INSTRUMENT, EXTENSION, CB No. 21

CONNECTIONS FOR LONG EXTENSION CIRCUIT, OR FOR AN EXTENSION OF A BRANCH EXCHANGE CIRCUIT. APPARATUS SCHEDULE

INSTRUMENT, EXTENSION, C.B. No. 21

The description includes Case for Extension Instrument, C.B. No. 21; fitted with Bell, Trembler, Circular, 25 ohms; Coil, Retardation, Type 9A (80 + 80 ohms); 2 Condensers, metal-cased, 2uf; Switch spring, 5, 8, and 5-point, Telephone, Exchange and Extension; Indicator, Drop, Tubular, 1,000ohms 2/2, or Indicator, Non-polarised, "C," 500 + 500 ohms, 2/2, Generator, Bracket, 3-terminal (Un-mounted); Coil, Induction, l50/1; and Suspender for Micro Telephone.

Telephone No. 28 (with Hook), which is superseded by Telephone, Table, P.B., Transmitter Type, Complete (Telephone No, 4).

Battery, Leclanche, Agglomerate, 2-block, 2-cell, No. 2.
Switch, 6-point, 2-position (see Notes).

The Exchange is automatically called by insertion of peg in the "Exchange" Switch spring; the Extension is called by Generator.


At the Extension a Telephone, Table (or Wall), with Generator, is required; the Generator being used both for calling and clearing. The clearing signal to the Exchange is sent by withdrawing the peg from the "Exchange" switch spring. When it is desired to extend the Exchange Circuit to the Extension Station at night and the length of the line will permit this to be done satisfactorily, it is necessary to ask for "Instrument, Extension, C.B. No. 21, with Switch, 6-point, 2-position."

If it is necessary to add such a Switch locally to an instrument not previously equipped, a " Switch, 6-point, 2-position, engraved 'Night' and 'Day' " should be demanded. It should be fitted immediately over the aperture for the Indicators, and connected to the six screw tags (as shown by the dotted lines in the diagram), the connections shown between tags 2 and 3, and 4 and 5, being removed. The wires must be taken round to the right hand inside the case, and carefully cabled up so as to allow the hinged front to be freely opened.

C.B. Diagram - No. 21.