690 BELL SYSTEM TECHNICAL JOURNAL 



matic controls are indicated because of their more exact performance, 

 with consequent reduction in variations and in interruptions and their 

 saving in maintenance, particularly with 24-hour operation demanded. 

 Developments are traced, showing an increasing trend toward auto- 

 matic regulation and control of main power supplies, ringing and other 

 signaling energy sources. 



The development of "unit type power plants" for telephone offices 

 is discussed and information is given on a number of standardized 

 plants which operate upon a full automatic or'a semi-automatic basis. 

 These furnish power supply for manual, toll, and telegraph central 

 offices, for magneto offices and for manual and dial system private 

 branch exchanges, also for small dial system central offices. 



Favorable operating experience points the way toward further intro- 

 duction of automatic devices which will place most telephone power 

 plants, except those in the larger dial system offices, in a position to 

 operate themselves over considerable intervals of time. 



