

On Contact Electricity of Metals. 13 f 



certain amount to that of the plate on which it lies. Thus two differ- 

 ent metals coated with layers of the same liquid do not, as a rule in 

 air, give zero potential-difference, but usually give nearly the same 

 potential-difference as the dry metals. For instance, the potential of 

 lead with an alcohol layer O'l cm. deep on it is about 0'13 volt higher 

 than that of dry lead, while that of wet copper is about the same 

 amount higher than the average value for dry copper. There is no 

 tendency shown for a liquid film to take up a definite potential inde- 

 pendent of that of the metal on which it lies, with any metal sepa- 

 rated from it by a dielectric, as a solid conducting film would do. 

 This seems to be the most important distinction between solid and 

 liquid conductors, and it is in accord with what is known of voltaic 

 cells; for if the potential-differences in the chain copper-water-zinc 

 were equal and opposite to that of zinc-copper, as copper-iron-zinc is- 

 to zinc-copper, we should have no electromotive force in the circuit, 

 when the materials are all at one temperature. In most of my 

 experiments the type is copper- water-nonconductor (air) -water-zinc,, 

 and not copper- water-zinc ; the members of the first and last pairs' 

 are in contact, while a non-conductor intervenes between the two free 

 water surfaces, these being able to take up their natural contact- 

 potential-differences with the metals they touch ; but the sum of the 

 potentials, copper- water and water-zinc, is not equal to copper-zinc, 

 thus proving that the two free surfaces of water are not at the same 

 potential. This is directly demonstrated by the experiments of 

 Professors Ayrton and Perry, and by the results given in Chaps. VI, 

 VII, and X of this communication. 



If the intervening layers of non-conductor be removed by joining 

 the liquid surfaces so that there is but one mass of liquid between 

 the plates, conduction at once tends to reduce the whole liquid to the 

 same potential, leaving the contact-potential differences, now un- 

 balanced by the removal of the non-conducting medium which was 

 capable of sustaining the stress, to act as external electromotive 

 force. This then shows the connection between contact-potentials, 

 measured electrostatically by the method described in this paper, and 

 the electromotive force of a voltaic cell. 



53. The con tact- potential of a liquid with a metal is clearly, if 

 the air-potentials be neglected, the difference between the potential 

 of the dry metal and that of the metal when wet with the liquid ; the 

 same standard plate being used as zero of potential in both cases. 

 The results given in the tables must no doubt be in some cases com- 

 plicated by the formation of solid compounds in the interface between 

 liquid and metal, so that the liquid is no longer in contact with clean, 

 metal, and the liquid must also displace any film of condensed air 

 which may exist on the metal. The. latter influence will be discussed 

 in Chaps. VIII and X. 



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