CATHODE EAYS. 161 



from tlie cathode and are deflected by a magnet, it does not show that 

 when the cathode rays are deflected by a magnet the path of the elec- 

 trified particles coincides with the path of the cathode rays. The sup- 

 porters of the theory that these rays are waves in the ether might say, 

 and indeed have said, that while they did not deny that electrified 

 particles might be shot off from the cathode, these particles were, in 

 their opinion, merely accidental accompaniments of the rays, and had 

 no more to do with the rays than the bullet has with the flash of a rifle. 

 The following modification of Perrin's experiment is not, however, open 

 to this objection: Two coaxial cylinders with slits cut in them, the 

 outer cylinder being connected with earth, the inner with the elec- 

 trometer, are placed in the discharge tube, but in such a position that 

 the cathode rays do not fall upon them unless deflected by a magnet. 

 By means of a magnet, however, we can deflect the cathode rays until 

 they fall on the slit in the cylinder. If under these circumstances the 

 cylinder gets a negative charge when the cathode rays fall on the slit, 

 and remains uncharged unless they do so, we may conclude, I think, 

 the stream of negatively electrified particles is an invariable accom- 

 paniment of the cathode rays. I will now try the experiment. You 

 notice that when there is no magnetic force, though the rays do not fall 

 on the cylinder, there is a slight deflection of the electrometer, showing 

 that it has acquired a small negative charge. This is, I think, due to 

 the plug getting negatively charged under the torrent of negatively 

 electrified particles from the cathode, and getting out cathode rays on 

 its own account which have not come through the slit. I will now 

 deflect the rays by a magnet, and you will see that at first there is little 

 or no change in the deflection of the electrometer, but that when the 

 rays reach the cylinder there is at once a great increase in the deflection, 

 showing that the rays are pouring a charge of negative electricity into 

 the cylinder. The deflection of the electrometer reaches a certain 

 value and then stops and remains constant, though the rays continue 

 to pour into the cylinder. This is due to the fact that the gas traversed 

 by the cathode rays becomes a conductor of electricity, and thus, though 

 the inner cylinder is perfectly insulated when the rays are not passing, 

 yet as soon as the rays pass through the bulb the air between the inner 

 cylinder and the outer one, which is connected with the earth, becomes 

 a conductor, and the electricity escapes from the inner cylinder to the 

 earth. For this reason the charge within the inner cylinder does not 

 go on continually increasing. The cylinder settles into a state of equi- 

 librium in which the rate at which it gains negative electricity from the 

 rays is equal to the rate at which it loses it by conduction through the 

 air. If we charge ui) the cylinder positively it rapidly loses its positive 

 charge and acquires a negative one, while if we charge it up negatively it 

 will leak if its initial negative potential is greater than its equilibrium 

 value. 



I have lately made some experiments which are interesting from the 

 SM97— 11 



