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BELL SYSTEM TECHNICAL JOURNAL 



for the discharging of the fluorescent screen so that the electrons can 

 drift back to the metallic circuit. 



The Electrode Unit 



With gas present in the tube, steps have to be taken to guard against 

 arcing and the injurious effects of positive ion bombardment on the 

 cathode. This is done by making the volume of gas surrounding the 

 electrodes very small. For this purpose the cathode and anode, 

 themselves small, are sealed into a short and narrow glass tube so 

 that the volume exposed to both electrodes in common is less than 

 1 cu. cm. All paths between the electrodes are then so short that at 

 this low pressure there is not sufficient ionization to build up an arc. 



The structure of this unit, or "electron gun" is shown in Fig. 2. 

 The cathode, /, is an oxide coated platinum ribbon of the same kind 



BRAON TUBE ON IT 



Fig. 2 



as the filament in our audion tubes. The anode, a, is a thin platinum 

 tube 1 cm. long and 1 mm. in diameter, one end of which is about 

 1 mm. from the top of the filament loop, the other end opening into 

 the main tube towards the fluorescent screen. Between the cathode 

 and the anode and connected to the cathode is a metal shield, s, with 

 a small aperture through which the electrons must pass in going to the 

 anode. Nearly all of the electrons must then go to the inside of the 

 tubular anode, and a small fraction of them pass through the whole 



