306 



Professor J. A. Fleming 



[March 6, 



Fio. 9. 



Placing the vase over an alternating magnetic pole, you see that the 

 magnetic induction creates a current in the coil which lights the 

 lamj3, and, moreover, that tlie electro-magnetic repulsion on the coil 

 causes the lamp and coil to rise upward in the water. 



§ 10. We must now pass on to study shortly another class of 

 actions, namely, deflections and rotations produced by electro-magnetic 



repulsion on highly conducting discs or rings. 

 If the conducting ring or disc which is pre- 

 sented to the alternating pole is constrained 

 by being fixed to an axis around which it can 

 rotate, the action may reduce to a deflective 

 force. Here, for instance, is a flat disc fixed 

 on a transverse axis. On presenting this disc 

 to the pole, the disc is prevented by its con- 

 straint from being repelled bodily ; so it does 

 the next best thing it can, it sets its plane 

 paiallel to the lines of magnetic force, and 

 gets into such a position that the induced cur- 

 rents in it are reduced to a minimum. On this 

 principle, before becoming acquainted with 

 Prof. Elihu Thomson's original work, I de- 

 vised a little copper disc galvanometer for 

 detecting small alternating currents. 



§ 11. More interesting than the deflective 

 actions are those which result in the produc- 

 tion of continuous rotation in highly conducting 

 bodies placed in an alternating field. This 

 electro-magnet in front of me, and which has 

 come from Prof. Elihu Thomson for the pur- 

 poses of this lecture, consists, as you see, of a 

 nearly closed circuit divided iron core wound 

 over with a coil (Fig. 10). The ends of the 

 iron circuit are provided with copper bars, 

 which embrace and cover portions of the polar 

 terminations of the magnet. When the magnet 

 is excited by a periodic current, these secondary 

 circuits become the seat of powerful induced secondary currents. 

 Taking in hand a large copper disc pivoted at the centre and held in 

 a fork, we hold this wheel so that part of the disc is inserted between 

 the jaws of the electro-magnet. Immediately, rapid rotation is pro- 

 duced. The reason is not far to seek. The alternating field induces, 

 both in the closed coils and in the neighbouring portions of the disc, 

 induced currents; these tend to cause the parts of the conductors 

 in which they flow to be pulled into parallelism, and if the polar 

 coils are so placed as to partly shield the poles these attractive 

 actions act unsymmetrically on the disc and pull it continuously 

 round. The action is, perhaps, better illustrated by a simpler 

 experiment. If we hold a pivoted copper disc (Fig. 11) symme- 



Incandescent lamp and 

 secondary coil floating 

 in water and repelled 

 by an alternating-cur- 

 rent electro-magnet, 

 placed beneath. 



