CBS is short for Central Battery Signalling which superseded the Magneto system.  The central battery being used for signalling only.

Diagram C.B.S. 3 is dated 1905.

CBS systems, of which there were three types, were designed by the Post Office and from 1923 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 CBS telephone had a local battery (two cells) to power the microphone and a the central battery in the exchange supplied power for signalling purposes.  Central Battery systems used the battery in the exchange for both signalling and transmission.

The CBS No. 1 was superseded by the CBS No. 2 and 3.  There were many reasons for this, but in practice the CBS No. 1 was found lacking due to unreliable earth provision at the customers premises and the 250 ohm bell ringer being part of the signalling system, which reduced the subscribers line to a loop resistance of only 500 ohms.  The CBS No. 1 floor pattern also used a combined lamp and relay unit on the subscribers line circuit, which was difficult to access for maintenance.

The CBS No. 3 operation was similar to CBS No. 2 but the system capacities were greatly reduced and consisted of small wall pattern switchboards with an ultimate capacity 2 or 5 junctions and 20 subscriber circuits.

Smaller CBS switchboards could be found in rural areas, residing in Post offices and the occasional private house.

The system is contained within a wooden switchboard or Multiple sections and the range extends from a small four line wall pattern model to a large floor pattern multiple exchange with a maximum capacity for 800 lines.  The switchboards are connected to a main trunk switchboard by means of Junctions and these circuits terminate on jacks with an associated calling indicator.  The subscribers line circuits also terminate on jacks and have an associated calling indicator which can be either "eyeball" or "drum" types.  The CBS No. 2 switchboards use a "disc" type indicator.

Connection of Junctions and Subscriber lines is by means of cords and these have associated supervisory indicators, generally of the "eyeball" type.  The cords are in pairs and are plug ended.  Larger switchboards use lamp signalling, which allows jacks to fitted per panel.  Ringing is by hand generator.

A CBS No. 1 Multiple switchboard suite is made up of two panel switchboard sections with the subscriber line and outgoing junction multiples every four panels.

Both the CBS No. 1 and No. 2 can be connected to Automatic exchanges.  When junctions to Automatic exchanges are provided a dial is fitted and on the CBS No. 1 special dialling jacks are associated with each outgoing junction.  On the CBS No. 2 it is possible to dial through the calling plug and a special dial key is provided for this purpose.  The CBS No. 3 had no provision for Automatic working.


All CBS switchboards that used primary batteries (24 volts DC) had a Milliammeter.  The Milliameter 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 system was designed to work in rural areas, where in some places there was no mains power supply.  The power requirements at the switchboard were reduced to a bare minimum by installing transmitter batteries at the customers instrument.  Where there was no mains supply the batteries were primary cells and required considerable maintenance, but if mains was available, a power unit would be installed and the maintenance overhead reduced.

Generally the main batteries were installed in two banks, working main battery and reserve battery, which were connected to the switchboard via a changeover switch.  No reserve batteries were installed on the smallest 10 line switchboard.  Each battery set consists of 20 cells in series (24 volts), but where current consumption was large two or more 20 cell battery sets could be joined in parallel on the working main battery. 

The battery change over switch was used to switch over the batteries if they went flat or required maintenance.  The change over switch can be seen in many of the pictures below either on the front or on the right of the switchboards.

There were also three other 2 cell sets of batteries and these powered, the operators transmission circuit, the operators reserve transmission circuit and the alarm bell circuit.

Telephones used on the CBS systems

The telephones needed a pair of batteries to power the transmitter and these would be either two R40 cells or two Leclanché cells, each cell providing 1.5 volts.

Telephones generally used on CBS systems are:- Telephone Nos. 3, 4, 11, 59, 69 and 88.

Party Lines
Party lines were also catered for and these could consist of 2 to 12 subscribers.  Party line subscribers are divided into two groups for exchange ringing purposes.  The stations in one group are known as X stations and in the other group as Y stations.  On two subscriber party lines there is only one X station and one Y station, but on 12 subscriber party lines the number in each group may be any number up to 6.

On a 2 subscriber party line an X station telephone is joined up in the same way as that of an exclusive line subscriber - that is, with the bell connected between the "A" line and earth, but at a Y station the bell is connected to the "B" line and a condenser is inserted between the bell and earth.

On 12 subscriber party lines the bells at all the X stations are connected to the "A" line and those at all the Y stations to the "B" line. At all the Y stations and at all the X stations except the last one, however, a condenser is inserted between the bell and earth at each station. At the last X station, which should be the station most distant electrically from the exchange, the condenser in the bell circuit is omitted in order that a clearing signal may be given to the exchange in the ordinary way at the close of a call from any of the stations.

The exchange rings 2 subscriber party line subscribers selectively and 12 subscriber party line subscribers semi-selectively, by code rings. An X station is called by ringing over the "A" line of the circuit and a Y station by ringing over the "B" line. The normal connections of the exchange ringing device are such that when a cord circuit calling plug is inserted in a subscriber's jack and a ringing key is depressed, bells connected to the "A" line will be actuated. X stations are, therefore, rung in the same way as exclusive line subscribers, but to enable stations with bells connected to the "B" line to be called, a reversal of the ringing connections is necessary. To provide for this a ringing reversing key is fitted on each operator's position in non-multiple exchanges and in many multiple exchanges, but in modern exchanges of the latter type this special key is dispensed with by allotting two jacks per multiple in the subscriber's multiple to each party line circuit. On one of these jacks the tip and ring connections are reversed. X stations can, therefore, be rung on one jack and Y stations on the other in the ordinary way.

On 2 subscriber party lines each subscriber hears only the rings intended for his own station. On 12 subscriber party lines each X station receives a ring for any X station, and similarly each Y station bell is rung whenever the call is for a Y station. Each station should, however, take notice of and reply only to its own distinctive number of rings.

