308 



Professor J. A. Fleming 



[March 6, 



cause attraction between that conductor and the shading conductor is 

 capable of being exhibited in various ways. We place on this copper 

 plate a light hollow copper ball, and support it in a little depression 

 in a copper plate. Holding the arrangement over the alternating 

 magnet, the ball begins to spin round rapidly when the magnet is 

 excited. This rotation is caused by the continual attraction of the 

 eddy currents induced in the fixed plate and in that part of the ball 

 which is not shielded from the pole by the plate. We may vary the 

 experiment, and exhibit many more or less curious and amusing 

 illustrations of it. If we float these copper balls in water (Fig. 12), 



Fig. 12. 



Fig. 13. 



Hollow copper ball floating in water Electro - magnetic gyroscope 



ovtT an alternate current electro- revolving over the pole of 



luagnnt, and caused to revolve by an alternate-current electro- 



the" interposition of a " shading " magnet. 

 plate. 



and place the glass bowl containing them over the alternating pole, 

 the interposition of a copper sheet between the pole and the balls 

 causes the latter to begin to spin in a highly energetic manner. 



§ 12. Prof. Elihu Thomson has invented a novel form of electro- 

 maf^netic gyroscope (Fig. 13). We have now suspended over the 

 alternating magnet a gyroscope of the usual form. The wheel of 

 the gyroscope is made of iron, and the tyre of the wheel is a thick 

 copj)er band. Immediately the magnet is energised, the gyroscope 

 bc'.ns to rotate with great rapidity over the pole. In this case the 

 unsymraetrical disposition of the eddy currents in the copper band 

 around the wheel is sufficient by itself to cause the rotation to occur. 

 The phenomenon which, however, lies at the bottom of all these 

 effects is that the self-induction of the secondary circuit causes the 



