432 BELL SYSTEM TECHNICAL JOURNAL 



of amplification adjustment to about 150 to 1. In addition a set of 

 constant resistance potentiometers in the relay control panel associ- 

 ated with the amplifier allows the current through any of the relays 

 to be varied in small steps without influencing the current in the other 

 relays or changing the impedance of the amplifier output circuit. 



The characteristic of the amplifier system may be measured by 

 applying a certain input voltage at varied frequencies, and noting 

 the corresponding output voltage. If the amplifier has been adjusted 

 to give a satisfactory signal when connected to a cable, measurements 

 will show that its amplification increases rapidly to a maximum which 

 occurs at about 1.5 times the signaling frequency, and then falls to 

 practically zero at about twice the signaling frequency. This elimina- 

 tion of the higher frequencies is effected by proper adjustment of the 

 inter-stage shaping networks, and it results in suppressing that portion 

 of the interfering currents received from the cable which lie above the 

 band of frequencies required to form the signal. 



The amplifier as described above is perfectly suitable for recorder 

 operation and will permit communication at speeds up to at least 

 ninety cycles per second which in cable code is equivalent to about 

 2,800 letters per minute, provided that a suitable recorder is employed. 

 It is, however, not entirely suitable for multiplex printing telegraph 

 operation under all conditions without the addition of apparatus to 

 prevent "zero wander." 



System for Correcting "Zero Wander" 

 In general, printing telegraph systems have been designed on the 

 assumption that they were to work over land telegraph lines and 

 they contain no provision for avoiding the effects of the "wandering 

 zero" which is caused by the inability of a practical cable transmission 

 system to transmit direct current. This inability to transmit direct 

 currents is due to the fact that there is usually present in a submarine 

 cable an earth current which is many times as strong as the signal, 

 and it is necessary to block out this earth current by series condensers 

 (as is usual in ordinary cable practice) or to keep it out of the amplifier 

 by a transformer. The syphon recorder does not require that the 

 zero of the signal be maintained closely but cable signal relays operate 

 on a fixed value of current of either polarity and are incapable of de- 

 termining whether or not part of this current is due to a displace- 

 ment of the zero. It is therefore necessary to reconcile in some way 

 the printing telegraph systems, which under some conditions require 

 the reception of a direct current, with the cable system which cannot 

 transmit a direct current. Several methods of doing this have been 



