1891.] on Electro-magnetic Bepulsion. 305 



down tlie copper ring upon the zinc or brass ring floating beneath it, 

 the rings are attracted together and the copper ring holds up the zinc. 

 This is obviously because the rings are all traversed by induced 

 currents circulating in the same direction. 



§ 9. It is, of course, an immediately obvious corollary, from all 

 that has just been said, that any cutting of a ring or disc which 

 hinders the flow of the induced currents causes the whole of the repul- 

 sion effects to vanish. We illustrate this by causing a ring of copper 

 wire to jump off the pole, and then cutting it with pliers, find it has 

 ceased to be capable of. giving signs of life. When the metallic 

 masses or circuits which are presented to the alternating magnetic 

 pole are of very low resistance, the electro-magnetic repulsion may 

 become very powerful, many pounds of thrust or push being produced 



Fig. 8. 



Copper ring " floating " in air over the pole of an alternating-current 

 electro-magnet, when restrained by strings. 



by apparatus of quite moderate size. It is, in fact, quite startling to 

 hold over the pole of such an alternating magnet as I have before me 

 a very thick plate of high conductivity copper. It would greatly 

 surprise any one not aquainted with these principles to be told that a 

 massive copper ring weighing eight or ten pounds could be made to 

 float in the air, but you have ocular demonstration before you that 

 this feat can be performed. The ring needs to be tethered by light 

 strings, to prevent it from being thrown off laterally, although these 

 strings in no way support its weight (Fig. 8). 



One of the most beautiful of Prof. Elihu Thomson's experiments 

 exhibits this effect of electro-magnetic repulsion on a closed coil, 

 which is buoyed up in water by a small incandescent lamp in circuit 

 therewith. In the glass vase before you floats a little glow-lamp like 

 a balloon (Fig. 9). The car consists of a coil of insulated wire, and 

 the ends of this coil are connected with the lamp. The whole arrange- 

 ment is accurately adjusted to just, or only just, float in water. 



