concerning Volta? s Contact Force. 365 



combine and for their heats of combination, as well as for the 

 value of their contact E.M.F. 



Now, that metals in contact attract each other I fully 

 admit. They must, because they are oppositely charged. 

 There was a time when people disputed the fact of the Volta 

 effect, but I am not one of those, and never was. All the 

 facts by which Lord Kelvin or anyone else properly dis- 

 plays the Volta effect I fully admit, and am accustomed to 

 show to my class ; though indeed in Lord Kelvin's Friday 

 evening Royal Institution discourse (Phil. Mag. July 1898) 

 they are described anew with admirable clearness and pre- 

 cision. To avoid any possible mistake on this score, I will 

 briefly recapitulate the facts. 



Dry zinc and copper in dry air brought into contact 

 become oppositely charged. If their surfaces are arranged 

 so as to form a condenser of reasonable capacity, the charges 

 may be large. When they are separated adroitly, the charges 

 remain on the metals and raise them to a high (numerical) 

 potential which may readily be displayed by an electrometer 

 or even a gold-leaf electroscope. 



A trace of moisture, i. e. of liquid electrolytic moisture, not 

 dry vapour, may be fatal to the success of this experiment; 

 for if a drop of liquid intervenes and connects the two metals 

 momentarily, after the true metallic contact has been broken, 

 nearly all their charge leaks across this conducting-bridge ; 

 and the resulting charge, and therefore the resulting poten- 

 tial of the separated plates, is extremely small. A water 

 junction between two plates has no appreciable power of 

 charging them oppositely ; practically no more power than 

 air has. In many respects a water intervention acts very 

 like an air intervention ; in one respect only does it strikingly 

 differ : — its conductivity enables it to abolish any effective 

 display of the opposite charges otherwise produced, e.g. by 

 metallic contact. The same detrimental influence is exerted 

 by the conductivity induced in air subjected to the action of 

 x- or of uranium rays; such air behaves precisely like a 

 badly conducting electrolyte. Nothing further need be said 

 to make clear the action of metallic uranium for present 

 purposes. 



It is remarkable, in practice, how imperceptibly minute the 

 quantity of liquid may be, in order thus to act detrimentally. 

 But after all it is only a question of arithmetic : — 



Let z be the average thickness of dielectric intervening 

 between the zinc-copper plates, during their so-called close 

 contact, everywhere except at the place where they really 

 effectively, i.e. metallically, last touch as they are being 



