716 BELL SYSTEM TECHNICAL JOURNAL 



acting as a balance coil for a three-wire direct-current system and the 

 secondary winding having taps to provide one or more of the four 

 alternating-current voltages used with 20-cycle ringing. The generator 

 can thus serve a toll installation at 105 volts, a dial or manual office 



Fig. 10 — Charging machines with remote controlled equipment at generator. 

 Overhead bus bar system. 



using 100 volts with the alternating-current — direct-current system 

 and two offices, having "superimposed ringing" for party lines, one 

 using 85 volts and the other 77 volts, the voltage used depending upon 

 the type of subscriber sets installed in the district. Positive and nega- 

 tive superimposed currents are obtained by small storage batteries 

 connected in series with the 77- or 85-volt tap of the transformer. All 

 four types of office ringing can be secured from one machine simultane- 

 ously, though more than two is unusual. The voltage is controlled 

 automatically within close limits, regardless of load or of normal 

 variations in the voltage and frequency of the supply power. 



In addition to ringing, the generator supplies approximately +110 

 volts direct-current for collecting coins and —110 volts direct-current 

 for refunding coins, the two voltages in combination also exciting the 

 generator field at 220 volts. 



Brushes bearing on sectional and solid rings mounted on the gener- 

 ator shaft interrupt battery current and provide a high tone of 480 



