MEASUREMENT OF TELEGRAPH TRANSMISSION 



175 



accuracy for some signal combinations because of the fact, as men- 

 tioned earlier, the distortion of the stop pulse is not added to the 

 distortion of other pulses. However for the purpose for which it is 

 used, namely to detect trouble conditions on working circuits, the 

 accuracy is believed to be adequate. 



In this set two measuring circuits are provided, one for measuring 

 marks and the other spaces. These are condenser-charging circuits 

 in which the charge on the condenser is an indication of the duration 

 of the pulse. These condenser voltages are compared to a reference 

 voltage and only those less than the reference voltage are permitted 

 to influence the distortion indicator. Therefore, only the shortest 

 pulses are measured and this permits observation on working tele- 

 typewriter circuits without involving a start-stop arrangement. By 

 adjustment of the time constant of the condenser-charging circuits, 

 as for instance by means of continuously variable resistances, the 

 percentage distortion for which an alarm is given may be varied at will. 



PEAK-DISTORTION 

 INDICATOR 



GAS TUBE 



C2 



Fig. 22— Distortion-measuring circuit of telegraph service-monitoring set. 



The distortion-measuring portion of the circuit used in the measure- 

 ment of marks is indicated by Fig. 22. Condenser C is charged during 

 marking intervals through high resistance R by voltage E; thus a 

 voltage is produced on the condenser which depends upon the duration 

 of the marking interval as is indicated by Fig. 23. At the time of the 

 transition from mark to space the condenser voltage is momentarily 

 compared to that of a reference source by way of the armature and 

 marking contact of relay 2 of Fig. 22. Immediately afterwards the 

 condenser charge is dissipated by the armature of relay 2 moving to 



