304 Professor J, A. Fleming [March 6, 



namely, the positive or repulsive impulses, have been enlarged at the 

 expense of the negative or attractive impulses. Theory, therefore, 

 points out that as a consequence of the self-induction of the ring the 

 balance between the attractive impulses and repulsive impulses is up- 

 set, and that the latter predominate. Our real ring behaves, therefore, 

 very differently to our ideal ring. The real ring is strongly rei^elled, 

 because the resultant action of all the impulses is to produce on the 

 whole an electro-magnetic repulsion. This repulsion is evidence of the 

 self-induction of the circuit exposed to the magnetic field, and it forms 

 a new way of detecting it. But although this is part of the truth, it is 

 not the whole truth. The lag of the induced current in the ring, and 

 hence the predominance of the repulsive imjjulses, depends on the 

 conductivity of the material of which the ring or circuit is made ; and 

 the better this conductivity the greater is that repulsion, because both 

 the induced current and the " lag " are thereby increased. Hence it 

 comes to pass that there are two factors involved in making this 

 repulsive effect, the conductance of the ring or disc and its inductance. 

 For equal conductivities, the greater the self-induction, the greater 

 the repulsion. For equal self-inductions, the greater the conductivity 

 of the circuit so much the more repulsive effect will be produced. 

 Time does not permit me to enlarge on the strict analysis of the effect. 

 Its broad outlines are indicated, perhaps, sufficiently, by what has 

 been said. 



§ 8. We can show the effect of the relative conductivity of discs 

 of equal size, and therefore of equal self-induction, by iveighing 

 similar discs of various metals over an alternating pole. Here, for 

 instance, are discs of cojiper, zinc, and brass of equal form and size. 

 Placing these discs on the scale pan of a balance, and suspending them 

 over the alternating pole, I am able to prove that the repulsion on 

 the copper disc is greater than that on the zinc, and that on the zinc 

 greater than that on the brass.* 



The same result can be illustrated by placing over the pole of our 

 alternating magnet a paper tube. Taking one of the copper rings in 

 my hand, and first exciting the magnet, I let the ring drop down the 

 tube. It falls as if on an invisible cushion that buoys it up, and it 

 remains floating in the air. If rings of different metals and equal 

 size are placed on the tube, they float at different levels like various 

 specific- gravity beads in a liquid. The greater the conductivity of 

 the ring, the greater is the repulsion on it, in any given part of the 

 alternating field, and hence the highly conducting rings will be 

 sustained on a weaker field than the feebler conducting rings, 

 assuming the rings to have about equal weights. Moreover, we are 

 able to show by another experiment the fact that these rings are 

 traversed, when so held, by powerful electric currents. If we press 



* Experiments of this kind liave been made by M. Borgman. See ' Comptes 

 Rendus,' No. IG, April 21, ISUO, p. 819 ; and also February 3, 1890, vol. ex. 

 p. 233. 



