626 



BELL SYSTEM TECHNICAL JOURNAL 



satisfactory from a transmission standpoint If it did not adversely 

 effect the important factor of received current wave shape. An ar- 

 rangement of the resistances was found which results in the maximum 

 steady-state received current and from oscillographic tests which were 

 subsequently made, this arrangement fortunately appears to improve 

 the wave shape of the received current as compared with that resulting 

 from other possible arrangements considered. It was also found from 

 field trials on a number of practical circuits that this arrangement 



Jp? ^ 



Fig. 1 — Bridge Duplex Telegraph Circuit 



results in improved transmission over other arrangements which have 

 been considered for the regulating resistances. 



Three different locations in the bridge duplex circuit are considered 

 for the regulating resistances. These locations are designated (1), 

 (2) and (3) in Fig. 1 and may be described respectively as follows: 



(1) A single resistance in series with the battery branch of the 



circuit. 



(2) Equal resistances in series with each of the bridge arms. 



(3) Equal resistances in series with the line and the artificial line 



of the duplex set. 



In considering locations (2) and (3), it is assumed that the resistances 

 are in the form of a double rheostat with the movable arms mechan- 

 ically connected to facilitate adding equal amounts of resistance 

 simultaneously. 



It will be seen from the circuit shown by Fig. 1 that of the three 

 locations for the regulating resistance, (3) might be expected to reduce 

 the received current most for a given line current, as that arrangement 

 introduces resistance directly between the receiving relays. However, 

 as that location for the resistances had been in general use, and since it 

 was not at all obvious which of the other two arrangements would be 

 the most favorable from the standpoint of received current, it seemed 

 desirable to set up line current and received current equations to de- 

 termine how the currents would be affected by the resistances in each 



