160 Prof. Oliver Lodge on the Seat of the 
Daniell cell, an absolute measure of the so-called contact-force 
was made ; and it was shown that, uniting the copper and 
zine by a drop of water, instead of by a metal, no deflection 
was produced. It was also shown that the deflection was 
greatest when the zine was clean and the copper oxidized”. 
But Sir William Thomson went further than this: he 
sounded a theoretic note, and in a sentence revived the whole 
controversy about the seat of power in the pile. The sentence 
is this:—‘* For nearly two years I have felt quite sure that 
the proper explanation of voltaic action in the common voltaic 
arrangement is something very near Volta’s, which fell into 
discredit. because Volta or his followers neglected the prin- 
ciple of the conservation of force. I now think it quite 
certain that two metals dipped into one electrolytic liquid 
will (when polarization is done away with) be at the same 
potential.” And then he goes on to one of those brilliant 
and extraordinary speculations characteristic of no one else, 
and applies this apparent contact-force to determine a lower 
limit to the size of atoms—an application obviously of tran- 
scendent interest, and of more importance than all the previous 
outcome of contact discussions put together f. 
The whole subject now acquired a fresh interest, and the 
new series of experimental determinations of contact-force 
began. 
3. Hankel’s and Gerland’s measurements belong to this 
period in point of date (1861-69), though in method and motive 
they probably are the outcome of the earlier period{. Hankel 
uses a modified Kohlrausch method and a Bohnenberger or 
* Proc. Lit. and Phil. Soc. Manchester. Letter from Prof. W. Thomson 
to the president, Dr. Joule, Jan. 21, 1862: “New Proof of Contact- 
Electricity.” See reprint of papers on Electrostatics and Magnetism, p. 317. 
+ “There cannot be a doubt that the whole theory is simply chemical 
action at a distance. Zinc and copper connected by a metal wire attract 
each other from any distance, so do platinum plates coated with oxygen 
and hydrogen respectively. I can now tell the amount of the force, and 
calculate how great a proportion of chemical affinity is used up electro- 
lytically before two such disks come within any specified small distance 
down to a limit within which molecular heterogeneousness becomes sen- 
sible. This of course gives a definite limit for the size of atoms.”— 
Letter to Dr. Joule, 1862, cited above. See also Thomson and Tait, Nat. 
Phil., Part II. Appendix F. 
t Hankel, Electr. Untersuchungen, Abh. der kinigl. Stchs. Gesellschaft, 
math.-phys. Klasse, i861 and 1865. See also Pogg. Ann. cxv. p. 57, 
and cxxvi. pp. 286 and 440; exxxi. p. 607. 
Gerland, “On the E.M.F. between Water and some Metals,” Poge. 
Ann. cxxxiil. 1868, p. 513; exxxvii. 1869, p. 552. In his second paper, 
to get over the air effect, Gerland joins two metals through a galva- 
nometer, and then dipping them into a liquid, observes the first swing of 
the needle. He also compensates the E.M.F. by Poggendortt’s method. 
He thus determines the value of 
M/M'+ M'/Liquid+ Liquid, M. 
