[ 15 ] 
II. The Contact Theory of Voltaic Action .—Paper No. III. 
By Professors W. E. Ayrton and John Perry. 
Communicated by Dr. C. W. Siemens, F.R.S. 
Received February 19,—Read March 13, 1879. 
[Plates 4-5.] 
I. 
Up to the commencement of 1876 direct experiments on the difference of potentials of 
solids in contact with liquids had been made by Volta,* Sir Humphry Davy,! 
MM. Buff,! Becquerel,§ Peclet,|| Pfaff,^ Sir William Thomson,** IIankel,!! 
and Gerland,|^ and of liquids in contact with liquids by Kohlrausch ; §§ but 
although extremely interesting they were all incomplete, for the following reasons :— 
1. The earlier experiments were not carried out with apparatus susceptible of giving 
accurate results. 
2. Owing to the incompleteness of the apparatus, assumptions had to be made not 
justified by the experiments. 
3. No direct experiments had been performed to determine the difference of poten¬ 
tial of two liquids in contact, with the exception of a few by Kohlrauscii, using a 
method which appeared to us quite inadmissible as regards accuracy of result 
With regard to the objection No. 1, it is needless to do more than to remark that 
some of the earlier experimenters thought it permissible to touch a liquid under test 
with the fingers, or assumed without experiment that a piece of blotting-paper, pre¬ 
viously soaked in a liquid, acted inductively on a distant metallic surface as a layer of 
the liquid would. 
* Voeta, Gehler’s Worterbnch, Bd. iv., s 616. 
t Davy, Ann. de Chim., t. Ixiii., p. 230, 1807. 
J Boff, Ann. d. Chim. n. Pharm., Bd. xlii., s. 5, 1842; u. Bd. xlv., s. 137, 1844. 
§ Becquerel, Ann. de Chim. et Phys., t. xxv., p. 405, 1824; Pogg. Ann., Bd. ii., s. 169. 
|| Peclet, Ann. de Chim. et Phys. [3], t. ii., p. 233; Arch, de l’El., t. i., p. 621, 1841. 
Pfaff, Pogg. Ann., Bd. Ii., s. 110 nnd 197, 1840. 
** Thomson, Proc. Lit. and Phil. Soc. of Manchester, Jan. 21, 1862: Papers on Elec, and Mag., § 400, 
p. 317. 
ft Hankel, Elektr. Untersuchungen, vi.; Abhandl. der Math.-Phys., Closse der Konigl. Sachs. Gesel!- 
schaft, Bd. vii., s. 585, 1865. 
+4 Gerland, Pogg. Ann., Bd. cxxxiii., s. 513, 1868. 
§§ Kohlraesch, Pogg. Ann., Bd. lxxix., s. 200, 1850. 
