100 BELL SYSTEM TECHNICAL JOURNAL 



it is possible to obtain from the output of the detector the difference 

 frequency of one cycle per second. This low frequency may be 

 readily separated by means of a filter. It is apparent, however, that 

 to maintain this difference frequency constant requires that the input 

 frequencies be held within a very narrow range of variation. 



61. Thermionic Valve as a High Tension Switch. If the plate 

 circuit of a valve is inserted in a high tension circuit, the flow of 

 current in the circuit may readily be stopped by cutting off the filament 

 heating supply, thus making unnecessary the breaking of any con- 

 tacts in the high tension circuit. In case the transmission of current 

 in both directions is necessary, two valves may be used. 



62. Devices Employing Secondary Emission. As pointed out in 

 Sec. 9, the grid current in a three-element tube shows a negative 

 resistance characteristic for a certain range of voltage, and various 

 uses of this fact have been pointed out. 50 



63. Electron Tube Oscillograph. A special type of thermionic tube 

 designed for oscillographic uses is of great importance as a laboratory 

 instrument. These tubes, using the hot filament as a source of 

 electrons, have certain marked advantages over the Braun tube 

 with its gaseous discharge. 51 One of the very successful thermionic 

 oscillographs has the following properties: anode potential 300-409 

 volts, sharp focus of electron beam, sensitivity of 1 mm. per volt be- 

 tween deflection plates and 1 mm. per ampere-turn when using mag- 

 netic deflection. Photographic recording is possible with relatively 

 short exposures by using suitable fluorescent material. 



< 9 Dushman, Phys. Rev., Oct., 1920, p. 854. 



60 See footnote 21. 



61 See J. B. Johnson, /. of Opt. Soc. of Amer., Sept., 1922, or Bell Syste:n Technical 

 Journal, Nov., 1922. 



