Electromotive Forces in the Voltaic Cell. 265 
x. Two metals in contact are at seriously different potentials 
(i. e. differences of potential greater than such millivolts as 
are concerned in thermoelectricity). [This is held by nearly 
everybody™. | 
xi. The contact-force between a metal and a dielectric, or 
between a metal and an electrolyte such as water and dilute 
acid, is small}. [Ayrton and Perry, Clifton, Pellat, and 
probably Sir William Thomson. | 
Chapter III. Theoretical. Page 
do, Statement of the writer’s OWN VIEWS .....cescce es eeneeenes 265 
14, Example of the calculation of a Volta-effect ........ ... ese 267 
15. Attempt to mentally represent the action of atmosphere on metals 
at the instant of contact. Mode in which two Volta plates may 
Peed AGA CONGONSED, oo. ew kc cine eset ence ee t 268 
16. Qualifications and doubts respecting the precise calculation of 
Volta-effects even for absolutely clean pure metals .......... 270 
17. Calculation of a series of metal-air contact-forces ; 2. e, of an abso- 
eem Oki SOMICS IN, AIT OF Wate? «6. .js0 ee eure es vale we eee ewes 272 
18. Calculation of Volta series in other media, such as chlorine and 
SLL br BiegW a sted Sea einaee tr Sama neta we taenrer 275 
PREM MM EMMBING ECM is os oe sls ak esse Ree bes Abie ta cae kk eben es 279 
13. Before proceeding to the statements embodying my 
own views, it will be more interesting if I try to explain in a 
fuller and more connected manner what they are tf. 
Let us regard the air as a dielectric bath of oxygen, in 
which metals are immersed, and picture a piece of zinc sur- 
rounded by oxygen molecules which are straining at it, and 
but the potential of the medium close to it, statements Nos. ix. and x. are 
undoubtedly true; and No. xi. is also true, I suppose, for it then only 
means that there is not much E.M.F. between the medium close to a 
metal and that at a little distance. 
* It is much more natural to suppose that the potential of a metallic 
conductor is uniform, whether it is homogeneous or not. Indeed, it is 
not only more natural, but it is true, that two parts of a conductor can 
only differ in potential by reason of an E.M.F. located at the junction. 
Now there usually isan E.M.F. ata junction, but itis only of such a 
magnitude as is concerned in thermoelectricity. This, indeed, does produce 
a difference of potential between the metals, but nothing else can. N.B. 
Always provided that by “the potential of a metal” is meant that potential, 
and not the potential of air near it. ; 
+ The experiments supposed to establish this, really prove only that 
there is very little difference between the air and the water in which a 
metal is partially immersed. I do not quite know how to understand, on 
Sir William’s plan, the potential of a metal which is half in one medium 
and half in another. 
{ The reason I set them forth at length is because I had no time at the 
meeting both to open the discussion and also to properly express my own 
ideas, and Sir William Thomson was kind enough to tell me to write out 
the paper completely, and to explain the position I took up fully. This, 
therefore, I have endeavoured to do. , 
