226 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



128. The Geissler tube and the Crookes tube. The discharge 

 of electricity through gases at low pressures is usually studied by 

 means of a glass bulb through the walls of which are sealed 

 platinum wires which terminate in metal plates called electrodes. 

 The current enters at one electrode, the anode, and passes out at 

 the other electrode, the cathode. This bulb, which is called a 

 vacuum tube, is filled with the gas to be studied and the pressure 

 is reduced to any desired value by exhausting the tube by means 

 of an air pump. 



Before exhaustion the discharge through the tube is in the form 

 of a sharply-defined spark similar to the spark in the open air. 

 When the pressure of the gas in the bulb has been 

 reduced to a few centimeters of mercury; the spark 

 begins to be nebulous and a continued reduction of 

 pressure causes the luminosity ultimately to fill the 

 entire tube. When the pressure has been reduced 

 to a few millimeters of mercury the discharge pre- 

 sents the following features, as shown in Fig. 159. 

 There is a thin layer of luminosity spread over the 

 surface of the cathode C, and beyond this there 

 is a comparatively dark space D called the Crookes 

 dark space, the width of which depends upon the 

 pressure of the gas, increasing as the pressure of 

 the gas diminishes. This Crookes dark space ex- 

 tends to a boundary which is approximately a sur- 

 face traced out by lines of constant length drawn 

 normally to the surface of the cathode. Beyond 

 the Crookes dark space is a luminous region N 

 called the negative glow, and beyond the negative 

 glow is another comparatively dark region F which is called the 

 Faraday dark space. Beyond the Faraday dark space is a lumi- 

 nous column P extending to the anode A and called the posi- 

 tive column. This positive column usually exhibits alternations 

 of bright and dark spaces which are called striations. The effects 

 here described are exhibited at their best in a vacuum tube in 





