96 



BELL SYSTEM TECHNICAL JOURNAL 



acteristic, acts as a detector and as pointed out in Sec. 37, the change 

 in space current is approximately proportional to the square of the 

 a.c. input voltage. For accurate work the circuit requires calibra- 

 tion but the calibration will in general remain good over long periods 

 of time. This method is particularly useful for small voltages. 



57. Power-Limiting Devices. As pointed out in Sec. 4 the total 

 emission from the filament at a given temperature is fairly sharply 

 defined regardless of the plate voltage, so long as this exceeds the 

 value required to give voltage saturation. The fact that the total 

 emission is limited by the temperature may be used to control the 



t I 8 n *> 20 



Grid Potenttol-Voib 



Fig. 72 



maximum current in a circuit. As an illustration, Fig. 71 shows its 

 application to an alternating current circuit, the performance of which 

 is illustrated in Fig. 72. The introduction of such a device into an 

 a.c. circuit will, of course, result in the generation of harmonics and 

 may therefore, be objectionable. 



There is almost no limit to the number of regulatory circuits which 

 can be devised to employ the three-electrode tube. 



High ( ~ 



^H,gh 

 voltage 



W 



5 Low 



Voltage 



Fig. 73 



58. Voltage and Current Regulation of Generators. The two-elec- 

 trode tube with tungsten filament has been used to great advantage 

 as a voltage regulator for a special airplane generator designed to de- 

 liver both 28 volts and 300 volts. The circuit arrangement is illustrated 



