1886.] 



A Theory of Voltaic Action. 



313 



37. The zinc sector of the electrometer and the zinc plate of the 

 condenser in connexion with it form an insulated conductor of zinc ; 

 the copper sector and the copper plate of the condenser form an 

 insulated conductor of copper. Consider first the hypothesis which 

 asserts that after recent metallic connexion, the potential of the zinc 

 exceeds that of the copper by a definite amount. The plates being 

 primarily at a distance, make them approach each other gradually. 

 As they do so the experiment shows that the difference of potential 

 between them gradually diminishes. Their behaviour is just that of 

 an ordinary charged condenser ; as the plates come closer together 

 their capacity is increased, being always inversely proportional to 

 their distance, and therefore for a given charge the difference of 

 potentials is diminished, being always directly proportional to their 

 distance. By bringing the plates very close together their difference 

 of potential may be reduced indefinitely. Having brought them 

 within a very minute distance of each other, establish metallic con- 

 nexion between them. According to the hypothesis this restores 

 them to their original difference of potential. This is done by the 

 separation of a quantity of electricity equal to the difference of 

 potential multiplied by the capacity of the condenser; the two plates 

 acquire charges of this amount but of opposite signs. This operation 

 involves a gain of energy to the system of amount equal to half the 

 capacity of the condenser multiplied by the square of the difference 

 of potentials. The plates being very close together these quantities 

 are of considerable magnitude. Now the operation of making 

 metallic contact was merely directive and involved no supply of 

 energy from without. This supply must therefore come from an 

 absorption of the heat of the system, or from chemical combination 

 either partial or complete. The first alternative is inadmissible, as it 

 would lead to a perpetual motion; we are, therefore, driven to that of 

 chemical combination, which may either be an action between the 

 zinc and copper at the point where the metallic connexion is made, or 

 an action between one or both of the metals and the surrounding 

 gaseous medium. In either case its amount, though very minute, is 

 not infinitesimal. As there is no evidence of any action between 

 clean zinc and clean copper at ordinary temperatures which involves 

 any liberation of energy worth considering, it seems nearly certain 

 that the actual liberation of energy is due to the other cause, which 

 in the atmosphere is oxidation whether nascent or complete. 



The plates with their acquired charges may now be separated by 

 mechanical energy, and their charges imparted to an electric receiver 

 by contact. The plates being now as at first the same cycle may be 

 repeated indefinitely, and a permanent supply of electricity may be 

 thus theoretically obtained at the expense of continued oxidation of 

 the plates in air. 



t 2 



