708 BELL SYSTEM TECHNICAL JOURNAL 



small percentage of outlying rural offices. For studies involving the 

 use of power furnished from a telephone power plant, it will, of course, 

 be necessary to consider the cost of the machine and battery equipment, 

 of the floor space and of the operating attendance in addition to the 

 cost of the "raw material" power as purchased. A fair overall 

 figure, including these charges, might approximate 30 cents per kw.-hr. 

 for a typical dial office, or 40 cents for a typical manual office, the 

 higher charge for manual offices, in general, being accounted for by the 

 fact that the quantities purchased and used are less, involving some- 

 what higher purchase price and overhead. It should, of course, be 

 appreciated that the amounts will vary considerably with local con- 

 ditions including the type of equipment used and the "load factor," 

 or the distribution of load throughout the day and night. In most 

 telephone power plants this factor is unfavorable for low cost power 

 since most of the traffic is concentrated within a few hours of the 

 twenty-four. The cost of energy varies also during the lite of the same 

 office, being higher during the early years and lower when load on the 

 power equipment more nearly approaches capacity. 



II. Some Developments to Meet the Power Plant Problems 



The objectives toward which development work is directed are 

 improved service, reduced cost, simplification of installation and de- 

 creased maintenance. Under these headings one of the most impor- 

 tant developments at the present time is the use of commercial type 

 charging generators. 



Commercial Type Charging Generators 

 Charging motor-generator sets furnish most of the energy used in 

 telephone power plants. Up to the present time "telephone genera- 

 tors" have been built to give an electrically smooth direct-current 

 output which will not cause interference with conversations when 

 furnishing current to the telephone circuits. They have also been 

 made mechanically quiet so as not to interfere with nearby testing. 

 They are quite special in construction, including smooth core armature 

 and brass gauze brushes, and are subject to certain limitations which 

 make them larger and considerably more expensive to build than or- 

 dinary machines of the same capacity. 



Filters consisting of choke coils and high capacity electrolytic 

 condensers have been developed, and with these filters commercial type 

 charging generators can be used to float or charge the central office 

 battery, and this type of generator is now being made available. 

 The purpose of the filter is to make the current from the discharge 



