ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 469 



actions going on there ; but it was supposed, and is still supposed (though, 

 as I venture to think, quite erroneously) to leave untouched the question 

 as to the precise seat of the E.M.F. in a battery. 



However that may be, the success of the chemical theory of the 

 electric current naturally caused it to be still more certainly assumed 

 that the apparent contact force of Yolta could also be accounted for by 

 accidental chemical action, and that without some chemical action some- 

 where no Yolta effect could be produced. This also I believe to be quite 

 false; provided always that the phrase 'chemical action' be used in its 

 ordinary sense as meaning combination, and that the word ' action ' be 

 not explained away as meaning anything whatever. 



2. The triumph of the chemical theorists with regard to the Volta 

 effect was, however, shortlived, for, from 1860, the invention of the 

 quadrant electrometer put into the hands of electrostatic experimenters a 

 far more refined and delicate instrument than could have been thought 

 possible a few years before ; and the illustrious inventor of that instru- 

 ment himself for ever put the truth of Volta's phenomenon beyond doubt, 

 by the most simple and beautiful device of suspending a charged torsion 

 arm over a zinc-copper junction. By comparing the deflection so pro- 

 duced with that caused by a Daniell cell, an absolute measure of the so- 

 called contact force was made ; and it was shown that on uniting the copper 

 and zinc 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 

 zinc was clean and the copper oxidised. 1 



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 principle 

 of the conservation of force. I now think it quite certain that two 

 metals dipped into one electrolytic liquid will (when polarisation 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 transcendent interest, 

 and of more importance than all the previous outcome of contact 

 discussions put together. 2 



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 (1861-1869), though in method and motive they probably are the 

 outcome of the earlier period. 3 Hankel uses a modified Kohlrausch method 



1 Proc. Lit. and Phil. Soc, Manchester. Letter from Prof. W. Thomson totbe 

 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. Zino 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 electrolytically before two such disks come within any specified small 

 distance down to a limit within which molecular heterogeneousness becomes sensible. 

 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. 



3 Hankel: Electr.Untersnchungen: Abh.derKonigl. Sachs. Gesellschoft. Math.- Phi 



