282 ADVANCED ELECTRICITY AND MAGNETISM. 



tube in which the pressure has been reduced to a few thousandths 

 of a millimeter of mercury. Such a vacuum tube is called a 

 Crookes tube. 



141. Cathode rays and canal rays. In order that a steady 

 discharge may flow through a vacuum tube, it is necessary that 

 the electric field intensity reach a value sufficient to impart to 

 the positive ions enough energy between collisions to enable them 

 to ionize the molecules of the residual gas, because if the electrons 

 (negative ions), only, produce ionization, the discharge through 

 the tube ceases very soon after all of the negative ions have moved 

 across to the neighborhood of the anode. In fact, ionization 

 by positive ions must take place in the neighborhood of the 

 cathode,* and it is this necessity which gives rise to the Crookes 

 dark space. The action which takes place in the Crookes dark 

 space is as follows : Electrons (negative ions) are thrown off from 

 the cathode at very high velocity by the intense electric field in 

 the Crookes dark space, very energetic ionization takes place 

 in the negative glow N, Fig. 206, and the positive ions that are 

 produced in this region attain sufficient velocity in traveling 

 towards the cathode to enable them to ionize the gas in the 

 immediate neighborhood of the cathode. That is, ionization by 

 positive ions takes place in the faint glow which covers the 

 cathode. The mutual dependence of the ionization which takes 

 place in the negative glow and the ionization which takes place 

 in the faint luminosity in the immediate neighborhood of the 

 cathode is shown by placing a small obstacle in the Crookes 

 dark space. This obstacle screens a portion of the cathode sur- 

 face from bombardment by the positive ions which move from 

 the negative glow towards the cathode so that the region so 

 shaded is free from even faint luminosity because ionization does 

 not take place there. 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 



* A detailed discussion of this matter may be found in J. J. Thomson's Con- 

 duction of Electricity through Gases, pages 529-603. 



