concerning Voltas Contact Force. 



473 



or boundary force as we might now call it, are accustomed 

 more or less completely to ignore the air boundaries ; employ- 

 ing an open circuit with the object of avoiding obvious 

 neutralising or compensating forces at other junctions, but 

 forgetting that opening a circuit necessarily introduces one 

 more boundary, not one less. 



Dr. Larmor has been good enough to tell me of a " proof" 

 in Parker's ' Thermodynamics ' (1894 edition) that on 

 Helmholtz's view the Peltier coefficient at a metallic junction 

 is proportional to the rate of change of the junction force there 

 with temperature ; a proof somewhat on the same lines as 

 that given independently by Lord Kelvin in the Proc. 

 Roy. Soc. Edin. vol. xxii. page 118, Session 1897-8 ; 

 but neither proof in reality establishes anything specially 

 limited to the metallic junction ; the whole of the junctions 

 are really laid under contribution, a fact which is half 

 recognised by Lord Kelvin, though not by Mr. Parker. 



In order to save other people from working through these 

 proofs I may reword them in a simpler and briefer fashion, 

 using language appropriate to the mass-affinity doctrine, but 

 incidentally throwing in the symbol a so as to include also 

 the essence of the specific heat idea. 



Let a charge of electricity given to a metal be held there 

 by an intrinsic attraction or mass-affinity ; it will have a 

 special affinity-energy in addition to the energy of its ordinary 

 electrical interactions with other charges : call this surplus 

 energy, at the absolute temperature 6, 



The corresponding entropy is 



so that 



o _,_ ( acW , Coin 



du = o~cW = 6ds. 



Consider an open circuit of two active metals and one neutral 

 substance, e.g. Zn, Cu, and Pt, or 1, 2, and 0, with air-gaps 

 forming condensers as in the diagram ; the condensers beincr 



