ELECTRON THEORY KLOEFFLER 



249 



vacuum depends first on the initial velocity of emission of the electrons 

 which is determined by the temperature of the cathode; second, upon 

 the attraction of the cathode for the electrons; third, upon the attrac- 

 tion of the plate which depends on the plate voltage; and, fourth, 

 upon the space charge. The fourth factor, space charge, is due to 

 the resultant charge in the space surrounding the cathode caused by 

 the presence of the electrons which are being emitted. A cloud of 

 electrons surrounding a cathode may be pictured as in figure 3. 

 Each of these electrons is a negative charge and as such exerts a re- 

 pulsion on all other electrons in its vicinity. At the instant a par- 

 ticular electron emerges from the cathode, all electrons in space are 

 exerting a repelling force but as it moves away from the cathode all 

 of those behind it are repelling or now aiding it toward the plate. 

 The influence of space charge is very effective in a high vacuum and 

 it was once believed that no emission would exist in a perfect vacuum. 

 The two-electrode high-vacuum tube with tungsten filament is used for 

 rectifying high voltages for use 

 in radio-transmitting stations, 

 X-ray equipment, smoke eli- 

 minators, and wherever a high 

 continuous voltage is desired. 

 The two-electrode vacuum 

 tube is used as a detector in 

 modern radio-receiving sets. 



The addition of a small 

 amount of gas to a two-elec- 

 trode tube changes its characteristic action. The presence of the gas 

 permits ionization to occur and the positive and negative ions thus 

 formed pass to the cathode and anode, respectively. The ions serve 

 as carriers of electrons and increase the current slightly but a large in- 

 crease in current results from another action. Thus the negative ions 

 formed are electrons and are attracted swiftly to the positive anode. 

 The positive ions are atoms (less one or more electrons) and because 

 of their large mass they move relatively slowly. Each positive ion 

 can neutralize the charge on one electron in the space charge at a 

 given instant, but because of its slow motion it serves to neutralize 

 many electrons while in transit. Thus it is possible for a single 

 positive ion to neutralize successively as many as 300 electrons in the 

 space charge. The neutralization of the space charge permits the 

 positive anode to attract many more emitted electrons and thus the 

 current through the tube is greatly increased. 



The gas-filled two-electrode tube is inherently a low-voltage device 

 (that is, it has a low voltage drop from cathode to anode). It rectifies 

 much larger currents than the vacuum type and is used for charging 

 storage batteries and similar low-power D. C. applications. 



Figure i. — Diagram of two-electrode tube or diode. 



