POWER PLANTS FOR TELEPHONE OFFICES 707 



electric power supply, all the equipment must be operated from pri- 

 mary batteries. The "magneto" office, so-called, is one which serves 

 "local-battery" subscribers each of whom has dry cells and a hand 

 ringing generator or magneto. In the larger "common-battery" 

 systems all the power for both talking and signaling is provided from 

 sources at the central office common to all subscribers. 



In dial offices it is evident that more power equipment is required, 

 since the processes of connecting through the circuits are performed by 

 machine instead of by operators. 



Cost of Power 



The cost of power as purchased from the public service companies 



varies largely, depending upon the location, the amount purchased and 



to some extent upon the characteristics of the load. In large cities, 



power is billed at from 2 to 5 cents per kw.-hr. Usually a sliding 



Fig. 4 — Frequency changer set for breakdown service, 650-h.p., 25-cyc!e, 2,200- 

 volt to 600-kv.-a., 60-cycle, 220-volt. Emergency power controlled by 11,000-volt 

 truck switch. 



scale is offered and the lower figures apply to purchases of alternating 

 current in large quantities. Cities near sources of soft coal supply or 

 near large water power developments get cheaper rates, in some cases 

 being nearer 1 cent than 2. In small offices, power usually costs 

 between 5 and 10 cents per kw.-hr., running as high as 15 cents in a 



