PHYSICS 363 



monstrating that their algebraic sum round a chain of them 

 in a cell was equal to the E.M.F. of the cell. The physical 

 chemists, however, whose point of view has been put most 

 completely by Ostwald and Nernst, but also had powerful 

 adherents long ago such as le Roux, Edlund, and Maxwell, 

 maintain that these contact potentials as measured are quite 

 illusory, and are really differences between the potential in the 

 air near one metal and that in the air near the other metal, 

 the films of air close to the plates adhering to them when they 

 are separated and causing the plates to behave as if a real 

 difference of potential existed between them. To this group 

 of workers the only real contact difference of potential between 

 two metals is represented by the Peltier effect, whose order of 

 magnitude is very much smaller than the " Volta " P.D.s, 

 and according to them the E.M.F. of a cell is built up of elec- 

 trode potentials between metal and electrolyte which owe their 

 origin entirely to chemical action in the cell. In defence of 

 their position they point out that when current passes round 

 a circuit of different metals there is no absorption or evolution 

 of heat at a junction at all consistent with a real Volta effect, 

 but only with the very much smaller and admitted Peltier 

 effect ; and further, that the application of the second law of 

 thermodynamics to their hypothesis undoubtedly yields certain 

 relations between the E.M.F. of the cell, the heat of reaction, 

 and the equilibrium constant of the chemical reaction involved 

 which have been verified by experiment. Possibly the most 

 notable of the contact theorists was Lord Kelvin, who, together 

 with Helmholtz, maintained that the Peltier effect does not 

 correspond directly to the contact potential, but is related to 

 the temperature coefficient of the contact potential, thus : 



P _^. 

 T dT' 



where P is the Peltier coefficient and V^ is the contact P.D. 

 A very complete and lucid account of all this earlier work will 

 be found in the Report to the British Association on the Seat of the 

 E.M.F. in the Voltaic cell, prepared by Sir Oliver Lodge in 1884, 

 and until recently it would seem that the contact theory had 

 " retired beaten," and the electrochemists held the field. 



However, as mentioned above, the appearance of thermionic 

 and photo-electric emission has changed things very consider- 

 ably, and, further, Nernst 's " solution pressures " do not 

 appear to be so much in favour with physical chemists as of old. 

 The main features of this new work are to be found in Richard- 

 son's papers, dating from 1902 onwards, on thermionic emission 

 (which are practically summarised in his books on the " Electron 



