|Various extension plan arrangements were available on all telephones prior to the introduction of
Plug and Socket (PST). This is the main reason for there being so many variations of each
telephone style. The introduction of the 700 type telephone reduced the amount
of different telephones needed as this telephone was adaptable and could be fitted with all manner of additional items. To this
end there were effectively only three styles of 700 telephones:-
Single or two button (746 only) = Tele 706 or 746
Four button = Tele 710 or 740
Wall type (could take four buttons) = Tele 711 or 741
Plug and socket effectively killed the special telephone and small telephone
systems took the place of most of the extension plans.
Not all extension plans can be fitted to every type of telephone exchange and the relevant BPO diagram must be referred to for
Extension Plan No. 1
This arrangement provides for one main telephone and from one to six internal extension
telephones connected in parallel. Each extension is provided with a DC trembler bell, the
operation of which is controlled by a press button on the main telephone. The DC to supply
these bells is line powered. The magneto bell is only fitted at the main telephone.
Extension Plan No. 1A
This arrangement provides for upwards to a maximum of five internal extensions in parallel
with the main telephone, with the facility for incoming calls to be received at any
station. Any station calls the exchange direct and no secrecy or inter-communication is
provided between any station. A bell cut off switch is provided on each extension (all
stations have a magneto bell).
Extension Plan No. 1B
This arrangement is the same as Plan No. 1 but without press buttons and extension bells.
It is therefore only suitable where the main bell can be heard at all extensions. i.e.
Extension Plan No. 1C
As Plan 1A except that the extensions have no bell on/off facility
Extension Plan No. 2
This plan allows the connection of 2 exchange lines to many telephones. Each telephone has
a switch that allows connection to only one line at a time. Two telephones are designated
as main telephones and these have their magneto bells permanently connected to their
respective exchange lines.
Extension Plan No. 3
The main can call an extension via a DC bell and the extension can press a button marked
SECRET that disconnects the main telephone from the line, thus affording secrecy.
Extension Plan No. 4
One telephone line provides service at several different points of the house or office. Up
to six telephones, each connected to a plug, can be provided and can be moved from socket
to socket, at will. Calls can be accepted at any point. There was always one fixed bell on
Extension Plan No. 5
Older version of the Plan No. 105. A switch unit or Bellset No's 20, 39 or 44 was used to switch the line to
one of two extensions.
This scheme caters for two internal extensions where communication between
the telephones is only occasionally required. Incoming calls are
received normally at the main instrument; either extension can be called by
pressing the appropriate button and turning the handle of the generator. When
the circuit is connected to a Magneto, Central Battery Signalling No. 2 or
Central Battery Signalling No. 3 exchange, the switch shown dotted replaces the
four-position switch, containing the bell, generator, and indicator. The
extensions call the main by a generator ring, which is received on the bell
associated with the switch. The purpose of the indicator is to show when an
extension is using the exchange line. For night service, calls can be received
on one or other of the extensions, but not both; a switch is fitted at the
selected extension and, when operated, the magneto bell at that extension is
connected to the exchange line, the switch at the main instrument being left in
the through position.
Extension Plan No. 5A
The lay-out of the apparatus is similar to that for Plan No. 5. Certain
modifications to the wiring are made with the object of providing secrecy
Extension Plan No. 7
Older version of the Plan No. 107. A switch unit or Bellset No's 20, 39 or 44 were used to switch the line to
Extension Plan No. 7A
This is similar to Plan No. 7, but certain modifications to the wiring are
made to provide secrecy.
Extension Plan No. 8
Two exchange lines are presented to two main telephones. One extension off of these two
telephones could speak to either line by means of a switch on the phone.
The main telephones have the ability to call the extension via trembler bells.
Extension Plan No. 8A
This version of the Plan No. 8 affords secrecy to the extension telephone. This is achieved by
the use of a switch unit that also has line busy indicators. In this instance the
extension has a magneto bell and the main telephones call the extension, one via a DC bell
and the other via a DC buzzer.
Available on Auto and CB systems only.
