580 HISTORY OF SCIENCE. 



connection of the coil A with the battery, a disturbance of the needle 

 again took place. Here the iron ring became an electro-magnet while 

 the current flowed, and in the act of becoming and of ceasing to be a 

 magnet, the ring so influenced the coil B that momentary currents were 

 induced in that circuit. Faraday's next step was to place the experiment 

 in such a form that no voltaic battery was used, the conversion of mag- 

 netism into electricity being distinct and direct. The experiment is 

 represented in Fig. 292. A coil of insulated copper wire was wound 

 round a cylindrical piece of soft iron, N s, and the ends of the coil were 

 connected with a galvanometer, G, placed at some distance. The iron 

 was then put between the contrary poles of a pair of bar magnets as 



FIG. 293. A MAGNET PRODUCING A CURRENT. 



shown in the figure. Every time the contact between the iron and the 

 magnets was made or broken the effect on the galvanometer indicated 

 that a momentary current passed through the wire. The current which 

 appeared on breaking the contact was in the reverse direction to that 

 which appeared on making the contact. In both cases the effect was 

 momentary, and the continuance of the magnetism in the iron cylinder 

 was not attended with any electrical effects. Faraday reasoned from this 

 that a current would be produced in a coil if a magnet were introduced 

 into the centre of the coil. Accordingly he took a coil made of about 

 200 feet of copper wire wound round a hollow cylinder, A B, Fig. 293, 

 and having connected its extremities with a galvanometer, D (placed at 

 a greater relative distance than shown in the figure), he put one end. 

 of a cylindrical bar magnet within the coil. While the bar magnet was 

 being quickly thrust in the whole length the galvanometer needle moved ; 

 it then returned to its former position, the bar magnet being at rest 

 within the coil. But while the magnet was being drawn out, the needle 

 moved again, but in the opposite direction. These effects were re- 

 peated as often as the magnet was put in or taken out of the coil, and 

 thus a wave of electricity was produced by the mere approximation, 

 and another wave by the recession of the magnet, but no effect from 

 the continuance of it in one position. Faraday soon succeeded in 



