280 ADVANCED ELECTRICITY AND MAGNETISM. 



approximately true in gases under moderate or high pressure, 

 and when the pressure is very low a greater electromotive force 

 is required to strike across a short gap than is required to strike 

 across a long gap. This curious behavior of gas at low pressure 



Fig. 205. 



is illustrated by a famous experiment due to Hittorf . Two elec- 

 trodes were sealed into the walls of two glass bulbs and the tips 

 of the electrodes were one millimeter apart, as shown in Fig. 205. 

 The two bulbs were connected together by a spiral tube 375 

 centimeters long, and, when the pressure of the gas in the bulbs 

 was reduced to a very low value, the discharge took place through 

 the long tube and not across the one millimeter gap space between 

 the points of the electrodes.* 



140. 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 



* This behavior of a gas at low pressure is fully explained by the atomic theory. 

 See J. J. Thomson's Conduction of Electricity through Gases, pages 430-527. 



