172 BELL SYSTEM TECHNICAL JOURNAL 



bauds) may exhibit a change in signal bias of 4 per cent or more for 

 each db change in input level. With a level compensator of the type 

 described herein, the bias may be kept within a range of ± 2 per cent 

 for a variation in input level of ± 8 db. 



The elimination of any considerable bias variations in individual 

 telegraph sections due to level changes which usually occur in practice 

 is particularly important in the case of multi-section * circuits. As a 

 result of this improvement it has been found feasible under test condi- 

 tions to operate satisfactorily as many as 10 telegraph sections in 

 tandem at 60 words per minute for long periods without objectionable 

 bias variations due to level changes and without the use of regenerative 

 telegraph repeaters. In practice, however, other considerations 

 usually make it desirable to use a regenerative repeater when the 

 number of sections in tandem exceeds 4. 



The greater part of the effective changes in received level in a given 

 circuit is due to temperature changes which are imperfectly compen- 

 sated for by the regulators, aggravated by the fact that the conditions 

 prevailing when the circuits are adjusted within the limits specified by 

 the maintenance routines may depart considerably from the average. 

 In addition to this, there are variations of considerable magnitude 

 between individual circuits due to structural differences. In view of 

 the fact that the variations over the whole frequency range are not the 

 same, there is a material advantage in a compensator which adjusts 

 the gain of each detector independently, a feature which could not be 

 secured with a pilot-channel regulator. 



Level Compensator 



The level compensator," shown diagrammatically in heavy lines in 

 Fig. 6, may be considered as functionally divided into two parts, one 

 of which is in series with the grid of the detector tube and the other 

 of which is connected to the armature of the receiving relay. The 

 first of these will be referred to as the grid-bias circuit and the second 

 as the compensator -relay circuit. 



The grid-bias circuit consists essentially of a condenser C shunted by 

 a high resistance R^ in series with a biasing battery Eq of fixed voltage, 

 the secondary of the interstage transformer T, and the grid-filament 

 terminals of the detector tube V. This arrangement functions to keep 

 the effective grid-filament voltage due to the signals nearly constant, 

 irrespective of their magnitude, by setting up a voltage on the condenser 

 which adds algebraically to the grid-bias battery and whose magnitude 



* By a multi-section telegraph circuit is meant a connection made up of 2 or more 

 telegraph lines in tandem with mechanical repetition between them. 



