328 Dr. S. L. Hart on some Capillo-electric Effects, 



This difference of potential, in air ; has usually been regarded 

 as that existing between the two metals ; but in experiments 

 similar in principle to Sir William Thomson's half-disk appa- 

 ratus, it is the potential of the air or gas that has been deter- 

 mined, and not that of the metal. 



In water, this difference of potential has been shown by- 

 Brown (Phil. Mag. Feb. 1879). 



A third, intermediate state of affairs exists, namely that in 

 which the metals are connected through a considerable external 



Fig. 4. 



resistance. Then the fall of potential is distributed according 

 to the external and internal resistances. 



Two suppositions have been made ; and these require to be 

 confirmed by experiment: — 



(1) Metals in contact are at the same potential, though dif- 

 ferences of potential exist in the medium (electrolytic) around 

 them. 



(2) Metals in the same electrolyte and in sufficient proxi- 

 mity, when insulated, are at different potentials, though the 

 medium may be at one intermediate potential between them. 



We have thus to prove : — 



I. The equality of potential of metals touching (say in air). 



I used the apparatus described in the first part of the paper, 

 and, as mentioned, obtained a deflection depending on the rate 

 of fall, the amount of oxidation, and consequently the medium 

 around the drops. The cause of this has been explained ; but 

 by substituting coal-gas for the air about the drops (and also 

 the exposed surfaces, as that of the mercury in the funnel) the 

 oxidation is more and more reduced, till the positive charge 

 passes into a negative one. (It is necessary, to obtain this 

 effect, that the drops run slowly.) That is to say, when 

 the medium is neutral and no chemical action is set up, no 

 deflection at all is observed. What is the condition of affairs 

 at this point ? 



The electrode B (fig. 1) is in connexion with an iron cylin- 

 der, which, not being oxidized in this medium, is, according 

 to the theory, at the potential of the neutral gas around it. 

 The change in the medium at this juncture is such that no 



