Electromotive Forces in the Voltaic Cell. 343 
with difficulties, because it is so difficult to make electricity 
pass across the junction. No limit to the force has 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 con- 
tact 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 insulator 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 com- 
parison. Tor intermediate substances they may have corre- 
spondingly important values; and it seems not unlikely that at 
the junction of metals with electrolytes, 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 H.M.F’. of a zinc-copper battery less than that 
of a zinc-platinum ? 
_ Why is the H.M.F. of a zine-lead or iron battery smaller 
than either ? 
