224 SCIENTIFIC RECREATIONS. 



until the working power or " potential " of one conductor is equal to that of 

 the other. When they become equal of course the action ceases, as there is 

 equilibrium. But when an apparatus like the galvanic battery is brought to 

 bear so that the force of electricity from one conductor is made always 

 greater than that of the other conductor, we have a continuous flow while 

 the action of the battery goes on. One view of the principle is thus 

 expressed by Professor Gordon :* 



" If two metals be placed near together, but not in contact, in a liquid 

 which acts chemically more upon one than upon the other, the metals 

 become charged, so that the one least acted on is of higher potential than 

 the one most acted on. The difference of potential produced depends only 

 upon the nature of the metals and of the liquid, and not on the size or 

 position of the plates. As soon as the difference of potential has reached 

 its constant value the chemical action ceases. 



" If now the metals are connected by a wire outside the liquid the 



Fig. 227. The Voltaic arc. 



difference of potential begins to diminish, and an electric current flows 

 through the wire. As soon as the difference of potential becomes less than 

 the maximum for the metals and liquid, chemical action recommences and 

 brings it up to the maximum ; and thus if no disturbing cause interferes the 

 current will continue until the metal most acted on is entirely dissolved." 



The metal most acted on is considered the " generating plate," and is 

 " positive." The other attacked less is " negative," and is known as the 

 " collecting plate," and the zinc is the positive plate. Sir W. Thomson has 

 shown that the electrical movement in the galvanic circuit is entirely due to 

 the electrical difference produced at the surfaces of contact of the dissimilar 

 metals. The electro-motive force obtained is not the same with all metals. 

 We have mentioned that some are electro-positive and some electro-negative, 

 and it is with reference to each other that the metals are considered to be 

 endowed with these properties respectively. It all depends how the metals 

 are arranged or coupled. With reference to their behaviour in this respect 

 scientists have arranged them in a series, as follows : 



1. Zinc. 5. Iron. 9. Copper. 



2. Cadmium. 6. Nickel. 10. Silver. 

 3- Tin. 7. Bismuth. u. Gold. 



4. Lead 8. Antimony. 12. Platinum. 



13. Graphite. 



o Treatise on Electricity and Magnetism." 