Extension Plan No. 9
Connected to two extension of a PBX this plan allows one telephone access to two lines.
Extension Plan No. 10
This is only applicable to a private branch exchange extension,
and provides facilities whereby the extension can be made secret against the
P.B.X. when connected to the exchange line. Each exchange line
is connected through a jack at the extension, and the extension line incoming
from the switchboard also terminates upon a jack. The extension telephone
terminates upon a plug and, by inserting the plug into the jack of an exchange
line, the switchboard is disconnected, and the extension telephone connected
direct to the exchange line.
Extension Plan No. 11
Similar to Plan No. 1A except that each telephone can call the next extension via a DC bell.
Only the main has a magneto bell.
Extension Plan No. 12
Similar to plan Nos. 1A and 1C except that the main has a switch that affords secrecy from
Extension Plan No. 12A
As plan No 12 except that main and extension telephones can call each other via DC bells.
Extension Plan Nos. 105 & 105A
Consists of a main telephone and two extensions (internal or external) connected in
parallel. A Plan set No. 625 is installed as a plinth to the
main telephone. This allows more complicated switching and signalling arrangements to give
the following facilities. Internal extensions can call each other directly without
intervention of the main. There is no direct signalling between external extensions, but
inter-communication between them may be established with the assistance of the main.
Incoming calls are usually received at the main, but can be received at either extension
Exchange calls to or from the main cannot be heard at the extensions. Plan No. 105A
provides for secrecy for extension to exchange calls from the main. A white lamp glows on
the main (Plan Set 625) when the exchange line is being used by
an extension. The main line can hold the exchange line and speak to the extension without
being heard on the exchange line (a red lamp glows during this condition). Bothway press
button signalling is provided between main and extensions.
Extension Plan No's 107 & 107A
Similar to Plan No's 105 & 105A but only one extension is provided.
Magneto System, Plan No. 7
The three-position switch employed is known as an inter-through switch, and
there are various forms of switch which will enable the required connections to
be made, but it will here suffice to describe the Ericsson pattern (Switch N.T. 14). The four springs rest on the inner contacts, and in the positions
shown the exchange line is connected through to the extension circuit, the
magneto bell being bridged across the lines. When the switch lever is moved over
to the right, the upper left spring makes contact with the top contact, whilst
at the same time the lower right spring is brought into contact with the bottom
contact. In this position the exchange line is connected to the main set
instrument, and the magneto bell is connected across the extension line.
If the handle of the cam be moved to the left the bell is placed across the
exchange line. In addition to the ordinary function of the magneto bell, an
extension of the hammer is so arranged as to release a drop shutter. This
shutter is restored to normal by a projection on the switch handle. If the main
station has been left unattended, the fact that a call has been received will be
obvious from the fact that the shutter is dropped. The connection shown dotted
is made when the shutter falls and is required if automatic clearing to the
exchange is effected by earthing the centre point of the magneto bell, a system
It is sometimes necessary to prevent the extension circuit from being
connected to the exchange line; for this purpose Switch N.T. 15 is employed,
having only two positions. Two extra springs and contacts are added to render it
electrically impossible to connect the exchange line to the extension. Similar
arrangements to those of Switch N.T. 14 can be made by using three pairs of
5-point jacks (Jack No. 14) and two mechanically-connected three-way circular
plugs (Plug 601).
Central Battery Signalling No. 1 System, Plan No. 1
All the telephones are connected in parallel to the exchange line and incoming
calls are received at the magneto bell at the main instrument, but any one of
the stations can call the exchange by lifting the receiver. A call having been
received at the main telephone for one of the extensions, the press button at
the main set in connection with that extension is pressed, thereby ringing the
trembler bell at the extension and indicating that attention is required. For
example, the main instrument may be placed in the general office of a firm of
solicitors or merchants, and the extensions in the private offices of the
partners or managers. Since the magneto bell is normally earthed, it is
necessary that it shall be disconnected when any station lifts the receiver, and
this is provided by taking the connection through contacts on each of the
telephone gravity switches. The extension call bells are actuated by the main
set speaking battery, together with the call extension speaking battery, which
are connected in series when the relative press button at the main telephone is
depressed. The trembler bell and the induction coil are contained in a separate
wooden case (Bell Set No. 6) when table telephones (Telephone No. 4) are used.