PBX Working
CBS subscriber switchboards were available but the extension telephones on the PBX had ringing generators, which were used to signal clear down to the PBX operator.

CB PBX Switchboards could also be connected the CBS exchange by the use of an Unit, Auxiliary Apparatus No. 18.

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 four 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 2uF 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 (two cells) 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 Bellset No. 15.

Ian Jolly comments
I remember as a youth, in my first year, I was with the lineman who looked after Rossett CBS No. 2 exchange between Chester & Wrexham - we spent many hours visiting subs with ‘faint speech’.   We would replace the two R40 cells or if they still had the wet cells, usually a case of topping up with water or changing the zinc rod.   CBS No. 2 exchanges were sometimes run off a bank of wet Leclanché cells or if there was mains available they used secondary cells charged from the mains.  Rossett CBS No. 2 was in the front bedroom of a terraced cottage - No 3 Sunny Villas -  worked by the people who lived in the house during evenings and nights and during the day by two operators who came from Wrexham auto-manual exchange on the bus.  It was in a private house because the exchange dated from National Telephone Company days.  In those days the village post office was on other side of the road with a K1 telephone kiosk outside.

The CBS No. 3 was used on small exchanges which could only have junctions to a manual exchange as there was no provision for dialing out from the switchboard.  The GPO had 472 of these exchanges, the last of which survived in use until 1964.  This is the only one to have survived - it was ‘Glenmoriston’ exchange installed in 1925 in the village post office in Invermoriston on the ‘Road to the Isles’ at the south west end of Loch Ness and survived in use until replace by a UAX12 exchange in 1955.  Originally it was a 3+5 board until it was expanded to its maximum of twenty subs around 1939.  See pictures below for more details.

GPO Exchange Codes
These are the codes, used in the List of Exchanges, to indicate the type of equipment in use.

Type of Exchange 1930 1936
Central Battery Signalling, Multiple B10 CBSm
CBS No. 1 - Lamp Signalling, Non-Multiple BL1 CBS1
CBS No. 1 - Indicator Type, Non-Multiple BX1 CBSL1
CBS No. 2 - Non-Multiple BL2 CBS2
CBS No. 3 BX3 CBS3

Switchboard Types

CBS No. 1 Switchboards (Most common types)

Designation Type Dimensions Subscribers Junctions Cord Circuits Features
ft    ins
ft    ins
ft    ins
Equipped Capacity Equipped Capacity Equipped Capacity
2+4 (10) CBS207 Wall 0   11 0    8 1    9 4 8 2 2 2 2 Eyeball inds
5+20 (45) Wall 1    8 1    1 2    2 20 40 5 5 5 5 Drum inds
5+10 (45) Floor 1    8 2    2 5    1 10 40 5 5 10 10 Eyeball inds
5+20 (45) CBS333 Wall 1    8 1    3 2    2 20 60 10 10 7 10 Drum inds
5+60 (135) Floor 1    8 2    2 5    1 60 120 5 15 14 14 Drum inds
20+60 (140) Floor 1    8 2    2 5    1 60 120 20 20 14 14 Lamp inds
CBS 1 Multiple Floor 1   11 3    5 6    5 60 200 30 70 17 17 Lamp inds


CBS No. 2 Switchboards (Most common types)

Designation Type Dimensions Subscribers Junctions Cord Circuits Features
ft    ins
ft    ins
ft    ins
Equipped Capacity Equipped Capacity Equipped Capacity
5+20 (50) CBS473 Floor 1    9 2    3 4    7 20 40 5 10 7 10 Disc inds
5+20 (70) CBS473 Floor 1    9 2    3 4    7 20 60 10 10 7 10 Disc inds
20+60 (160) CBS481 Floor 2    2 2    3 4   11 60 100 20 60 12 14 Disc inds
30+60 (160) CBS481 Floor 2    2 2    3 4   11 60 100 30 60 12 14 Disc inds
20+60 (240) Floor 2    1 2   10 6    4 60 180 20 30 14 16 Disc inds


CBS No. 3 Switchboards

Designation Type Dimensions Subscribers Junctions Cord Circuits Features
ft    ins
ft    ins
ft    ins
Equipped Capacity Equipped Capacity Equipped Capacity
2+5 (22) CBS488 Wall 1    4 1    3 2    8 5 20 2 2 3 5  
2+20 (22) Wall 1    4 1    3 2    8 20 20 2 2 3 5  
5+20 (25) Wall 1    4 1    3 2    8 20 20 5 5 3 5  


Switchboard Pictures

CBS No. 1 - 2+4 (10)
Picture dated 1921
CBS No. 1 - 5+20 (45)
CBS No. 1 - 5+10 (45)
Picture dated 1926
CBS No. 1 - 20+60 (140)
Picture dated 1921
CBS No. 2 - 5+20 (50)
Picture dated 1924
CBS No. 2 - 30+60 (160)
Picture dated 1928
CBS No. 2 - 5+20 (50)
Internal view with the front swung open
Picture dated 1924
CBS No. 2 - 30+60 (240)
Picture dated 1926
CBS No. 3 - 2+5 (22)
Picture dated 1926


CBS No. 1 at Grantham
Picture dated 1947


CBS No. 1
Five position multiple suite using lamp signalling


CBS No. 2 at Four Elms, Tonbridge, Kent
The switchboard is a 20+60 (160)
Picture taken 1935


CBS No. 2 at Littlewick Green
Picture taken 1947


Glenmoriston CBS No. 3 Exchange

Picture dated 1924


CBS No. 3 5+ 20 (25)
Note the alarm bell, battery changeover and docket holder switch on the right hand side of the case



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Last revised: December 23, 2021