CENTRAL BATTERY (C.B.) SYSTEM
|The Central Battery (CB) 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 then 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 early on 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 1980.
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 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.
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.
Today all telephones are C.B. except that they have the addition of a dialling mechanism.
Last revised: October 07, 2020