520 repoet— 1884. 



which prevents their reduction to exactly the same potential, but the 

 outstanding difference is small and vai'ies with temperature. It can be 

 measured thermoelectrically by the Peltier effect, but in no other known 

 way. It is probably entirely independent of surrounding media, metallic 

 or otherwise. 1 



xxii. If two metals are in contact, the potential of the medium sur- 

 rounding them is no longer uniform : if a dielectric it is in a state of 

 strain, if an electrolyte it conveys a current. 



xxiii. In the former case the major part of the total difference of 

 potential is related closely to the difference of the potential energies of 

 combination, and is approximately calculable therefrom. In the latter 

 case the total E.M.F. is calculable accurately from the energy of the 

 chemical processes going on, minus or plus the enei'gies concerned in 

 reversible heat effects. 2 



xxiiia. ' 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.' (Thomson.) 



xxiiii. ' If the action in a cell consists in part of irreversible processes', 

 such as : (1) frictional generation of heat ; (2) diffusion of primary or 

 secondary pi'oducts ; (3) any other action which is not reversed with the 

 current; there will be a certain dissipation of energy, and the E.M.F. 

 of the circuit will be less than the loss of intrinsic energy corre- 

 sponding to the electrolysis of one electro-chemical equivalent. It is only 

 the strictly reversible processes that must be taken into account in calcu- 

 lating the E.M.F. of a circuit.' (Maxwell : 'Elementary 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 commonly observed Volta series minus the Peltier, the other 

 is the Peltier or thermoelectric series. To reckon up the total E.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 chemical data, because they depend on chemical 

 tendencies. By ' Peltier or thermoelectric series ' I mean those giving a 

 purely physical E.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 con- 

 veniently represented by a number of curves, because temperature varie3 

 continuously; Volta series, on the other hand, can hardly be represented 

 geometrically, because the transition from one medium to another is 



1 To distinguish between Peltier force and Volta force henceforward it will be 

 best to write Bi/Sb or Zn/Cu for the former, and Zn/Air/Cu or Fe/VVater/Pt for the 

 latter. The force electroscopically observed is Air/Zn/Cu/Air, but this involves both ; 

 the right way of denoting the Volta effect pure and simple is Zn/Air/Cu. 



2 Such, for instance, as we have been discussing under the head of inconstant or 

 simple voltaic batteries (sections 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. 



