228 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



ionization which takes place in the faint luminosity in the imme- 

 diate neighborhood of the cathode is shown by placing a small 

 obstacle in the Crookes dark space. This obstacle screens a 

 portion of the cathode surface from bombardment by the positive 

 ions which move from the negative glow towards the cathode so 

 that in the region so shaded ionization does not take place. In 

 the same way the obstacle also screens a certain portion of the 

 negative glow from bombardment by the electrons which are 

 thrown from the cathode and this portion of the negative glow 

 ceases to exist because ionization is no longer produced there. 

 That is to say, the obstacle casts a shadow on the cathode and it 

 also casts a shadow into the negative glow. 



The electric field intensity in the Crookes dark space, being 

 necessarily sufficient to enable the positive ions to produce 

 ionization at the surface of the cathode, is able to impart very 

 much greater velocity to the electrons than is necessary to enable 

 them to produce ionization. The result is that the electrons 

 which are thrown off from the cathode travel in straight lines 

 through a long portion of the tube. These high velocity elec- 

 trons constitute what are called cathode rays. The cathode rays 

 are faintly visible throughout the tube because of occasional col- 

 lisions with the molecules 

 of the gas. 



When the cathode has 

 a small hole through it, 

 the positive ions which 

 move towards the cathode 

 from the negative glow 

 pass through this hole in 

 the form of a stream of 



Fig. 160. , . , . . , 



rays which is made visible 



by the luminosity which accompanies the collisions of the posi- 

 tive ions with the molecules of the gas. Such a stream of posi- 

 tive ions constitutes what has been called the canal rays. 



An object of any kind placed in the Crookes tube casts a 

 sharp shadow upon the wall of the tube, as shown in Fig. 160. 



