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BELL SYSTEM TECHNICAL JOURNAL 



System is 170 lines. At the end of 1928 there were about 3,500 mag- 

 neto offices with approximately 5,500 operators' positions, serving 

 1.1 million telephones. 



Common Battery 



At the end of 1928 there were 2,036 common battery offices, the 

 maximum size office serving 10,500 lines and the average being 3,700 



Fig. 14 — Cable vault in Central Office, St. Louis, Missouri, showing entrance of 

 cables through ducts and connecting to silk and cotton insulated cables extending 

 up through the building. 



lines. There was a total of 46,000 switchboard positions where the 

 subscribers' lines are answered. In addition, there were 13,000 so- 

 called trunk positions which are required where it is necessary to 

 trunk calls from one office to another in areas having more than one 

 central office. 



The trend in development in manual switchboard has been toward 

 performing more of the necessary switching and signaling operations 

 automatically by means of somewhat more complicated circuit and 

 equipment arrangements and less and less by the operator. These 

 changes have resulted in less manual operating labor and in an im- 



