470 



REPORT 1884. 



and a Bohnenberger or Hankel electroscope ; lie makes measurements 

 of Volta force for nnmerons metals, but his special merit is the deter- 

 mination of metal-liquid contact forces without introducing blotting 

 paper, glass, or fingers, as the earlier experimenters had done. 



Fig. 3.— Hankel's Arrangement for observing the Yolta Effect between a Metal and 



a Liquid. 



The liquid i. ia in a funnel tube A b, c is a copper plate, and m the metal under observation. 

 First r touches < for an instant, then c is raised and made to touch p', which leads to a 

 Ilankel electrometer. 



This gives cu/w + m i<=ka. 



Then the liquid is run out of the funnel, a plate of the metal M is placed on its mouth, 

 and the experiment repeated. 



This gives cu,'.M = k;8 



To eliminate k substitute a plate of zinc for M, and get 



CU zn=k-y. Then finally M L — "_ cu zn. 



7 

 In all these expressions air contact forces are as usual neglected, lint 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 tal e zinc instead of M, and can also make the condenser plate of zinc instead of 

 co] per, and .so on ; but we git no forwarder, fresh unknowns appear as fast as addi- 

 tional equations, and some of the equations are liable to degenerate into identities. 



So far back as 1824 Becquerel 1 attempted the investigation of metal- 

 liquid contacts, and Buff 2 made some measurements 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 E.M.F. He has 

 only published a preliminary paper, 3 in which he overlooks minutia 1 such 

 as change of contact force by time and adventitious circumstances. 



JUasse — 1861 and 1865. See also Pogg. Ann. cxv. p. 57, and exxvi. pp. 286 and 440 : 

 exxxi. p. 607. 



Gerland : 'On the E.M.F. between Water and some Metals,' Pogg. Ann. exxxiii. 

 1868, p. 513; exxxvii. 1869, p. 552. In his second paper, to gel over the air effect, 

 Gerland joins two metals through a galvanometer, and then dipping them into a 

 liquid observes the first swing of the needle. He also compensates the E.M.F. b}- 

 Poggendorff's method. He thus determines the value of — 



MM' -^ M'/Liquid + Liquid M. 



1 Becquerel : Ann. de C/iimie, 1824. He put the liquid in a copper capsule on the 

 plate of an electroscope, and connected it with the condenser plate by his lingers. 



2 Buff: Liebig, Ann. Chun. v. Pharm. 1842. He made the lower plate of his con- 

 denser 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 s 

 metal. 



3 Clifton : Proc. Royal Soc. xxvi. p. 299, 1877. 



