520 



hepokt — 1884. 



■which, prevents heir reduction to exacihj the same potential, but the 

 outstanding difFcrence is small and varies with temperature. It can bo 

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

 way. It is probably entirely independent of surrounding media, metailif; 

 or otherwise.' 



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

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

 stx'ain, if an electrolyte it conveys a current. 



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

 potential is i-elated closely to the diflerence of the potential energies of 

 combinatio: and i.s apjiroximatcly calculable therefrom. In the latter 

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

 chemical processes going ou, minus or plus the energies concerned in 

 reversible heat effects.'^ 



xxiiirr. ' The E.M.F. of an electro-chemical apparatus ' whose eneray n 

 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 a« : (1) frictional generation of heat; (2) diffusion of primary or 

 Becondary products ; (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- 

 spending 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. lib.) 



xxiv. There are two distinct and independent kinds of series in Avhich 

 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 Ave may take differences of numbers from each series and add 

 them together. 



23. It is necessary to illustrate the meaning of this last statement, 

 IS^o. xxiv. By ' real Volta series ' I mean such series as we have attempted 

 to calculate from purely chemical data, because they depend on chcraical 

 tendencies. By ' Peltier or thermoelectric senes ' I mean those giving ;i 

 purely physical E.M.F., produced we know not quite how, whose enerfry- 

 BOurce 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 bj- 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 anotler is 



' To distinguish between Peltier force and Volta force henceforwani it will be 

 best to write Ui/Sb or Zii/Cu for the former, and Zn/Air/Cu or Fe/ Water/ I'fc lor the 

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

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



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

 simple voltaic batteries (sections 19- l.'I). These reversible heat effects indicate tlio 

 presence of thermal contact forces which, wherever they exist, prevent clieinical (lat,"i 

 from giving E.M.F. accurately : they also must be taken into account. We i.ave 

 called them Joule or Couty effects. 



