ADVANCES IN CARRIER TELEGRAPH TRANSMISSION 



195 



this circuit acts like a capacitance, while at lower frequencies it acts 

 like an inductance. There is thus a rapid change in both the voltage 

 and in the phase thereof across T2 whenever there is a variation in 

 frequency on either side of the specified value. The output current 

 from V2, combining with the current impressed directly through T3, 

 produces corresponding abrupt changes in the d-c. component of the 

 resultant which in turn varies the bias of tube Vz and hence the current 

 through the regulating field of the motor. 



In order to assure a rapid change in impedance with frequency, the 

 anti-resonant circuit referred to above comprises a carefully shielded 

 air-core coil having a very small resistance relative to its inductance. 



The frequency indicator shown in Fig. 20-4 provides means for easily 



note:-arrows show direction 



of current and associated 

 letters show to which 

 contact armature is urged 



Fig. 20 — Frequency indicator. A. Schematic diagram. B. Attenuation of 



two input-paths. 



observing any departures of the carrier frequency from its nominal 

 value, as well as an automatic maximum-minimum alarm to warn the 

 attendant if these variations become excessive. Since all the carrier 

 currents are derived from generator elements which are mounted on a 

 common shaft, it is sufficient to observe the frequency of a single 

 channel. Current from channel 10 (1955 cycles) is impressed simul- 

 taneously on two vacuum tube circuits which are identical except for 

 the fact that there is a low-pass filter in the input of one while there is 

 a simple pad in the input of the other. As indicated in Fig. 20-B, the 

 loss through the pad is the same at all frequencies and equal to that of 

 the filter when the generator speed is correct. If the frequency in- 

 creases, the attenuation of the filter branch goes up; while if it de- 

 creases, its attenuation goes down ; but in any case the loss through the 

 pad remains constant, of course. The net resulting ampere-turns 



