476 



REPonT — 1884. 



a heterodox notion ■which I do not find in any other of hi;i writings, and 

 wliich I beliovc ho has abandoned even if he ever really hold it. 



In 1879 Brown tried a copper-nickel divided ring, substitutinj; HCI 

 for air, and hero also succeeded in obtaining a reversal of sign. He also 

 arranged a divided ring of wet blotting paper, and showed that then; 

 was a differenco of potential when the two halves were touched with a 

 zinc-copper couple (which is not remarkable), but ho then goes on to 

 draw a moral, and to say that the slit' of the divided ring corresponds 

 to the air-film, and the wet paper to the moisture film in the ordinary 

 Volta condenser experiment. The film of moisture on the zinc plate is 

 tlm'! shown to have a -t- charge, and that on tho copper a negative. If 

 it bo objected that the better the plates fit, tho better the manifestation of 

 contact E, it is ^o bo replied that it is not to be supposed that there is no 

 air between them anyhow (says lb-own). Probably, he says, gas pro- 

 duces the difttTcnce of potential only so far as it forms a film on the 

 surface. When a metal and a liqnid are experimented on it is probably 

 really a 2-fluid cell, tlie other fiiiid being that condensed on the surface of 

 the metal. 



lb-own thus goes strongly for the activity of the films, or condensed 

 air-sheets, which certainly exist on the surface of solids, and which may 

 play an important part in the matter ; but he supposes that these filra-4 

 act by corroding or attacking the jjlates, and that such a film is neces- 

 sarily existent between sui-faccs nominally in contact if any Volta cft'eet \-^ 

 to be produced, so that if tho metal faces really and truly touched all 

 over, tiiey Avould show no charge when separated. ]\Ioreovcr, he lays it 

 down that the potential difference is only observed while chemical action 

 is going on, but that so soon as it ceases, from any cause, at once the 

 Volta etfect ceases too. In all this I entirely differ from him, but liis 

 ex)ieriments are very interesting and much to the point. 



riiey cannot, however, be regarded as settling the question — tho very 

 important and fundamental question — as to whether the Volta clfect 

 depends on the atmosphere or medium surrounding the plates, or wliethcr 

 it is an absolute effect depending on contact alone. Experiments on this 

 point are absolutely discordant, and it seems to be one of those points 

 which it is very difficult to settle by direct experiment. For if by 

 using a chemically-active gas instead of air, you get a positive result or 

 change in the Volta effect, the answer from the other side is: 'Yes, of course. 

 because your plates arc corroded and coated with sulphide or chloride, 

 or something whose contact forces come in and modify everything.' It, 

 on the other hand, you get a negative result when you substitute sorao 

 inert gas like hydi-ogen for air, then it is objected that yon haven't 

 remoA'cd the air film which tho plates had contracted from long ptaiiding' 

 in the air, and if you answer indignantly that you did, and that your 

 hydrogcn was perfectly pure, it is replied with a sneer: 'Oh yes, it is 

 not so easy to get pure hydrogen as you seem to think.' 



Moreover, suppose a positive effect on changing the ^as /'■">' esta- 

 blLshed, what then ? Nothing is settled except that the metal /air con- 

 tact force is proved to be somewhat different from the metal/gas contact 

 force. There seems to be really no way of knocking contact force on tlic 

 head experimentally, and this pi-obably because it is a reality : th<:rc 

 really is a contact force at every juiiclion of dii^slmllar siihstanccs; awl 

 the J'J.M.F. of a circuit, lahefher it he inductive or roinliiclive, />' (dicivjs 

 the sum of sitcit contact forces. I do not say that the contact force at 



