfaces 
* Wied. Ann. xxxiv, p. 755; 1, p. 189. The second paper corrects some errors in the 
first. 
7 Wied. Ann. x], p. 115. 
