. American telephone practice . Y SUB-SCRIBERS WHEREIN THE PRIVATE-BRANCH SUBSCRIBER CON-TROLS THE SUPERVISORY SIGNAL AT THE MAIN OFFICE, 27 417 418 AMERICAN TELEPHONE PRACTICE. in series with a two-microfarad condenser between the two sides ofthe trunk line. Both sides of the signal circuit are broken at the jackwhen the plug, is inserted. The cord circuit uses two batteries, feeding respectively theanswering and calling cords, through the double-wound magnets ofthe supervisory signals, A and B. These coils are not differentiallywound, and the signals are therefore displayed as long as curren
. American telephone practice . Y SUB-SCRIBERS WHEREIN THE PRIVATE-BRANCH SUBSCRIBER CON-TROLS THE SUPERVISORY SIGNAL AT THE MAIN OFFICE, 27 417 418 AMERICAN TELEPHONE PRACTICE. in series with a two-microfarad condenser between the two sides ofthe trunk line. Both sides of the signal circuit are broken at the jackwhen the plug, is inserted. The cord circuit uses two batteries, feeding respectively theanswering and calling cords, through the double-wound magnets ofthe supervisory signals, A and B. These coils are not differentiallywound, and the signals are therefore displayed as long as current istraversing them. In addition to the supervisory signals and theusual ringing and listening keys, each cord circuit is provided withtwo keys, K and K, each serving, when operated, to cut off batteryfrom its cord and at the same time to close together the two coils ofthe supervisory signal so that they form a bridge across the cordcircuit. Battery may, therefore, be cut off from that plug which isinserted into the trunk FIG. CIRCUIT FOR PRIVATE-BRANCH EXCHANGE SYSTEM. When two private branch lines such as are shown at the leftof Fig. 313 are connected by the private branch cord circuit, thekeys, K and K are left in their normal positions. Current is thenfed to each subscriber for talking through the two coils of the grid-iron signals, A and B, which signals are displayed as long as thesubscribers are talking, but are effaced as soon as they hang up. If,however, the connection is between a trunk line and a main officesubscribers line, as shown in the figure, that cord which is insertedinto the jack of the trunk line has its battery cut off by the key Kor K, in which case the corresponding supervisory signal is ener-gized by current flowing from the main office as long as the con-nection is complete at both ends of the trunk line. It is evident thatthe presence of the signal A or B bridged across the trunk line atthe private branch end will keep the supervisory
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