and a Tlieory of Contact-Potential. 329 



oxidation or reduction takes place ; the mercury therefore 

 (according to the theory) receives neither positive nor negative 

 charge, and is therefore likewise at the potential of the air. 

 The only remaining possible cause of deflection being the con- 

 tact of iron and mercury (to both electrodes are attached iron 

 wires) — since there is no deflection observed, there is no dif- 

 ference of potential between the iron and the mercury (except 

 perhaps that answering to the slight Peltier effect), 



II. The influence of the presence of one metal on the poten- 

 tial of the other, and the difference of potential thereby set up. 

 In the case of the liquid cell, this influence has accidentally 

 been proved by experiments previously performed. Thus, 

 Perry and Ayrton find that the air near copper touching 

 water in which is also dipped an iron bar, is at a higher 

 potential than the air about the iron ; for the air oxidizes both 

 metals, though unequally (for the copper is out of the reach 

 of the influence of the iron through air) ; whereas the water 

 oxidizes the iron, rendering it negative to itself, and therefore 

 renders the copper positive by the deposit of hydrogen which 

 clings to it. The air being positive to both metals, is thus 

 necessarily at a higher potential about the copper than about 

 the iron. Thus, though copper would get oxidized in water 

 alone (as it does in the air), the presence of iron prevents its 

 oxidation in the water, whence the difference of potential 

 amounting to '16 volt. 



Now this same influence ought to be felt in air, if the dis- 

 tance is sufficiently small to overcome to a sensible extent elec- 

 trolytic diffusion ; that is, iron, brought quite close to a less 

 electro-negative metal, ought to reduce the oxidation going 

 on at the surface of that metal. For this purpose the same 

 apparatus was used ; and when the deflection was steady, a 

 clean iron bar was put to earth and brought up to the nozzle 

 and laid as close as possible to the falling drops. For each 

 approach of the iron, the deflection, which, as shown, depends 

 on the oxidation of the drops, was reduced by thirty divisions, 

 equivalent to about '15 D. Thus, through a film of air of less 

 than 1 miilim. thickness, this influence of one metal on the 

 other can be made evident. 



I repeated this in another way ; for I put the bar to the 

 electrode B (fig. 1) and connected the electrode A to earth : 

 the change in the deflection showed that the iron is rendered 

 more negative (by about *15 D) by being brought quite close 

 up to the mercury. The oxidation of the electro-negative 

 element is increased by the presence of another less negative 

 metal ; which proposition is the counterpart of the preceding 

 one. 



