POWER PLANTS FOR TELEPHONE OFFICES 



709 



leads of the power plant sufficiently quiet for talking battery 

 supply. It is also possible to use a somewhat higher speed 

 machine which is smaller than the present type. The usual slotted 

 mica commutator construction and self-lubricating carbon brushes are 

 employed. While the mechanical noise tends to be greater because of 

 the higher speed and the carbon brushes, it has been found that this is 

 not a factor of importance under present conditions where power plants 

 can usually be located more or less by themselves and well removed 

 from the Wire Chief's testing equipment. 



Filters 



A new type of choke coil has been developed foi the filter used with 



commercial type generators. It is of the enclosed shell type design 



having short air gaps, using the materials more economically and 



having a higher inductance than the coils which have been available 



Fig. 5 — Part of power room for two large panel units as provided five years ago. 

 Twelve motor-generators for both alternating-current and direct-current service with 

 power switchboard at left and battery control board in background. This was an 

 alternate arrangement to use of large motor-generator to make both kinds of power 

 available. 



heretofore. Associated with this coil is a group of electrolytic con- 

 densers each of which has upwards of 1,200-mf. capacity on 24 volts, or 

 roughly half this amount on 48 volts. 



