Prof,, J. J. Thomson on Cathode Rays. 297 



At high exhaustions the rays were deflected when the two 

 aluminium plates were connected with the terminals of a 

 battery of small storage-cells ; the rays were depressed 

 when the upper plate was connected with the negative 

 pole of the battery, the lower with the positive, and raised 

 when the upper plate was connected with the positive, the 

 lower with the negative pol^. The deflexion was propor- 

 tional to the difference of potential between the plates, and I 

 could detect the deflexion when the potential-difference was 

 as small as two volts. It was only when the vacuum was a 

 good one that the deflexion took place, but that the absence 

 of deflexion is due to the conductivity of the medium is 

 shown by what takes place when the vacuum has just arrived 

 at the stage, at which the deflexion begins. At this stage 

 there is a deflexion of the rays when the plates are first 

 connected with the terminals of the battery, but if this 

 connexion is maintained the patch of phosphorescence gradu- 

 ally creeps back to its imdeflected position. This is just what 

 would happen if the space between the plates were a conductor, 

 though a a ery bad one, for then the positive and negative ions 

 between the plates would slowly diffuse, until the positive plate 

 became coated with negative ions, the negative plate with 

 positive ones ; thus the electric intensity between the plates 

 would vanish and the cathode rays be free from electrostatic 

 force. Another illustration of this is afforded by what happens 

 when the pressure is low enough to show the deflexion and 

 a large difference of potential, say 200 volts, is established 

 between the plates ; under these circumstances there is a large 

 deflexion of the cathode rays, but the medium under the large 

 electromotive force breaks down every now and then and a 

 bright discharge passes between the plates ; when this occurs 

 the phosphorescent patch produced by the cathode rays jumps 

 back to its undeflected position. When the cathode rays are 

 deflected by the electrostatic field, the phosphorescent band 

 breaks up into several bright bands separated by comparatively 

 dark spaces ; the phenomena are exactly analogous to those 

 observed by Birkeland when the cathode rays are deflected 

 by a magnet, and called by him the magnetic spectrum. 



A series of measurements of the deflexion of the rays by 

 the electrostatic force under various circumstances will be 

 found later on in the part of the paper which deals with the 

 velocity of the rays and the ratio of the mass of the electrified 

 particles to the charge carried by them. It may, however, be 

 mentioned here that the deflexion gets smaller as the pressure 

 diminishes, and when in consequence the potential-difference 

 in the tube in the neighbourhood of the cathode increases. 



