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BELL SYSTEM TECHNICAL JOURNAL 



Fig. 5 shows the circuit combination employed when a local ad- 

 dress is to be given, while at the same time the same address is deliv- 

 ered to one or more distant points. In order to allow the local audi- 

 ence to hear the address by means of the loud speakers, the power 

 amplifier B supplying energy to these is bridged across the output of 

 the amplifier A associated with the transmitter T. A volume indicator 

 V, connected across the circuit at the point where the bridge is made, 

 makes it possible to maintain constant volume both for the local loud 

 speakers and for the transmission applied to the toll lines by suitable 

 adjustment of amplifier A. At the toll office means are indicated for 

 connection to two distant points X and Y. Owing to the fact that 

 amplifiers C\ and C% are one-way devices, no inter-actions can occur 

 between lines X and Y or between these lines and the local loud 

 speaking system. The arrangements for reaching the distant points 

 X and Y are similar to the one illustrated in Fig. 4. 



All of the circuit arrangements which have so far been described 

 are arranged simply so that a speaker may address one or more local 

 or distant points. When it is desired that the speaker and the audi- 

 ence at the sending end also be able to hear a speaker at the distant 

 point, more complicated arrangements are required. 



Fig. 6 shows a circuit arranged for such two-way service, the line 

 being operated on the four-wire principle, i. e., two separate transmis- 



Fig. 6 — Twc-Way Four-Wire Connection for Addressing Local and Distant Audiences 



sion paths are provided, one for transmission in each direction. The 

 circuits connecting transmitter 7\ with the projector group R 2 and 

 transmitter Ti with the projector group i?i are similar to the circuit 

 in Fig. 4. By-pass connections Fi and F2 are added at the two ends 

 which allow part of the output of each transmitter to pass into the 

 local loud speakers. These by-pass connections are so arranged 

 that transmission can pass only in the proper direction. Two volume 

 indicators are provided at each end. Referring to the left-hand ter- 

 minal, volume indicator V\ is provided to insure that power is supplied 

 to the toll line within the proper limits of volume, as explained above. 



