COOLIDGE HIGH-VACUUM TUBE 3 



is increased the electron current does not increase as rapidly as the 

 increase in number of thermoelectrons. The electron current moving 

 from filament to plate is said to be limited by " space charge." 



Repulsion by the thermoelectrons surrounding the filament prevents 

 all the emitted electrons from moving towards the collecting plate. The 

 electron current has a maximum or saturation value expressed by the 

 relation deduced by Langmuir, 



_ V2 [7 v 3/2 



where e and m are the charge and the mass of the electron, x the distance 

 between the electrodes, and V their difference of potential. It is essen- 

 tial to have the electrodes quite close together in order to have electron 

 currents of appreciable magnitudes flow between the electrodes. A close 

 approximation for tungsten is i = 2.33 X 10 -6 v 3/2 /x 2 amperes per 

 square centimeter of thermoelectron-emitting surface. 



Scale 



~l — r- — 1 



12 3 inches 



Hot-filament cathode^v V-Tungsten target 



Fig. 1-1. A universal Coolidge tube which may be used up to 200,000 volts with 

 an 8-ma current between target and hot-cathode filament. Solid tungsten target. 



Coolidge [1913] designed an x-ray tube using the thermoelectron 

 current from a hot filament in a high vacuum to replace the cathode-ray 

 stream for bombarding the target in order to generate the x-rays. 



Using a heated tungsten filament as his source of electrons, and a 

 disk of tungsten, backed by copper for heat conduction, as a target, 

 Coolidge found that it was possible to obtain a very stable discharge 

 which permitted more accurate reproducible results, that the tube 

 could be made smaller, that a greater flexibility could be obtained since 

 current through the tube and the voltage across the tube could be varied 

 independently, and that the tube had a comparatively long life. 



The commercial form of Coolidge tubes varies in size and shape de- 

 pending on operating conditions. Figures 1-1, 2, and 3 show some 

 available commercial forms. Figure 1-1 is an air-cooled type that 

 will carry 8 ma between hot filament and its solid tungsten target when 

 excited at a difference of potential of 200,000 volts. This design of 

 Coolidge tube is used primarily for therapeutic work. Figure 1-2 shows 

 an x-ray tube having a copper-backed tungsten target with an air-cooled 



