Electromotive Forces in the Voltaic Cell. 161 
Hankel electroscope ; his special merit is the determination 
of metal-liquid contact-foreces without introducing blotting 
paper, glass, or fingers, as the earlier experimenters had done. 
Fig. 3.—Hankel’s Arrangement for observing the Volta Effect between a 
Metal and a Liquid. 
The liquid, L, is in a funnel-tube, A, B; C is a copper plate, and M the 
metal under observation. First, » touches C for an instant, then C is 
raised and made to touch p’, which leads to a Hankel electrometer. 
This gives Cu/M+M/L=kaea. 
Then the liquid is ren out of the funnel, a plate of the metal M is placed 
on its mouth, and the experiment repeated. 
This gives Cu/M=A48. 
To eliminate & substitute a plate of zinc for M, and get 
Cu/Zn=ky. 
Then, finally, M/L=%—* ou/m. 
Y 
In all these expressions air contact-forces are, as usual, neglected. But 
it is very tempting to try if, by increasing the number of such equations, 
one cannot calculate some metal/air contact-forces. Thus the special 
case when M is copper gives one more equation. We can then take 
zine instead of M, and can also make the condenser-plate of zine 
instead of copper, and so on; but we get no forwarder, fresh unknowns 
appear as fast as additional equations, and some of the equatious are 
liable to degenerate into identities. 
So far back as 1824 Becquerel* attempted the investiga- 
tion of metal-liquid contacts ; and Buffft made some measure- 
ments in 1842, but his results are scarcely likely to be reliable 
considering the poor experimental resources of that date. 
Professor Clifton employed Kohlrausch’s method in 1877, 
using a Thomson electrometer and a Clark cell as standard 
of H.M.F. He has only published a preliminary papert, in 
which he overlooks minutiz such as change of contact- 
* Becquerel, Ann. de Chime, 1824. He put the liquid in a copper 
capsule on the plate of an electroscope, and connected it with the con- 
denser plate by his fingers. 
+ Buff, Liebig’s Ann. Chem. u. Pharm. 1842. He made the lower plate 
of his condenser the metal to be examined; on it he placed glass, and 
then filter paper soaked in the liquid, which he connected with the metal 
plate by a wire of the same metal. 
} Chfton, Proc. Royal Soc. xxvi. p. 299 (1877). 
