CBS is short for Central Battery Signalling which superseded the Magneto system.

CBS systems, of which there were three types, were designed by the Post Office and CBS No's 2 and 3 were standard for all exchanges that were not large enough to justify the provision of Central Battery equipment or where a Rural Automatic Exchange would not meet the requirements.  The CBS systems provide automatic signalling similar to that of the Central Battery system.  The difference being that the telephone had a local battery to power the microphone and that the central battery in the exchange supplied power for signalling purposes.

CBS No. 1
This system used telephones with a 250 ohm ringer connected to earth.

CBS No. 2
This system used telephones with 1000 ohm ringer connected in series with a capacitor across the line.

CBS No. 3
This was similar to CBS No. 2 but had small switchboards with an ultimate capacity 2 or 5 junctions and 20 subscriber circuits.

All CBS switchboards that used primary batteries had a milliammeter. This was used to indicate current drain and when the drain was over permissible limits the operator would plug out the subs circuits one at a time to see which one was leaking to earth.

The telephone still needed a battery to power the transmitter and lifting the phone called the exchange operator.

Central Battery Signalling No. 1 Instruments
In this system a local battery supplies the current for the transmitter, whilst the current required for signalling is derived from a central battery located at the exchange.  The signalling conditions in the No. 1 system are that the A-wire is connected to earth through the gravity switch and magneto bell when the receiver is on its rest, whilst with the receiver removed the A and B wires are looped.  The calling signal is thus given by looping the lines and the clearing signal by earthing the A-wire.

The standard wall set Telephone No. 3 is similar to the Telephone No. 11, but the generator is omitted and the internal connections modified, so that when the receiver is on the rest the earthed magneto bell is connected to the A-wire.  The bell coils are connected in parallel, so as to offer a resistance of 250 ohms only for signalling purposes.

The standard table telephone consists of a Telephone No. 4 and a Bell Set No. 5 which contains the induction coil and 250 ohm magneto bell (500 ohm coils in parallel).  The bell set is fitted at some convenient point adjacent to the position of the table telephone, and is joined up by a length of 4-conductor flexible cord.

An earlier pattern of table set is the Telephone No. 26. This instrument is entirely self contained, except for the local battery.  The magneto bell is of the single dome pattern and its coils are wound to a resistance of 100 ohms.  The cradle rests upon a spring and the adjustment of this spring is somewhat difficult, a good deal of care in bending being necessary to ensure reliability in action.

Central Battery Signalling Nos. 2 and 3 Instruments
In this system the method of signalling is by means of a loop when the receiver is removed from its rest which is substituted by a 1,000 ohm magneto bell in series with a 2 F condenser when the receiver is replaced.  The system is thus the same as the Central Battery system so far as the signalling arrangements are concerned.  The transmitter current is, however, supplied from a local primary battery as in the Magneto and Central Battery Signalling No. 1 systems.

The standard wall telephone was the Telephone No. 69, which is similar to the Telephone No. 59, except that the generator is omitted.

The standard table telephone was a Telephone No. 4 together with a Bell Set No. 15.

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Last revised: March 30, 2020