168 



BELL SYSTEM TECHNICAL JOURNAL 



RECTIFIER 

 TUBE 



3 



•o-o 



100 TO 130 oS 



VOLTS 

 50 TO 60 

 CYCLES <■ 



-nm^ 



} ] Hi 



50 2^ 



LTS VOL 

 •C A( 



160 24 



VOLTS VOLTS 



DC AC 



- FILAMENT 



- + PLATE 



T 



20 TO 40 

 VOLTS DC 



i 



SIGNAL 



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. 



