THE PHENOMENA OF ELECTROSTATICS. 22/ 



which the pressure -has been reduced to a few millimeters of 

 mercury. Such a vacuum tube is called a Geissler tube. When 

 the exhaustion of the vacuum tube is carried further, the dark 

 space which surrounds the cathode (the Crookes dark space) ex- 

 pands until it fills the entire tube. The glass walls of the tube 

 then show a yellowish-green or blue luminescence (according as 

 the tube is made of soda glass or lead glass) and a slight nega- 

 tive glow may remain in portions of the tube remote from the 

 cathode. These effects, which were first studied by Crookes in 

 England and by Pliicker and Hittorf in Germany, are exhibited 

 at their best in a vacuum 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. 



129. Cathode rays and canal rays. In order that a steady dis- 

 charge 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 gas, because if the electrons (negative ions), only, pro- 

 duce ionization, the discharge through the tube ceases very soon 

 after all of the negative ions have moved across to the neighbor- 

 hood 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 gJow N, 

 Fig. 1 59, 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 



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

 of Electricity Through Gases, pages 529-603. 