Central Battery Signalling No. 1 System, Plan No. 7
The four-position switch used in this plan is known as Bell Set No. 7, and the various combinations of connections are determined by the four positions of the pointer of a rotary switch.
Position 1: The main set (Telephone No. 3) is connected to the exchange line
and the extension to a magneto bell in the bell set. Thus the main set can call
or be called by the exchange, and the extension can call the main station.
Position 2: The main instrument is connected to the extension line, which is
called by the generator provided in the bell set. The exchange line is held by
the magneto bell placed across the lines (i.e. the switching over of the main
set to the extension does not disconnect the exchange line), and thus does not
give a clearing signal while the extension is being called by the main station.
Position 3: The main instrument is connected to the extension line and the
exchange line to an earthed magneto bell for the receipt of calls from the
exchange during the progress of a conversation with the extension.
Position 4. The extension is put through to the exchange, and in order to
give a clearing signal to the main set on conclusion of the conversation, a
4,000 ohm eyeball indicator is bridged across the lines in series with a magneto
bell. When the extension restores the receiver, thus earthing the A-wire through
the magneto bell, a clearing signal is given to the exchange, and on withdrawal
of the plug the relay and battery are again joined up. The current flowing
through the eyeball indicator is insufficient to operate the line relay (biased
against 10 mA), but operates the indicator. The main station cannot overhear the
conversation, but if secrecy against the main station is not desired, the main
set is placed in bridge by joining the terminals marked A and A1, and B and B1
respectively. The night switch is for the purpose of disconnecting the eyeball
signal, and, consequently, the local alarm circuit, when the extension is left
connected to the exchange line during periods for which the main instrument is
Central Battery Signalling No. 1 System, Plan No. 4
The circuit arrangement is similar to that described for Central Battery
circuits (see page 331), using Jacks No. 13, Plug No. 407, and Telephone No. 90
or Jacks No. 9, Plug 701, and Telephone No. 4.
Central Battery Signalling Nos. 2 and 3 Systems Plan No. 4
The arrangement of the plug and jacks is shown in (diagram) 235. The portable
telephone is a Telephone No. 4 connected by means of a Cord Instrument No. 422
to a Plug No. 404. Jacks No. 21 are used at the extension positions and a Bell
Set No. 5 is permanently connected to the first position. The magneto bell shown
dotted may be omitted at any position from which the magneto bell in the Bell
Set No. 5 is audible; when this is done, springs 1 and 4A on the jack are
connected together. When the instrument plug is inserted in any jack, the
connection between spring 4 and spring 4A is broken, so connecting the magneto
bell at that position in circuit, in addition to the magneto bell of the bell
set. (diagram) 236 illustrates the connection made when the plug is in a jack at
an intermediate or last position.
Central Battery System, Plan No. 1
A Telephone No. 1 is used at the main set, and the extensions, which in
(diagram) 237 are table instruments, use the induction coil and condenser of the
main set. The bells for gaining the attention of the extensions are joined
across the A and B wires, when the press buttons are operated and are rung by
the exchange central battery.
If one of the extension telephones is of the wall type, the magneto bell is
disconnected and it is connected in parallel across the exchange lines. In this
case the extension uses its own induction coil and condenser and does not make
use of that at the main set, as is the case of table instruments. The apparatus
- Main set: Telephone No. 1 or equivalent (two press buttons).
- Extension 1: Telephone No. 1 or equivalent (magneto bell disconnected),
Bell 13A or 56A.
- Extension 2: Telephone No. 2 or equivalent (uses main set induction coil
and condenser), Bell 13A or 56A.
