136 PHYSICAL SCIENCE 



of an influence electric machine. As in liquid 

 conductors, the electrode by which the current 

 enters is called the anode, and that by which it 

 leaves, the cathode. Starting from the cathode, 

 we first see a bright glow covering its surface, then 

 a dark space, succeeded by a second dark space, 

 beyond which is a luminous column reaching to 

 the anode. Within certain limits of pressure and 

 strength of current, this positive column, as it has 

 been called, shows fluctuating striations. If the 

 length of the tube be increased, it is this positive 

 column alone which increases with it ; the two 

 dark spaces, and the negative glow, vary very 

 little with the length of the tube. 



The effect of very high vacua on the electric 

 discharge was first systematically investigated by 

 Sir William Crookes. As the air is gradually 

 removed, it is found that the dark space nearest 

 the cathode, known as Crookes' dark space, 

 gradually extends, until eventually it fills the whole 

 tube. At this stage, green phosphorescent effects 

 begin to appear on the anode and on the glass 

 opposite the cathode. If a solid object, such as a 

 screen of mica, be interposed between the glass 

 and the cathode, a sharp shadow is seen, showing 

 from its position that rays capable of producing 

 phosphorescence proceed in straight lines from the 

 cathode. These cathode rays possess momentum, 

 for a light windmill placed in their path can be 

 made to rotate ; moreover, they are deflected by 

 a magnet, in the same direction as would be 

 negatively electrified particles, travelling in the 

 course of the rays. For these reasons, the cathode 

 rays must be regarded as a flight of negatively 

 electrified material particles. 



