Dec. I, 1887] 



NATURE 



107 



Nor is the revolving action confined to metallic con- 

 ductors and to true conduction. Liquids and gases, 

 although they convey electricity by something of the 



.^^ 



Fig. 21.— Gore s circular railway. The light spherical metal ball revolves 

 round the two concentric metal hoops or rails whenever it is made to 

 convey a current between them in a vertical magnetic field. 



nature of convection, are susceptible to rotation in a 

 precisely similar manner. 



Fig. 22. — Rotation of a liquid disk conveying a radial current in a vertical 

 magnetic field. 



To show the rotation of liquid conductors under the 

 influence of a magnet, take a circular shallow trough of 



Fig. 23. — A falling stream of liquid conveying a current between two mag- 

 netic poles, and being thereby twisted into a spiral. (Copied from a 

 paper in Phil. Mag. by Dr. Silvanus Thompson). 



liquid, supply it with electrodes at centre and circumfer- 

 ence, and put the pole of a magnet below it. The liquid 

 at once begins to rotate, and by using a magnet and 



cunent of fair strength, can easily be made to whirl so 

 fast as to fly over the edge of the trough (Fig. 22). The 

 experiment is plainly the same as Fig. 19, except that a 

 liquid disk is used in place of a solid one. Or, again, it 

 may be considered the same as Fig. 20. Reverse the 

 magnet, and the rotation is rapidly reversed. 



Another method is to send a current along a jet of 

 mercury near a magnet and note the behaviour of the jet. 

 It twists itself into a flat spiral as shown in Fig. 23. 



Fig. 24.- 



-Induction coil discharge from a to ^ through rarefied gas, rotating 

 round a glass-protected magnetized iron rod. 



The rotation of a gas discharge is most commonly 

 illustrated by an arrangement like Fig. 24, where the 

 terminals of the induction coil are connected to the 

 rarefied gas respectively above one pole and round the 

 middle of a magnetized bar. If the discharge can be got 

 to concentrate itself principally down one side, the line of 

 light so formed is seen to revolve. 



Action between a Magnet and an Electric Charge in 

 Relative Motion. 



From all this it is not to be doubted that a charged 

 pith ball moving in the neighbourhood of a magnet is 

 subject to the same action. There is no known action 

 between a magnet and a stationary charged body, but 

 directly either begins to move there is an action between 

 them tending to cause one to rotate round the other. It 

 is true that for ordinary speeds of motion this force is 

 extremely small ; but still it is not to be doubted that if a 

 shower of charged pith balls or Lycopodium granules are 

 dropped on to a magnet pole, they will fall, not perfectly 

 straight, but slightly corkscrew fashion. And again, if a 

 set of charged particles were projected horizontally and 

 radially from the top of a magnet, their paths would 

 revolve like the beams of a lighthouse. And if by any 

 means their paths were kept straight, or deflected the 

 other way, they wolild exert on the magnet an infinitesimal 

 "couple," tending to make it spin on its own axis. 



Conversely, if a magnet were spun on its axis rapidly 

 by mechanical means, there is very little doubt but that 

 it would act on charged bodies in its neighbourhood, 

 tending to make them move radially either to or from it. 

 This, however, is an experiment that ought to be tried ; 

 and the easiest way of trying it would be to suspend a 

 sort of electrometer needle, electrified positive at one end 

 and negative at the other, near the spinning magnet, and 

 to look for a trace of deflection— to be reversed when the 

 spin is reversed. A magnet of varying strength might be 

 easier to try than a spinning one. 