Central Battery System, Plan No. 2
It is often desired that the extension station may speak without the
possibility of the conversation being overheard at the main set located, say, in
the general office. The principle of the arrangement consists in
placing both telephones in parallel on the exchange line through a six, point,
two-position switch, which, when moved to the secrecy position, cuts off the
main instrument. Both instruments are provided with their own induction coil and
condenser, and since failure to restore the secrecy switch prevents calls from
being received at the main set, the switch in this position joins up the magneto
bell in the extension bell set, so that a ring will be received on this
instrument. The apparatus required is:
- Main set: Telephone No. 1 (press button).
- Extension: Telephone No. 2, Bell Set No. 1, Bell 13A or 56A, Switch
six-point two position.
Central Battery System, Plan No. 4
To avoid the use of extension circuits and switches, a table telephone (No.
2) is connected to a Plug 407, and wall sockets are fixed in the various rooms
where it is required to use the telephone (e.g. dining-room, study, and
bedroom). A Bell Set No. 1 and a socket (Jack No. 13) is fixed in the
dining-room, and in each of the other rooms a socket and a magneto bell. These
bells are, however, short-circuited until the plug is inserted. Thus, the
instrument may be carried to and connected in whichever of the rooms the
convenience of the moment dictates. The arrangement will be readily appreciated
from a consideration of (diagram) 239. A later arrangement consists in the use
of Plugs 404 and Jacks No. 3 and 21.
Central Battery System, Plan No. 7
The arrangements previously described do not provide for intercommunication
between the extensions and, where this facility is required, recourse must be
had to switchboards. It is, however, possible to provide intercommunication
between the main set and one extension by the aid of Bell Set No. 4, The
conditions which the arrangement must fulfil are:
- Main set connected to exchange. Extension connected on extension bell at
- Main set connected to extension. Exchange connected on extension bell.
- Main set connected to extension. Exchange line held.
- Exchange line connected to extension.
These conditions are effected in the four positions of the switch on Bell Set
4, and are shown in skeleton in (diagram)s. 241-244. The Bell Set No. 4
((diagram) 240) comprises a 4-position switch, indicator 1,000A, generator No.
4, 1,000 ohm magneto bell, 1F condenser, and a 2-cell battery. The main
instrument may be a Telephone No. 1, but at the extension set a generator is
required for calling the main set, and therefore the apparatus fitted consists
of a Telephone No. 1 or No. 2, with Bell Set No. 1, the line circuit passing
through the back contacts of a generator No. 4, so that when the generator is
used the extension telephone is disconnected. The switch pointer on the bell set
normally remains at position 1, and thus a ring from the exchange is received on
the bell of the main set. Raising the receiver extinguishes the supervisory lamp
at the exchange in the ordinary way. The extension is connected to the extension
bell, and thus provision is made at the main set for the receipt of a call from
the extension. The extension is called by the generator fitted in the bell set.
In position 2, the positions of the main set telephone and the extension
bell, with its condenser, are reversed; but, in addition, a 2-cell battery is
placed in the circuit to the extension to furnish speaking current between the
main and extension. Calling, in both directions, is by generator. A ring from
the exchange is now received on the extension bell.
Position 3 holds the exchange line while speaking to the extension. The
necessity for this condition may arise when the main set receives an inquiry
which necessitates consultation with the extension before replying. The only
change, as compared with Position 2, is that one coil of the bell set magneto
bell is joined across the exchange line, thus preventing the lighting of the
supervisory lamp at the exchange. Position 4 puts the extension through to the
exchange, but inserts an eyeball indicator in the main circuit. At the
conclusion of the conversation, when the extension replaces his receiver it
disconnects the circuit, and thus the indicator, which has been operated while
the call was in progress, falls back to normal. This is known as a negative
clearing signal, to distinguish it from a positive clear, which may be defined
as a signal which is operated to give the clearing indication. Failure to
restore the switch to normal involves the receipt of a subsequent ring from the
exchange on the bell set and on the bell at the extension set. On the Bell Set
No. 4, links are provided so that the main set may, in Position 4, be connected
in parallel across the circuit while the extension is speaking to the exchange,
or, by their removal, secrecy of conversation between the extension and the
exchange is secured.