174 BELL SYSTEM TECHNICAL JOURNAL 



receiving relay. A quantitative discussion of the operation of the 

 compensator is given in the Appendix. 



In order that the voltage across the condenser may not decrease 

 during spacing signals, an auxiliary polar relay, called the compensator 

 relay, is provided which derives its operating current from the arma- 

 ture of the receiving relay and serves to disconnect the resistance Re 

 during spacing signals. As discussed in the appendix, the unbiased 

 operation of the compensator relay would cause a noticeable decrease 

 in the condenser voltage during the rapid transmission of signals, 

 because the wave shape of the signals impressed on the grid circuit of 

 the detector tube is not square but considerably rounded. In other 

 words, there is a portion of a marking signal during which the receiving 

 relay is operated to marking but the grid is non-conducting. Hence 

 more charge would leak from the condenser during the time the re- 

 sistance is connected across the condenser than would be replaced by 

 rectification. To prevent this, the compensator relay bridges the dis- 

 charge resistance Re around condenser C for a period of time which is 

 shorter than that during which the receiving relay is on its marking 

 contact. The amount by which the compensator condenser must be 

 biased depends on the signaling speed and other factors. It is de- 

 termined by observing the "drift" in bias suffered by reversals when 

 these are suddenly switched on after a long marking interval. 



The operation of the level compensator will be more readily under- 

 stood by referring to Fig. 7, which shows diagrammatically the manner 

 in which received impulses of different magnitudes are made to operate 

 the receiving relay for equal time intervals. In this diagram, the posi- 

 tive halves of the envelopes of three received marking impulses * of 

 different amplitudes are shown in relation to the grid-voltage plate- 

 current characteristic of the detector tube. For normal input level 

 the sensitivity of the detector is made sufficiently great so that the 

 amplitude of the impulse which is impressed on the interstage trans- 

 former is of the magnitude shown at N. The envelope of the received 

 carrier current is symmetrical about the line En, which is located at 

 the net value of grid biasing voltage due to the battery voltage Eq 

 and the grid condenser voltage Cc. The latter voltage, as previously 

 noted, is produced in the grid circuit by rectification of that part of the 

 received carrier current which lies on the positive side of the zero grid- 

 voltage axis 00. By properly adjusting the bias of the receiving relay 

 the latter may be made to operate at a value AA which is one-half the 

 crest value of the envelope. Signals having amplitude N will thus be 

 repeated unbiased by the receiving relay, since the ascending and de- 



* In practice such pulses would usually reach the steady state. 



