RELAYS /iV TfllL HULL SYSTF.M 



33 



interrupt the (.inuit ihroujili tlic wiiuiiiij; by Dpeiiin^ .i pair of con- 

 tacts in the operated position to advance the selector by self inter- 

 ruptions. 



Ri:l.\ys in Toi.i. Circiits 



Supervision on all of the longer toll circuits and on most of the 

 shorter ones is provided on what is known as a ringtiown basis. This 



Fig. 23 — Stcp-hy-step rotary switch 



usuallv- involves a ringup relay at each end of the liiir. w liich operates 

 in response to 20-cycle signaling impulses. These impulses may be 

 transmitted over the line from one otifice to the other or they may 

 originate in the same office as the relay and be impressed on the line 

 by the operation of a so-called composite ringer in response to signals 

 of a different frequency. The ringup or drop relay provides the 

 signal in the toll switchboard. It is usually removed from the cir- 

 cuit when the line is taken up by the operator and the supervision 

 is then transferred to the toll cord. 



The toll cord supervisory circuit is shown in Fig. 24 and illustrates 

 a typical condition which has imposed ()articular re(|uirements on 

 the relays involvetl. The signal receiving relay .1 mav' be bridged 



