96 



PHYSICS OF MATTER 



on a gas-pipe. If a pencil-mark be made upon the plate near the anode 

 it will be acted upon inductively, and a ball discharge will pass from 

 it to the anode pin-point. The positive discharge will go in the oppo- 

 site direction from the pencil-mark, but it leaves no trace. It appears 

 that this ball discharge upon the surface, which results in a destruc- 

 tion of the insulation of the surface, is a characteristic of the negative 

 current. 



What would be the result if a suspended Maxwell coil were to be 

 looped into either of these unipolar circuits? Would this case neces- 

 sarily give the same result that Maxwell obtained? 1 Of course we 

 know that the result which Maxwell sought to detect is very small. 

 We are more particularly concerned with the nature of the action 

 than with the magnitude of the result. If the a particles are so large 

 that they can contribute little or nothing to the current through 

 a metallic conductor, then the positive current may practically be 

 left out of consideration. But it seems doubtful whether the a par- 

 ticles are ultimate in their character, and here is where experimental 

 work is yet needed. It would be exceedingly interesting to study 

 these ball discharges upon a photographic plate under diminishing 

 pressures, as they gradually become a cathode discharge, in a Crookes 

 tube. A Crookes tube may be connected by only one of its terminals 

 to the Holtz machine. The free terminals of the machine and tube 

 may be connected to wires hung on silk fibres and making contact 

 with many pointed ground plates hung on long silk fibres in air. 

 The terminals are then in fact grounded on the dust particles in the 

 air. Either one of these air contacts may be replaced by a ground on 

 the gas-pipe. In all of the possible arrangements covered in this 

 description the tube will give excellent X-ray pictures. 



+H-H- 



contact 



One of these arrangements is represented in the annexed figure, 

 where the cathode terminal of the tube is grounded on the gas-pipe, 



1 Electricity and Magnetism, n, p. 201. 



