TOLL SWITCHBOARD NO. 3 



21 



operation of the talking key. This connects an additional 600 ohms 

 in the sleeve circuit, which releases relays which are held operated in 

 the line circuit and control the lamp. 



^^l^ 



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Line 



Cord 



Locking Line Relays and 

 arrangement for Flashing 

 cord supervisory lamp 

 omitted for sake oF 

 clearness 



Switching Trunk 



Fig. 4 — Schematic: Toll switchboard No. 3 circuits; monitoring and positional cir- 

 cuit keys are not shown 



Composite Ringer Simplified 



In order that the toll lines may be used for telegraph as well as 

 telephone service, composite sets are often connected into the line 

 circuit at each end. These composite sets are electrical filters which 

 separate the telephone and telegraph currents and direct the telephone 

 currents to the switchboard and the telegraph currents to the telegraph 

 equipment. 



When composite sets are connected in the lines terminating in a No. 

 1 switchboard, it is also necessary to connect a composite ringer in the 

 circuit between the composite set and the switchboard. This is nec- 

 essary because the 20-cycle current, which is used as ringing current 

 from the switchboard, is in the telegraph range of frequencies and 

 consequently will not pass through the telephone branch of the com- 

 posite set. The composite ringer substitutes for the 20-cycle outward 

 ringing current received from the switchboard, a higher frequency 

 current which will pass through the telephone path of the composite 

 set. Likewise on incoming ringing signals, the ringer substitutes for 

 the higher frequency current which comes over the line and through 

 the telephone path of the composite set, a 20-cycle current which will 

 operate the ringing relays of the line or cord circuits. A schematic of 

 the composite set and composite ringer, as used with the No. 1 board, 

 is shown in Fig. 5. 



In general, the composite ringer for the No. 3 switchboard has been 

 greatly simplified and made a part of the terminating line equipment. 

 This has been accomplished, as illustrated schematically in Fig. 4, by 

 arranging the line circuit so that a relay may be cross-connected in the 



