ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 503 



the electricity in the body and giving it an absolute charge. Bat directly 

 the copper touched the zinc the oxygen atoms were cleared away at the 

 point of contact, and the stress of those at the rest of its surface was no 

 longer counterbalanced. Moreover, they can now all move nearer to the zinc 

 because a way of escape for electricity is provided into the copper, whose 

 surrounding oxygen atoms will be thus driven back somewhat further from 

 the surface, until the dielectric strain, assisting the chemical strain on the 

 copper surface and opposing it on the zinc surface, prevents further dis- 

 placement, and equilibrium is again attained. The electricity which 

 escaped from the zinc to the copper was negative electricity (oxygen 

 being essentially an electro-negative element), the negatively charged 

 oxygen atoms have moved a little nearer to the zinc than their normal 

 distance, i.e. the thickness of its layer of negative electricity is reduced, 

 or its surface is positively charged ; the negative layer on the copper has 

 been slightly thickened — its surface is negatively charged. 



This is a pictorial way of representing the process, and may be 

 regarded as somewhat fanciful ; it is, however, the way in which the 

 theory originally occurred to me, and it permits more insight into the 

 processes than a mere statement in terms of potential can ; though it may 

 well be that the imagined processes are but distant likenesses of the real 

 ones. 



The oxygen atoms have moved nearer to the zinc, it is now more 

 easily oxidised than before ; the copper, on the other hand, is by contact 

 with zinc somewhat protected. 



Observe that the contact has not developed any force ; it has only, by 

 sweeping away the oxygen from the point of contact, enabled previously 

 existing forces to do work and produce their effect. 



The air surrounding the metals in contact is in a state of slight dielec- 

 tric strain, such as would be produced by two pieces of any one metal of 

 similar size and position, charged so as to differ in potential from each 

 other by a volt. 



Zinc and copper plates in contact may therefore be regarded as the 

 plates of a condenser, but they form a peculiar condenser, for they are not 

 really at different potentials ; the whole step of potential which throws 

 the air into its state of dielectric strain is located on their bounding 

 surfaces. 



Let s and g be the electrostatic capacity of the zinc and the copper 



respectively — think of them as two independent spheres united by a fine 



wire — and let x be their common potential ; then the zinc has, by the fact 



of contact, gone up 1*8 — .v, and the copper has gone -down x — '8; and, 



since the quantity of electricity which left the one went to the other, it 



follows that — 



a (1'8-a}) = s' 0--8), 



l-8s + -8.s' 



or x = 7—j — ' 



s + s 



There is no necessary relation between s and s' in general, but in the 

 ordinary form of the Volta experiment the two plates are of equal size 

 and shape ; in which case s = s', and x = 13 volts below the potential of 

 the unconstrained air. 



All this is wholly nnlike a condenser investigation. To treat it as 

 a condenser we must consider the air surfaces close to the two metals as the 

 plates of the condenser, and we can then speak of its electrostatic capacity 



