154 BELL SYSTEM TECHNICAL JOURNAL 



sides of short four-wire circuits. The conditions of balance between 

 the networks and the toll lines prevent more than a very small amount 

 of the direct transmission from each local transmitter from entering 

 the local loud speading receiver circuit at the points where the local 

 circuits connect to the toll line. Practically all of the transmission 

 from transmitter T t to projector group R\ and from transmitter jf 2 to 

 projector group R 2 is delivered through the adjustable by-pass cir- 

 cuits Fi and F 2 , respectively. 



For connections requiring to and fro conversations between three 

 or more points, all of which may be equipped with loud speakers, in- 

 termediate points may be connected to a two-wire telephone 

 circuit by employing the arrangement shown in Fig. 8. A three- 

 winding transformer is inserted in the toll line which is so con- 

 structed that the impedance which it introduces into the circuit is 

 small enough to avoid a serious irregularity. Talking currents are 

 put out on the toll line through this transformer. The received trans- 

 mission is obtained from a high impedance bridge across the midpoints 

 of two of the windings of the three-winding transformer. Amplifiers 

 C\ and Ci introduce sufficient gain to overcome the losses due to the 

 inefficient coupling with the telephone line. The rest of the circuit at 

 the intermediate point is the same as Figs. 6 and 7, the local speaker 

 being heard by his own audience by means of transmission delivered 

 through by-pass F. A modification of the arrangement of Fig. 8 can, 

 of course, be used with a four-wire toll circuit. 



Arrangements for Armistice Day, 1921 



Fig. 9 shows the circuit which was employed on Armistice Day, 1921, 

 when audiences of 100,000 people at Arlington, 35,000 people at New 

 York and 20,000 people at San Francisco, joined in the services at 

 the burial of the Unknown Soldier. This was the first time that audi- 

 ences at more than one distant point were simultaneously addressed 

 from one point by means of the public address system. 



At Arlington three different transmitters T 2 , T 3 and T 4 were used 

 for the different parts of the ceremonies. T% was used for the musical 

 selections, T 3 for the speeches made in the amphitheatre, and T 4 for 

 the speeches at the grave of the Unknown Soldier. Another transmit- 

 ter Ti was provided for the use of an announcer who kept the audi- 

 ences at New York and San Francisco advised of the proceedings. 

 The speech currents leaving these transmitters were brought up to 

 moderate volume by means of amplifiers A% and A\, the former tak- 

 ing care in turn of the three different transmitters employed during 

 the ceremonies. 



