and made of 7/8-inch round iron. He wound two sets of coils round it, one occupying nearly half the ring, and the other filling most of the other half. One of these he connected with a galvanometer, the other could be connected at will with a battery. On sending the battery current through the latter coil, the galvanometer needle swung completely round four or five times, and a similar action took place, but in the opposite direction, on stopping the current. Here it was clearly the magnetism induced in the iron ring which produced so powerful a current in the galvanometer circuit. Next he wound a quantity of covered copper wire on a small iron bar, and connecting the ends to a galvanometer, he placed the little bobbin between the opposite poles of a pair of bar magnets, whose other ends were in contact. As soon as the iron core touched the magnets, a current appeared in the galvanometer. On breaking contact, the current was in the opposite direction. Then came the experiment above mentioned, in which no iron was employed. After this, one end of a cylindrical bar magnet was introduced into a helix of copper wire, and then suddenly thrust completely in. The galvanometer connected with the coil showed a transient current. On withdrawing the magnet, the current appeared in the opposite direction; so that currents were induced merely by the relative motion of a magnet and a conductor. A copper disc was mounted so that it could be made to rotate rapidly. A wire was placed in connection with the centre of the disc, and the circuit completed by a rubbing contact on the circumference. A galvanometer was inserted in the circuit, and the large horseshoe magnet of the Royal Institution so placed that the portion of the disc between the centre and the rubbing contact passed between the poles of the magnet. A current flowed through the galvanometer as long as the disc was kept spinning. Then he found that the mere passage of a copper wire between the poles of the magnet was sufficient to induce a current in it, and concluded that the production of the current was connected with the cutting of the "magnetic curves," or "lines of magnetic force" which would be depicted by iron filings. Thus in the course of ten days' experimental work, in the autumn of 1831, Faraday so completely investigated the phenomena of electro-magnetic induction as to leave little, except practical applications, to his successors. A few weeks later he obtained induction currents by means of the earth's magnetism only, first with a coil of wire wound upon an iron bar in which a strong current was produced when it was being quickly placed in the direction of the magnetic dip or being removed from that position, and afterwards with a coil of wire without an iron core. On February 8, 1832, he succeeded in obtaining a spark from the induced current. Unless the electro-motive force is very great, it is not possible to obtain a spark between two metallic surfaces which are