ELECTUOMOTIVE FOIICES IX THE VOLTAIC CELL. 



493 



of intcv- 

 pment "1 

 >o change 

 -in ollnv 

 finda tlie 

 at at ilio 

 I volts : - 



In 



.•2 



cal sketch 

 ly to the 

 metals ami 

 Vltiei- co- 

 yr tinits M 

 s bismutb- 

 I'LM.F. tit 

 lie formub 



cases also. 



I it for ilis- 



Tics of com- 



'idation and 



mst give up 



aical action, 



Lces, anil by 



,ot produci', 



lysical. 



iiigly great. 



iccms to iB'^ 



:o may bavc 



ion repeatcJ, 



•fectly safe." 



)f discussion 



[M.F. at the 



mbstancc in 



^rcnit is tb*-' 



the circuit. 

 Lw.co?i.,bat 

 ]e dissenting 

 [fcssor Exner 

 Vc else. 

 [U cororaanfl 



propositi(iit 



10 ; csp. r; f ;: 



:stcd by Cler^ 



Uc in Uio series 

 Chonipson, ' <-'" 



instead of tlio sum of the contact forces at even/ junction, we attend only 

 to the contact forces at the ?»t'teZ//(' junctions, the proposition will no 

 longer bo true. This fact, that the metallic junctions are insuilicient to 

 account for all the JvAI.F., was established by Ik'cquerel, Di; la Kivo, and 

 t»thers, and still more thoroughly and exhaustively by Fai-aduy. It is the 

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

 current without any metallic junction whatever. Faraday gives some 

 thirty of them.' 



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

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

 :il).sori)tion of heat, or by chemical a.ction, or by gravity, or by some other 

 such agent — not by mere contact. 



So much being agreed to, what remains as subject-mat tci- 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 obsei-ved 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 ([uestion. The old chemical and contact controver.sy has died out, 

 hut another controversy remains. !Most physicists probably wonld say 

 to-day that the major part of the E.M.F. of the cell i-csides at the zinu- 

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

 hook writers taught by Sir William Thomson. Some few would say nt 

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



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

 iiiwcen a feeble energy-less metal-metal contact, and an active energetic 

 ii;(.'tal-lluid contact with potentialities of chemical action straining across 

 the junction. What is there to distingui.sh between the two? Electro- 

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

 E.M.F.'s, air/]\I + ^l/M' + M7air, and give you the .'lum. 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 eli'ect remains almost unaltei'od : 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 

 e.^cperinients 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 ab the total E.M.F. of tlid 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 yon call Z/A is air/Z + Z/A -f A/air . 



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



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



' I'.rp. lies. ii. 2020. Dr. J. A. Fleming de.scribos another of these batteries in Phil. 

 *%., June 1871, and gives some very cogent and readable arguments in favour of 

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

 hotwecii tlio terminals of a battery on open circuit is duo to potential chemical com- 

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

 Volta experiment; and, as I'rof. Chrystal has pointed out (^Ency. JJrit. p. Dt)), up- 

 lioldors of the chemical theory arc bound to expl.iin this. 



''Ill I 



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