1891.] 



on Electro-magnetic Mepulsion. 



301 



Fig. 4. 



circuit. The exact nature of tlie electro-magnetic repulsion it is our 

 business now to explore. 



Taking in my hands a copper ring, I hold it over the pole of this 

 powerful vertical alternating electro-magnet, and find at once that 

 there is a perceptible and strong repulsion. Letting the ring go, it 

 jumps up into the air, impelled so to do by the electro-magnetic 

 repulsion acting upon it (Fig. 4). All good conducting rings will 

 execute this gymnastic feat, and rings of copper and aluminium are 

 found to be most nimble of all. Eings of zinc and brass are sluggish, 

 and a ring of lead will not jump at all. We 

 owe the discovery of this striking effect to 

 Prof. Elihu Thomson, and he has explored 

 in all directions the consequences and nature 

 of this interesting effect. Why is it that our 

 real copper and metal rings behave so dif- 

 ferently, when immersed in an alternating 

 field, to ideal rings of conducting matter? 

 The explanation is not very difficult to find. 

 The real ring possesses a quality, called its 

 inductance, of which we took no account in 

 our examination of the case a moment ago. 



§ 6. Before me lies a very large bobbin 

 of wire (Fig. 5), and the ends of this coil are 

 connected to an incandescent lamp. We send 

 a current of electricity through the bobbin 

 and the lamp, and have arranged matters as 

 in the diagram before yoU, so that the cur- 

 rent divides through the two circuits of lamp 

 and bobbin. Under these circumstances, the 

 divided current is just sufficient to illuminate 

 faintly the lamp. I break the circuit of the 

 battery, and you see that the lamp flashes up 

 for a moment, and we are well aware that 

 this effect is due to the electro-magnetic 

 momentum of the coil, or to its inductance, 

 in virtue of which the current continues to 

 flow on in the bobbin for a short period after 

 the impressed electromotive force is with- 

 drawn. Also, we know that there is a small 

 but definite time required before the current 



practically reaches its full strength in the bobbin when the electro* 

 motive force is again applied. These effects are the self-inductive 

 effects of a coil first noticed by Prof. Joseph Henry in 1832, and 

 subsequently fully examined in the ninth series of Faraday's ' Experi- 

 mental Researches in Electricity.' Every circuit of any kind — disc, 

 ring, or coil of wire — possesses a certain degree of this quality of 

 self-induction, and, as a natural consequence it follows therefrom 

 that if that circuit is subjected to a periodically varying electro^ 



Vol. XIII. (No. 85.) x 



Alumiuium ring project^ 

 ing from the pole of an 

 alternating electro-mag- 

 net, and floating over 

 the pole when restrained 

 by three strings. 



