168 



BELL SYSTEM TECHNICAL JOURNAL 



RECTIFIER 

 TUBE 



100 TO 130 

 VOLTS 

 50 TO 60 

 CYCLES 



N5-0 

 O 



o-o 



^15^^5^ 



"1 T 



160 24 



VOLTS VOLTS 



DC AC 



~~\ GROUND 



FILAMENT 



+ PLATE 



20 TO 40 

 VOLTS DC 



Fig. 5 — Schematic of a-c. power supply for terminal. 



Freedom from false operation on speech is obtained by tuning the 

 receiving circuit to the rate of interruption and by providing a slow 

 operating relay such that the signal must persist for several tenths of a 

 second before 20-cycle current is passed to the switchboard. 



Fig. 6 — Terminal panel — front view. 



