176 BELL SYSTEM TECHNICAL JOURNAL 



The condenser is the essential element of the compensator, and it is, 

 of course, able to accumulate a charge in the absence of the resistance 

 Rc\ the only function of the latter being to dissipate this charge quickly 

 when the input level drops. If there were only a resistance and no 

 condenser there would be a simple rounding off of that part of the 

 positive crests of the carrier waves which cause grid current to flow 

 but no transfer of the operating axes as a whole to new bias values, 

 such as El, En and Eh in Fig. 7. 



Before leaving Fig. 7, it may be interesting to note parenthetically 

 that a signal with large bias, such as H is much more immune to 

 distortion due to changes in its own magnitude, or variations in relay 

 bias, than a signal such as L. This can readily be seen if we imagine 

 the line A A , which corresponds to the operating point of the receiving 

 relay, to be moved up or down and note the relative lateral displace- 

 ment of the intersections on these two envelopes. Another advantage 

 of the stronger signal is that the rate of change of energy at the mo- 

 ment when the relay operates is much greater, insuring a more positive 

 operation of its armature. The concave character of the detector 

 characteristic is also favorable to securing a desirably shaped pulse for 

 relay operation. Advantage was taken of these wave shaping possi- 

 bilities in the design of detectors antedating the use of the level com- 

 pensator to minimize the effect of circuit and battery variations. 



In the absence of resistance Rs, Fig. 6, there would be a tendency dur- 

 ing a long spacing interval for leakage in the wiring connected to the 

 grid, to reduce the grid bias to ground potential. Before such discharge 

 had gone very far, however, the receiving relay would close and re- 

 charge the condenser. This would give rise to periodic operation or 

 pulsing of the relays. The purpose of Rg is to prevent this undesirable 

 effect by making the negative bias voltage approximately equal to E^ 

 during long spaces. Since this resistance is large compared to R^ 

 it has a negligible effect during the reception of signals. 



Where a circuit is exposed to transient additions of energy from 

 external sources such as lightning, the operation of the level com- 

 pensator may be stabilized by bridging a large capacitance in series 

 with a resistance around condenser C. 



Sending Circuit 

 In the voice-frequency telegraph system the spacing signals are 

 produced at the transmitting end by short circuiting that portion of 

 the circuit which supplies the sending filter with power, and the 

 marking signals by allowing the current from the generator to flow 

 through freely. This operation is performed by the sending relay. 



