ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 



513 



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^[oasnvoniont of rontact force between insulators is besot witli dif- 

 ticultii's, liccauso it is so diHiouIt to make cU'ctricity pass across tho 

 junction. No limit to the force has at present bicn observed: \vlienever 

 an electrical nuichine reaches its limit and refuses to charge its prinio 

 conductor, ov a Leyden jai-, to a Iiit^hcr potential, it is ac(;ounted for by 

 s-:iyiii'4 that the rate of leakat^o is now cfiual to the rjite of production 

 \ which is niuleniably true), but nothinf* is said about whether the rate of 

 production is the smne as it was when the jar was uiicharrfcd. It is 

 ;i ditlieult matter to si-ttle, because most of the Uaknjifo takes phico closo 

 to the rubber; and, tbou<,di it is (piito possil)lo, it is unlikely that a limit 

 t.) the force will be discovered, by nndinj:^ the activity of a frictional 

 machine less at high potentials than at low. When the substances in 

 contact are two metals it is impossible for them to drive electricity very 

 hiird. for it would, so to apeak, slip tbron,c;;b tbeir finpfers ; but when an in- 

 sulator is concerned, its grip is so great that probably there is no limit to tho 

 ■i'orco until its insulating power is overcome, and through it also electricity 

 begins to slip. C'ertaiidy any upper limit must be a very high one, for tho 

 force can readily pile np a charge till it produces sparks a foot or more long. 



Whether Vul/a forces, or contact-forces between substances and tho 

 racdium surroumling them, exist for insulators also we do not know ; wo 

 Jiiive \w reason Avliatevcr to deny their existence; but whereas in the case of 

 viietuls these exceeded tho forces acting between the substances themselves, 

 iiere in the case of insulators they arc absolutely negligible by comparison. 

 For intermediate substances they may have correspondingly important 

 values, and it seems not unlikolv that at the junction of metals with elec- 

 trolytes, and of electrolytes with one another, the total contact force may 

 be a complex one ; partly eheniical, and due to tho possibilities of chemical 

 fiction straining across the junction ; and jiartly physical, due to different 

 velocity of tho molecules. 



-<J. The preliminary experiments of ]3outy have caused him to 

 believe in tho existcnco of ])hysical contact forces, at tho junction of 

 metals with electrolytes, which cannot bo brought into harmony with 

 energies of chemical action. And though tho subject is too unexplored 

 in this direction to be ripe for discussion, it may be well to point out 

 that these contact forces aro important in the theory of the voltaic cell 

 even in its simplest form. 



Why is the E.M.F. of a zinc-copper battery less than that of a zinc- 

 platinum ? 



Why is tho E,!\[.F. of a zinc-lead or iron battery smaller than either ? 



Tho same chemical action goes on in each, zinc is dissolved at one end 

 and hydrogen liberated at the other ; how then can tho E.M.F. be different 

 if it is calculable from the chemical reactions ? ' 



If we picture to ourselves the actual forces in action we shall get a 

 l^ind of answer indicated to us. In a zinc-iron cell the E.^I.F. is duo to 

 tlie zinc pulling at oxygen harder than tho iron does ; but, since the iron 

 does pull too, with no inconsidferable strength, the balance of force is 

 not so great as if the iron -were replaced by copper, -which pnlls less, or 



' I'vofossor Exner c\its tliis knot in characteristic fashion by assert inp; roundly 

 tliat tho I'J.^LF. of all such cells is tho same, and that it iiiattovs nothins? what metal 

 >•* opposed to the zinc of a cell so long as it does not alter the chemical action <joing 

 "". He further asserts that all batteries are iion-polarisable and (|uite censtaiit as 

 soim as I hey have got rid of dissolved air, and before siiljjhate of zinc lias aecuuuilated. 

 He verities these extraordinary statements, to three significant figures, hy straight- 

 lorwanl fxpcriment. .See his paper ' On Inconstant Voltaic Batteries,' cited above. 

 ISvt. L L 



