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BELL SYSTEM TECHNICAL JOURNAL 



balanced circuit analogous to that of a Wheatstone-bridge galvano- 

 meter as regards outgoing signals, and is therefore operated only by 

 incoming signals. The fact that the home relay is not influenced 

 by the home battery will be appreciated by considering the battery 

 at the distant station to be short-circuited and the artificial line to 

 balance the line and distant apparatus perfectly. When battery 

 is introduced at the distant station however it causes a signal to be 



Fig. 1 — Line equipped with composite sets and ground-return polar duplex sets 



received at the home station as it corresponds to inserting a battery 

 in one arm of a Wheatstone bridge. The " bridge coil " is connected 

 so as to be series-aiding for incoming signals, the inductance being 

 about 75 henries in this case, and parallel-opposed for outgoing 

 signals, the inductance then being about 3 henries. The composite 

 set serves to separate the telephone currents from those of the two 

 telegraph circuits by "filtering" action or frequency discrimination, 

 the telegraph employing a frecjucncy range below that of the tele- 

 phone. 



The oscillograms which are shown in Fig. 2 illustrate the wave 

 shape and magnitude of crossfire impulses in comparison with the 

 normal operating currents, in a typical composited large gauge cable 

 circuit. Trace A shows the wave-shape of the normal current in 

 the line at the sending end (reduced to about one-seventh as com- 

 pared to the other waves), and B shows the current in the receiving 

 relay at the distant station. Although it is not outstanding in the 



