FIRST PERIOD OF ELECTRICAL DISCOVERIES But Faraday's chemical work was soon overshadowed by his electrical discoveries. The first experiment which he has recorded was the construction of a voltaic pile with seven halfpence, seven disks of sheet zinc, and six pieces of paper moistened with salt wa ter. With this pile he decomposed magnesium sulphate (first letter to Abbott, July I2, His first notable discovery was the pro duction of the continuous rotation of magnets and of wires con ducting the electric current round each other. The consequences deducible from the great discovery of H. C. Oersted (July 21, 182o) were still imperfectly understood. In the summer of 1821 Faraday wrote for the Annals of Philosophy an historical sketch of electro-magnetism, and he repeated the experiments he de scribed. This led him in the beginning of September to discover the method of producing the continuous rotation of the wire round the magnet, and of the magnet round the wire. He did not suc ceed in making the wire or the magnet revolve on its own axis.
Although the fact of the tangential force between an electric current and a magnetic pole was clearly stated by Oersted, and clearly apprehended by Ampere, Wollaston and others, the reali zation of the continuous rotation of the wire and the magnet round each other was a scientific puzzle requiring no mean ingenuity for its original solution. For on the one hand the electric current always forms a closed circuit, and on the other the two poles of the magnet have equal but opposite properties, and are inseparably connected, so that whatever tendency there is for one pole to cir culate round the current in one direction is opposed by the equal tendency of the other pole to go round the other way, and thus the one pole can neither drag the other round and round the wire nor leave it behind. Faraday solved the problem by causing the current, in some part of its course, to run in two parallel chan nels, one on each side of the magnet, in such a way that during the revolution of the magnet the current is transferred from the channel in front of the magnet to the channel behind it, so that the middle of the magnet can pass across the current without stopping it, just as Cyrus caused his army to pass dryshod over the Gyndes by diverting the river into a channel cut for it in his rear.
Faraday's crowning discovery was the induction of electric currents. In Dec. 1824 he had attempted to obtain an electric current by means of a magnet, and on three occasions he had attempted to produce a current in one wire by means of a current in another wire or by a magnet. On Aug. 29, 1831, he obtained the first evidence that an electric current can induce another in a different circuit. In nine more days of experimenting he had arrived at the results described in his first series of "Experimental Researches" read to the Royal Society on Nov. 24, 1841.
During his first period of discovery, besides the induction of electric currents, Faraday established the identity of electrifi cation produced in different ways ; the law of the definite elec trolytic action of a current ; and the fact, upon which he laid great stress, that every unit of positive electrification is related in a definite manner to a unit of negative electrification, so that it is impossible to produce what Faraday called "an absolute charge of electricity" of one kind not related to an equal charge of the opposite kind. He also discovered the difference of the capacities of different substances for taking part in electric induc tion. Henry Cavendish had before 1773 discovered that glass, wax, rosin and shellac have higher specific inductive capacities than air, and had actually determined the numerical ratios of these capacities, but this was unknown both to Faraday and to all other electricians of his time, since Cavendish's Electrical Researches remained unpublished till 1879.