ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 



•Ill 



Hypothetical Yalta Series, in Sulphur or Sulphuretted "Hydrogen, of Metallic 

 Oxides, and possibly of Air-coated Metals. 



The series so obtained gives copper and iron in their proper order ; 

 but it is scarcely likely to be really correct, because it assumes that the 



les of the metals are exposed to the gas rather than the metals 

 themselves. It is quite possible that it is not veiy incorrect for tarnished 

 metals— i.e. metals coated with a film of oxide ; but for ordinarily clean 

 metals, coated, not with a film of oxide, but with a film of oxygen, it is 

 nothing but a rough approximation, given because we have no better data. 



It is to be noted that, as the film of oxygen diffuses away, the Volta 

 effect depending on it must diminish ; until at length the active affinity caus- 

 ing the chemical strain is nothing more than M,S, or perhaps M,S — H 2 ,S. 

 A gradual falling off and ultimate even reversal of sign was observed by Mr. 

 Brown in both HC1 and H 2 S. In so far as actual chemical action occurs 

 and a film of chloride or sulphide forms, so far, of course, also will the effect 

 diminish ; because it depends essentially on the unsatisfied chemical strain, 

 not on the accomplished chemical action. 



For a summary of the views here expressed see section 22. 



19. Having now explained why I believe the main part of the Volta effect 

 to take its rise at the surface of contact between metal and medium rather 

 than between metal and metal, it remains to consider whether this belief 

 requires one to assert that there is no true contact force at all at the 

 junction of two metals. By no means ; the existence of such a force is 

 undoubted ; but for metals it is usually very small and may be neglected 

 in comparison with the Volta force, though, strictly speaking, what is 

 observed electroscopically is a mixture of the two. It is the true contact 

 force which gives rise to the Peltier effect, and its variation with tempera- 

 ture (assisted by the Thomson effect) causes thermo-electric currents. 

 A contact force exists, as Thomson has shown, not only at the junction of 

 two different metals, but also between parts of the same metal at different 

 temperatures. 



In another place x I have endeavoured to gain some insight into the 

 nature of this true contact force and to suggest its cause. This has been done 

 by many others ; but I may be permitted to repeat my own notion — vague 

 and incomplete though it avowedly is. Molecules of matter do not move 

 in independence of electricity; at any rate, the converse is certainly true — ■ 

 electricity does not move independently of matter. Electricity, in flowing 

 through a wire, meets with resistance ; there is something analogous to 

 friction between the matter and the electricity, and the opposing force is 

 precisely proportional to the strength of the current. This much is 



1 Phil. Mag. December (suppl.), 1876, < On a Mechanical Illustration of Thermo- 

 electric Phenomena.' 



