ADVANCES IN CARRIER TELEGRAPH TRANSMISSION 



175 



scending curves bounding the envelope represent identical time 

 sequences of events. 



If the level increases so that the amplitude of the received signals 

 has a value indicated by curve H, a much larger part of the signal 

 current momentarily flows through the grid-filament space and is 

 rectified, thereby increasing the charge on the condenser in such a 



CURRENT TIME 



Fig. 7 — Principle of level-compensator action. 



way as to increase the negative bias of the grid and shift the signal 

 bodily towards the negative side of 00. This shift continues until the 

 total effective grid voltage has reached a point at which the rectified 

 current is just sufficient to compensate for the increased discharge 

 through resistance i?« due to the higher voltage across the condenser. 

 The system is then in equilibrium at the new input level. If the 

 constants of the circuit have been properly chosen, the new grid 

 voltage Eh will be such that the middle of the positive envelope of the 

 received signal again passes through the line A A, thereby giving 

 unbiased signal reception. 



The corresponding condition for an input level below normal is 

 shown by curve L. 



