CENTRAL BATTERY (C.B.) SYSTEM

Technical Pamphlet for Workmen - CB

The first central battery exchange in Europe was opened in Bristol in 1900 and was supplied by Western Electric.

The Central Battery (C.B.) system is defined as a system in which the whole of the energy required for the transmission and signalling is drawn from the exchange.  No batteries or hand generators are used at the telephone end and the calls are automatically signalled, by means of lamps, on the exchange switchboard.  Supervisory lamps indicate to the operator when callers have cleared down.

This system achieves substantial savings and good quality transmission by means of the central power source.  The need for local power and hand generators was only retained for test telephones, large PMBX's, certain extension plans and some private circuits, although today none of these are generally in use.

In the early days all telephones had a transmitter fixed to them and there were different types and makes.  The problem was they were either big, bulky and generally required adjustment to keep them serviceable.  The Carbon Granule transmitter was later invented and these were small and light.  This meant that a hand held device could be used for both transmitter and receiver - thus the handset was born.

But because Carbon Granule transmitters were prone to packing, line transients and noise they were better suited to Local Battery phones or fixed to the phone.  North America, in the early days, did not use handsets and therefore the change to C.B. working was accelerated in towns and cities.  The first C.B. exchange was installed in 1880.

Around 1930 the GPO did a cost benefit analysis of the Carbon Granule transmitter against the early types.  They concluded that all early types should be replaced with the Carbon Granule type which would eventually save money.

Another reason the telephone administrations moved to C.B. was the cost of servicing the local batteries.  A central battery could be charged and maintained easily.

At first, C.B. exchanges could only support telephones at short distances but this was later extended and a line resistance of around 800 ohms was the maximum.  In the case of a line being of a longer length, a local battery telephone could be used.  In the 1950's more efficient telephones were introduced which allowed a line resistance of up to 1000 ohms and this would approximate to a radius of 3 miles around a telephone exchange.

But, in North America where some rural lines could be up to 25 miles long (with multiple users sharing the line)  the Magneto telephone was king and remained so well into to the 1970's in some areas.  Magneto phones used local batteries either inside the telephone or close by, in a box.

Today all telephones are C.B. except that they have the addition of a dialling or press button mechanism and are connected to Automatic systems.

Connecting a CB subscriber to a subscriber on an automatic exchange

GPO Central Battery (Central Battery) Sets

Wall Instruments
The standard wall telephone was the wooden Telephone No. 121CB.  With the spread of the automatic system since 1912, when the first automatic public exchange in Britain was installed at Epson, it is desirable that the telephones used on automatic and Central Battery manual systems should be interchangeable, and this telephone provides this facility.  Behind the notice frame is an aperture for accommodating a dial which is required when the instrument is to be used on an automatic system and the internal connections of the instrument are so arranged that the dial may be readily connected.  Terminals marked l, 2, 3, 4, and 5 are those required for the dial connections.

Most C.B. telephones were similar in design, whether wall or table types; the action of the induction coil connected across the A and B wires, whilst, when the receiver is removed, the wires are looped through the primary winding of the induction coil and the transmitter.  The impulsing springs of a dial would occupy the position marked X in circuit diagrams when a dial is fitted so the telephone could be used on an automatic system.

An earlier form of wall set is the Telephone No. 101; it is similar in appearance to the Telephone No. 121CB, except that no facility is provided for the addition of a dial.  A still earlier form of wall set is the Telephone No. 1.

Table Instruments
The standard table telephone was the Telephone No. 150CB Candlestick telephone.  This instrument is incomplete, and the induction coil, magneto bell, and 2uF condenser are provided by a Bell Set No. 1.  This wooden cased telephone contains space for a dial, required for use when the telephone is connected to an automatic system; when used on a Central Battery system, this space is covered by a dummy dial which accommodates the instruction label.

An earlier form of table set is the Telephone No. 2.  This pedestal type telephone is used with a bell set and the connections are similar to those of the Telephone No. 150 except that no facility for the addition of a dial was provided.  Many No. 2 telephones were converted to No. 150 telephones in the GPO repair factories.
 

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

Last revised: February 25, 2024