. American telephone practice . reason why such a jack should not be made to give per-fectly satisfactory operation. A system wherein the second method of supervisory signaling isemployed will now be considered. In this it will be remembered thatthe supervisory signal at the private branch exchange board, and notthat at the central office, is under the control of the private branchexchange subscriber. A private branch line circuit is shown at the left of Fig. 313. Thesignal in this is of the mechanical or gridiron type, already discussedin a previous chapter. This line is shown connected with
. American telephone practice . reason why such a jack should not be made to give per-fectly satisfactory operation. A system wherein the second method of supervisory signaling isemployed will now be considered. In this it will be remembered thatthe supervisory signal at the private branch exchange board, and notthat at the central office, is under the control of the private branchexchange subscriber. A private branch line circuit is shown at the left of Fig. 313. Thesignal in this is of the mechanical or gridiron type, already discussedin a previous chapter. This line is shown connected with a regular exchange line at theright of the figure, the connection being made through the cord cir-cuit at the branch exchange, the private branch trunk, and the regu-lar A operators cord circuit at the main exchange. The arrange-ment at the main exchange end of this trunk is that of the standardKellogg line circuit already described. At the private branch end anordinary ring-down drop is used for the signal, this being bridged. FIG. BETWEEN PRIVATE-BRANCH AND CITY 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. Batte
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