Rni..iys IX THE BELL SYSTEM 2i 



Rl.l.AVS IN I'iNDAMI NTAL SkI.IX TINC; C'llKLIT 



For another iiUiTcstiiig fxample of the importance of relay opera- 

 tion in machine switching circuits assume tiiat it is desired to select 

 the fourth terminal in a particular group of a final selector bank as this 

 terminal represents a subscriber's line which has been called from 

 another station. A schematic illustrating the principle of the funda- 

 mental circuit for selecting this terminal is shown in Fig. IG. The 

 calling subscriber, by dialing the numl)er of the called station, lias 



Commutator |~ Terminalinq ORfce 



Originating Office 



Fig. 16 — Schematic of selecting circuit 



established the circuit condition shown in this figure through the 

 medium of the sender, so that both the line relay on the final frame 

 in the called office and the stepping relay in the calling otifice are 

 operated. The circuit is also closed through the up-drive magnet 

 of the final frame, and the selector multiple brush is advancing toward 

 the bank terminal to be chosen. As the selector is driven upward, 

 the commutator brush making contact with the first commutator 

 segment, of the particular group desired, places ground on the inter- 

 office trunk in the called office which shunts down the stepping relay 

 in the calling office. This releases the stepping relay, which had 

 established a circuit when operated through the lower relay of the 

 fourth pair of counting relays and had shunted the upper relay of 

 the pair so it would not operate. The release of the stepping relay 

 removed this shunt and permitted the upper relay to operate, locking 

 both relays through the make contact of the lower relay and trans- 



