POWER PLANTS FOR TELEPHONE OFFICES 705 



provided. For other types of toll circuits "voice-frequency" ringing 

 at 1000 cycles must be furnished. 



Message registers in manual ofifices use direct current at 39 volts, 

 coin collect and refunding operations require positive 110-volt and 

 negative 110-volt direct current, while "tones" of approximately 160 

 and 480 pulsations per second are needed for giving various signals 

 to the operators and the subscribers. A graduated tone, like a siren, 

 is required for the "howler" used to call the attention of a subscriber to 

 a telephone receiver left off the hook. Various flashing signals and 

 combined tones and flashes are also used, such for example as the 

 "busy" signal. 



Operation and Maintenance 



In addition to being designed for furnishing power of the required 

 characteristics, the machines and apparatus must operate for long 

 periods with a reasonably small amount of operating attention and 

 maintenance. Due to the narrow requirements being placed by the 

 circuits upon the power equipment and the more frequent readjust- 

 ments required, it is becoming necessary in many cases to furnish 

 automatic voltage regulation. As the cost of labor increases, it will 

 become still more desirable to provide equipment which will largely 

 run and regulate itself. 



Sizes of Power Equipment 



It has been stated at different times by people connected with the 

 telephone companies and also by outsiders that the amount of power 

 required to carry on telephone conversation is microscopically small, 

 if not negligible. This perhaps is true when considering merely the 

 small amount of alternating current which travels over the line and 

 operates the diaphragms of the receivers. The great sensitivity of this 

 instrument permits operation on very small energy. 



There is, however, a large amount of equipment in the central office, 

 including relays, lamps, and other apparatus, which must function in 

 order that this small talking current may be provided and may go from 

 the subscriber who wishes to talk to the subscriber he desires to reach. 

 When this apparatus is multiplied for the thousands or even hundreds 

 of thousands of conversations per day which may be supplied from a 

 power plant servdng two, three or perhaps more central offices, the 

 size of equipment needed becomes quite substantial. In these multi- 

 office power plants several of the largest charging generators each 

 driven by an 80 h.p. motor, as well as a number of smaller charging 

 sets may be required, while two batteries of the largest storage battery 

 cells manufactured may be used in parallel to give the necessary 

 battery reserve. 



