ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 513 



Measurement of contact force between insulators is beset with dif- 

 ficulties, because it is so difficult to make electricity pass across the 

 junction. No limit to the force lias at present been observed : whenever 

 an electrical machine reaches its limit and refuses to charge its prime 

 conductor, or a Leyden jar, to a higher potential, it is accounted for by 

 saying that the rate of leakage is now equal to the rate of production 

 (which is undeniably true), but nothing is said about whether the rate of 

 production is the same as it was when the jar was uncharged. It is 

 a difficult matter to settle, because most of the leakage takes place close 

 to the rubber ; and, though it is quite possible, it is unlikely that a limit 

 to the force will be discovered, by finding 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 

 hard, for it would, so to speak, slip through their fingers ; but when an in- 

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

 force until its insulating power is overcome, and through it also electricity 

 begins to slip. Certainly any upper limit must be a very high one, for the 

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



Whether Volta forces, or contact-forces between substances and the 

 medium surrounding them, exist for insulators also we do not know ; we 

 have no reason whatever to deny their existence ; but whereas in the case of 

 metals these exceeded the forces acting between the substances themselves, 

 here in the case of insulators they are absolutely negligible by comparison. 

 For intermediate substances they may have correspondingly important 

 values, and it seems not unlikely 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 chemical, and due to the possibilities of chemical 

 action straining across the junction ; and partly physical, due to different 

 velocity of the molecules, 



20. The preliminary experiments of Bouty have caused him to 

 believe in the existence of physical contact forces, at the junction of 

 metals with electrolytes, which cannot be brought into harmony with 

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

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

 that these contact forces are 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 the E.M.F. of a zinc-lead or iron battery smaller than either ? 



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

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

 if it is calculable from the chemical reactions ? l 



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

 kind of answer indicated to us. In a zinc-iron cell the E.M.F. is due to 

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

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

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



<i I SS'S Exn er cuts this knot in characteristic fashion by asserting roundly 

 iliat the h.M.F. of all such cells is the same, and that it matters nothing what metal 

 is opposed to the zinc of a cell so long as it does not alter the chemical action going 

 on. He further asserts that all batteries are non-polarisable and quite constant as 

 soon as they have got rid of dissolved air, and before sulphate of zinc has accumulated. 

 lie verities these extraordinary statements, to three significant figures, by straight- 

 torward experiment, See his paper ' On Inconstant Voltaic Batteries,' cited above. 

 lb - 4 - L L 



