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Small automatic private branch exchange

01 January 1926

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This article describes a small automatic P.B.X. made by the Bell Telephone Mfg. Co. of Antwerp. It has a capacity for 21 lines and is supplied in three sizes arranged with 3, 5, or 6 connecting circuits and for 2, 3, or 5 exchange lines respectively. It uses a battery of 24 volts and requires three wires to each instrument. The apparatus is compactly mounted on a frame capable of being fitted on a shelf extension of an ordinary office desk. The extension instruments are of two types, a set for ordinary service and one with call-back facilities by means of which the user can hold an existing connection and originate a new call, afterwards returning to the original connection. Supervisory and busy signals are received on a disc signal fitted in each instrument. A user desiring an exchange connection is connected automatically with the first exchange line if it is free. If it is busy he must dial each other exchange line in turn until he secures one that is free. Incoming exchange calls are reported by the attendant to the required extension, who connects himself with the exchange line concerned by dialling. The attendant is thus prevented from listening on the connection. Tie-line operation on a both-way basis is provided. The circuit arrangements comprise one cut-off relay per line with a call-seeker and a one-motion switch stepped by the dial impulses per connecting circuit.