426 Professor J, J. Thomson [April 30, 



sliowing 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 electro- 

 meter, 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 per- 

 fectly 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 con- 

 tinually increasing : the cylinder settles into a state of equilibrium 

 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 up 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 equili- 

 brium value. 



I have lately made some experiments which are interesting from 

 the bearing they have on the charges carried by the cathode rays, as 

 well as on the production of cathode rays outside the tube. The 

 experiments are of the following kind. In the tube (Fig. 10) A and B 

 are terminals. C is a long side tube into which a closed metallic 

 cylinder fits lightly. This cylinder is made entirely of metal except 

 the end furthest from the terminals, which is stopped by an ebonite 

 plug, perforated by a small hole so as to make the pressure inside the 

 cylinder equal to that in the discharge tube. Inside the cylinder 

 there is a metal disc supported by a metal rod which passes through 

 the ebonite plug, and is connected with an electrometer, the wires 

 making this connection being surrounded by tubes connected with 

 the earth so as to screen off electrostatic induction. If the end of 

 the cylinder is made of thin aluminium about -^Q^h. of a millimetre 

 thick, and a discharge sent between the terminals, A being the cathode, 

 then at pressures far higher than those at which the cathode rays 

 come off, the disc inside the cylinder acquires a positive charge. And 

 if it is charged up independently the charge leaks away, and it leaks 

 more rapidly when the disc is charged negatively than when it is 

 charged positively ; there is, however, a leak in both cases, showing 

 that conduction has taken place through the gas between the cylinder 

 and the disc. As the pressure in the tube is diminished the positive 

 charge on the disc diminishes until it becomes unappreciable. The 



