ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 493 



instead of the sum of the contact forces at every junction, we attend only 

 to the contact forces at the metallic junctions, the proposition will no 

 longer be true. This fact, that the metallic junctions are insufficient to 

 account for all the E.M.F., was established by Becquerel, De la Rive, and 

 others, and still more thoroughly and exhaustively by Faraday. It is the 

 easiest possible thing to make a number of batteries which shall give a 

 cnrrent without any metallic junction whatever. Faraday gives some 

 thirty of them. 1 



One more certain proposition we can lay down — viz., that whenever a 

 current is produced, the energy of the current must be maintained by 

 absorption of heat, or by chemical action, or by gravity, or by some other 

 such agent — not by mere contact. 



So much being agreed to, what remains as subject-matter for con- 

 troversy? This : A voltaic circuit contains at least three junctions ; what 

 is the value of the contact force at each of them, and especially to which 

 junction is the major part of the observed E.M .F. due ? Is it the zinc 

 acid ? or is it the copper acid ? or is it the zinc copper ? There is no 

 other question. The old chemical and contact controversy has died out, 

 but another controversy remains. Most physicists probably would say 

 to-day that the major part of the E.M.F. of the cell resides at the zinc- 

 copper junction. This was Volta's view, and this is the view of the text- 

 book writers taught by Sir William Thomson. Seme few would say at 

 the zinc-acid junction, and among them I must confess myself. 



It is no question between contact and something else ; it is a question 

 between a feeble enei'gy-less metal-metal contact, and an active energetic 

 metal-fluid contact with potentialities of chemical action straining across 

 the junction. AVhat is there to distinguish between the two ? Electro- 

 static experiments with air condensers prove nothing. They add up three 

 E.M.F. 's, air/M + M/M' + M'/air, and give you the sum. The experi- 

 menters usually assume that M/M' is what they are measuring, but 

 there is no proof to be given in support of the assumption, except that if 

 you substitute water for air the effect remains almost unaltered : but then 

 water contains oxygen as the active element the same as air does. Well, 

 then, it may be urged, the effect is the same in vacuo and in hydrogen as 

 in air ; and to this I answer, Not proven. 



Can any further assertions be made with reference to electroscopic 

 experiments as bearing on voltaic theory ? Yes ; it can be asserted that by 

 adding up the Volta effects for A/B, for B/C, for C/D . . ., and for Z/A, 

 you arrive at the total E.M.F. of the circuit A, B, C . . . A. True ; 

 but what then ? 



The Volta effect you call A/B is really air/A + A/B + B/air; 

 that you call B/C is air/B + B/C + C/air; 



and that you call Z/A is air/Z + Z/A + A /air . 



Add them up, and you get A/B + B/C +. . . + Z/A, 



which must be the E.M.F. of a circuit by common sense: — i.e. without 



1 Exp. Res. ii. 2020. Dr. J. A. Fleming describes another of these batteries in Phil. 

 Mag.. June 1874, and gives some very cogent and readable arguments in favour of 

 the ' chemical theory ' of battery E.M.F., suggesting that the difference of potential 

 between the terminals of a battery on open circuit is due to potential chemical com- 

 bination of the metals and electrolytes. He does not, however, explain the old 

 Volta experiment ; and, as Prof. Chrystal has pointed out (Ency. Brit. p. 99), up- 

 holders of the chemical theory are bound to explain this. 



