296 Prof. J. J. Thomson on Cathode Bays. 



charge and acquires a negative one ; while if the initial charge 

 is a negative one, the cylinder will leak if the initial negative 

 potential is numerically greater than the equilibrium value. 



Deflexion of the Cathode Rays by an Electrostatic Field. 



An objection very generally urged against the view that 

 the cathode rays are negatively electrified particles, is that 

 hitherto no deflexion of the rays has been observed under a 

 small electrostatic force, and though the rays are deflected 

 when they pass near electrodes connected with sources of large 

 differences of potential, such as induction-coils or electrical 

 machines, the deflexion in this case is regarded by the sup- 

 porters of the setherial theory as due to the discharge passing 

 between the electrodes, and not primarily to the electrostatic 

 field. Hertz made the rays travel between two parallel 

 plates of metal placed inside the discharge-tube, but found 

 that they were not deflected when the plates were con- 

 nected with a battery of storage-cells ; on repeating this 

 experiment I at first got the same result, but subsequent 

 experiments showed that the absence of deflexion is due to 

 the conductivity conferred on the rarefied gas by the cathode 

 rays. On measuring this conductivity it was found that it 

 diminished very rapidly as the exhaustion inci'eased; it seemed 

 then that on trying Hertz's experiment at very high exhaus- 

 tions there might be a chance of detecting the deflexion of the 

 cathode rays by an electrostatic force. 



The apparatus used is represented in fig. 2. 



Fie:. 2 



The rays from the cathode C pass through a slit in the 

 anode A, which is a metal plug fitting tightly into the tube 

 and connected with the earth ; after passing through a second 

 slit in another earth-connected metal plug B, they travel 

 between two parallel aluminium plates about 5 cm. long 

 by 2 broad and at a distance of 1*5 cm. apart ; they then fall 

 on the end of the tube and produce a narrow well-defined 

 phosphorescent patch. A scale pasted on the outside of 

 the tube serves to measure the deflexion of this patch. 



