SURFACES OF DISCONTINUITY 683 



normal concentration reduced at the interface thus producing 

 the positive side of the layer there, while the excess electrons 

 go to increase the local concentration in the other body, produc- 

 ing the negative side of the layer. It will be seen that this 

 contact potential Vc = Vi — Vz depends on temperature. 

 Now long ago Lord Kelvin and Helmholtz in combating the 

 view that Volta potentials could be identified with the Peltier 

 effect, showed that the latter is really dVc/dt being thus simply 

 the temperature coefficient of the Volta effect. (See for exam- 

 ple Lord Kelvin's paper, Phil. Mag., 46, 82 (1898).) If this 

 is so we see that the Peltier effect, i.e., the "thermoelectric 

 power" of two metals is (k/e) (log Wi — log W2). But we know 

 that this is very feeble compared to Vc, and there is also evi- 

 dence from the values of electric conductivities and from recent 

 work on the electron theory of metals that the electron concen- 

 trations in different metals are of the same order of magnitude, 

 so that the term (kt/e) (log ni — log 712) is negligible. Thus, 

 practically, 



Vc = <l>2 — <^l. 



This is the modern formulation of Volta's theory, expressing the 

 contact potential as the difference of two electron affinities, 

 each one a characteristic of its metal. 



As regards the production of current, suppose the metals 

 to be in contact at a pair of faces, and bent so as to face each 

 other across a relatively wide gap at another pair. If an 

 ionizing agent were placed near the air gap, ions would be 

 created in the gap and be driven one way or the other by the 

 electric field between the two faces at differing potentials, thus 

 tending to annul the field. If the ionization ceases, the P.D. is 

 restored in the air gap ; fresh ionization will create fresh current 

 and so on. It will be observed that the energy of the currents 

 is not obtained from the surface of contact of the metals but 

 from the ionizing agent. This vitiates at once one of the 

 implicit assumptions of earlier generations of workers, viz., 

 that one must look for the source of the E. M. F. at the same 

 place as one finds the source of the energy changes. The 

 function of the electrolyte, as Lord Kelvin always emphasized, 



