|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.
The standard wall telephone was
the Telephone No. 121CB. With the spread of the automatic system since 1912, when the first automatic
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 CB 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.
The standard table telephone was the Telephone No. 150CB. 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.