476 report — 1884. 



a heterodox notion which I do not find in any other of his writings, and 

 which I believe he has abandoned even if he ever really held it. 



In 1879 Brown tried a copper-nickel divided ring, snbstituting HC1 

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

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

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

 zinc-copper couple (which is not remarkable), but he 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 

 thus shown to have a + charge, and that on the copper a negative. If 

 it be objected that the better the plates fit, the better the manifestation of 

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

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

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

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

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

 the metal. 



Brown 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 films 

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

 sarily existent between surfaces nominally in contact if any Volta effect is 

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

 over, they would show no charge when separated. Moreover, 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 effect ceases too. In all this I entirely differ from him, but his 

 experiments are very interesting and much to the point. 



They cannot, however, be regarded as settling the question — the very 

 important and fundamental question — as to whether the Volta effect 

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

 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 are corroded and coated with sulphide or chloride, 

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

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

 inert gas like hydrogen for air, then it is objected that you haven't 

 removed the air film which the plates had contracted from long standing 

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

 hydrogen 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 gas was esta- 

 blished, 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 the 

 head experimentally, and this probably because it is a reality : there 

 really is a contact force at every junction of dissimilar substances; and 

 the E.M.F. of a circuit, whether it be inductive or conductive, is always 

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



