THERMIONIC VACUUM TUBES 



97 



in Fig. 73 in which M represents the main field winding, and D the 

 differential winding which opposes M. The high voltage given by 

 the generator when applied to the plate of the valve is sufficient to 

 produce a condition of voltage saturation. As the speed of the 

 generator increases, the current through the main field winding and 



Fig. 74 



the filament increases, thereby giving rise to greater emission from 

 the filament and a larger current through the differential winding. 

 It was found possible to so design the valve as to yield the very close 

 regulation illustrated in Fig. 74. 47 



The three-electrode tube can also be used as a voltage regulator 

 for a generator as shown in Fig. 75. It is apparent that an increase 



Fig. 75 



in the voltage across the line tends to increase the current through 

 the tube and resistance R. This in turn lowers the grid potential and 

 tends to prevent an increase in current through the field winding. 

 The circuit shown in Fig. 76 illustrates an arrangement for main- 



n 



DC 



7 



Fig. 76 



Fig. 77 



taining a constant current from a generator. The operation of the 

 device is apparent. 



Fig. 77 shows another arrangement for maintaining a constant 

 generator voltage. In this circuit an increase in voltage tends to 



" Radio Telephony by Craft & Colpitis, Trans. A. I. E. E., Vol. 38, p. 330. 1919 



