704 



BELL SYSTEM TECHNICAL JOURNAL 



The low voltage charging generators furnishing the bulk of the 

 power must be electrically quiet so that they will not cause disturbing 

 noises in the telephone circuits. It is, of course, economical to furnish 

 most of the energy required by the telephone equipment directly from 

 the motor-generator sets rather than from the reserve battery, since 

 the conversion efficiency is substantially greater and the battery in- 

 vestment is much less. While various direct -current voltages are re- 

 quired, 24 volts and 48 volts predominate. 



Fig. 2 — Building switchboard to distribute incoming power shown in Fig. 1. The 

 5,000-ampere circuit breakers switch the important load circuits from this panel to a 

 similar reserve panel fronting on a different street. 



The signaling machines and batteries, while of relatively small out- 

 put, are subject to rather exacting performance limitations. Twenty- 

 cycle alternating current of approximately sine-wave form, at nominal 

 voltages of 105, 100, 85, 77, etc., is needed for ringing on different types 

 of circuits. For four-party selective ringing, positive and negative 

 direct currents are superimposed upon the alternating current to secure 

 wave shapes especially suited to the operation of biased ringers. For 

 machine ringing, the 20-cycle current is divided into one or two second 

 ringing periods separated by silent intervals during which direct 

 current is provided for operating the tripping relay and stopping the 

 ringing when the called subscriber answers. 



For ringing over composited toll lines a higher frequency, which will 

 not interfere with telegraph operation, is required and 135 cycles is 



