Eleetromotive Forces in the Voltaic Cell. 3903 | 
processes going on, minus or plus the energies concerned in 
reversible heat-effects*. 
xxiii a. “The E.M.F. of an electro-chemical apparatus ”’ 
whose energy is entirely expended in maintaining a current 
“is equal to the mechanical equivalent of the chemical action 
on one electro-chemical equivalent of the substance.”’ (Thom- 
son. 
xxiii b. “ If the action in a cell consists in part of irreversible 
processes, such as (1) frictional generation of heat, (2) dif- 
fusion of primary or secondary products, (3) any other 
action which is not reversed with the current, there will be 
a certain dissipation of energy, and the H.M.F. of the circuit 
will be less than the loss of intrinsic energy corresponding to 
the electrolysis of one electro-chemical equivalent. It is only 
the strictly reversible processes that must be taken into account 
in calculating the H.M.F. of a circuit.” (Maxwell : ‘ Hlemen- 
tary Electricity,’ p. 148.) 
xxiv. There are two distinct and independent kinds of 
series in which metals (and possibly all solids) can be placed : 
one kind depends on the dielectric or electrolytic medium in 
which the bodies are immersed, the other kind depends on 
temperature. The one is the real Volta series, but it is the 
eommonly observed Volta series minus the Peltier; the other 
is the Peltier or thermoelectric series. ‘To reckon up the total 
.M.F . of a circuit, we may take differences of numbers from 
each series and add them together. 
23. It is necessary to illustrate the meaning of this last 
statement, No. xxiv. By “real Volta series’ I mean such 
series as we have attempted to calculate from purely che- 
mical data, because they depend on chemical tendencies. 
By “ Peltier or thermoelectric series’’ I mean those giving a 
purely physical H.M.F., produced we know not quite how, 
whose energy-source is not chemical but thermal. We have 
on the one hand a number of Volta series, each for a special 
medium, and on the other a table of thermoelectric powers at 
different temperatures. The latter can be conveniently repre- 
sented by a number of curves, because temperature varies 
continuously ; Volta series, on the other hand, can hardly be 
represented geometrically, because the transition from one 
medium to another is probably per saltum; at least, it is not 
* Such, for instance, as we have been discussing under the head of in- 
constant or simple voltaic batteries (sects. 19-21). These reversible heat- 
effects indicate the presence of thermal contact-forces which, wherever 
they exist, prevent chemical data from giving E.M.F. accurately: they 
also must be taken into account. We have called them Joule or Bouty 
effects. 
