PUBLIC ADDRESS SYSTEM AND LONG LINES 149 



ably constant at some deflection determined by previous calibration, 

 it is practicable to keep the telephone power within the required 

 limits. Obviously, this same device may also be employed to keep 

 the telephone power constant at any other point in the system. 



While the necessity for keeping the power applied to the toll lines 

 within proper limits cannot be over-emphasized, it should also be 

 noted that this is not sufficient. It is also essential that all parts of 

 the toll circuit, including the repeaters, be maintained at prescribed 

 efficiency so that the power levels at all intermediate points in the 

 circuit will also be kept within proper limits. Long telephone lines 

 are designed with special emphasis on this matter of constant efficiency 

 so that, in general, no special precautions are required when using 

 these circuits in connection with loud speakers. 



In another paper, 4 the "echo" effects which may occur on long tele- 

 phone circuits are explained. When setting up two-way circuits for 

 loud speaker use, it is necessary to pay particular attention to effects 

 of this sort. Furthermore, there is another source tending to produce 

 echoes in circuits arranged for two-way use with loud speakers. This 

 is the tendency for the sound delivered from the loud speaker pro- 

 jectors to enter the sensitive transmitter and be returned to the dis- 

 tant end of the circuit as an echo. Owing to the relatively slow 

 velocity of ttansmission of sound through air the lag in such an echo 

 may be great enough to be serious, although the line is a short one 

 with high transmission velocity. It is, therefore, evident that this 

 coupling through the air between the loud speaker projector and the 

 transmitter must be kept small. If a very sensitive transmitter 

 arranged so that a speaker may stand several feet away from it is 

 employed, this problem becomes even more difficult. 



There is one thing more that remains to be considered: the neces- 

 sity for special operation. When a large number of people are assem- 

 bled at some point to hear an address delivered at a distant point, it 

 is evident that delay in establishing the connections cannot be tol- 

 erated. It is, therefore, necessary to establish such connections ahead 

 of time and it is usually also necessary to set up spare circuits for 

 use in case of failure of the regular circuits. A special operating force 

 is required for checking up the circuits, establishing the connections 

 when required, and making the necessary adjustments. Rehearsals 

 are necessary on important occasions to insure proper functioning of 

 the circuits and proper co-ordination of the handling of the circuits 

 with the programs at different points. 



4 Clark, loc. cit. 



