“If now the metals are connected by a wire outside the liquid the 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. 2. Cadmium. 3. Tin. 4. Lead. 5. Iron. 6. Nickel. 7. Bismuth. 8. Antimony. 9. Copper. 10. Silver. 11. Gold. 12. Platinum. 13. Graphite. Each metal in the list is arranged so that it is electro-positive to any one below, and electro-negative to any one above it. There is another curious fact which should be mentioned. In associating these metals it has been found that when two are brought into contact the electro-motive force becomes greater the more distant they are in the series given above; in other words, the force between any two is equal to the sum of the forces between those intervening between those two. So when zinc is used with copper its force is not so great as when used with platinum. It was Herr G. S. Ohm who laid down the law that the strength of the electric current is equal to the electro-motive force divided by the resistance, for he proved that the “resistance was inversely proportional to the strength of a current.” There are two other laws respecting currents; viz.,— (1.) Parallel currents in the same direction attract each other. (2.) Parallel currents in opposite directions repel each other. [Illustration: Fig. 228. Chemical action of electricity. Fig. 229.]