|In 1955 GEC decided to renumber all their telephones with the prefix
TEL. The MT 41xx, MT 42xx, MT 43xx and MT 44xx series, which was the original part number
range for the
this series of phones were part of this renumbering. Please see the table below for more
The GEC Extension Telephone System
The G.E.C. Extension Telephone System
permits two or three telephones to be associated on one exchange line, for exchange calls to be made or received at any of them and for
intercommunication between them. One instrument - the switching telephone
- is fitted with a four position switch; this allows calls to be extended to the
other two, which are called extension telephones. Inter-telephone calls are initiated by operation of
push buttons and signalled by means of buzzers, or, alternatively, can be signalled over pole routes by
- Calls from exchange can be signalled and answered at the switching telephone; if required, they can be instantly switched to either
- Outgoing calls can be made from any of the telephones, because, at any time, the exchange line can be left switched through. Straps in the switching telephone can be removed to prevent the overhearing of through calls at the switching telephone.
- An intercommunication call between switching and extension telephones is made at the touch of a pushbutton. An exchange call can be held whilst conversation takes place from the switching telephone to an extension. Such a conversation is entirely secret from the exchange caller.
- An extension user can call the switching telephone, even though the extension instrument may be already engaged on an exchange call. Calls can thus readily be transferred from extension telephone to switching telephone.
- In whichever position the switch is left, the bell of the switching telephone is connected across the line to signal incoming
- The system can be used on either automatic or C.B. manual lines, and requires no modification to the exchange.
For the Telephone Administration
The system replaces the old switching bell set, with its multiplicity of parts, by a compact telephone which is simple to use and pleasingly modern in appearance.
Subscribers will welcome the special facilities offered, with resultant increased revenue to the Administration.
The switching telephone is already widely used by the British Post Office, for whom it was designed by the G.E.C., and is standardised as
Telephone No. 248 with Bell Set No.
For the Business House
In business or commercial premises, the system can be connected to any type of exchange line and proves a valuable aid to speed and efficiency. Incoming calls can be filtered through the switching telephone, under the control of a secretary, and be instantly switched, as required, to the executive or his assistant.
Simplicity of Installation
The system is simple to install, requiring only the following apparatus and connexions:-
- At the switching position - a switching telephone, with an external bell to signal exchange calls and a 3-volt battery for inter-telephone calls.
- Between positions - a 4-wire interconnecting line of any reasonable length. Line, bell and battery connexions are made at moulded plastic terminal blocks, which have robust construction and pleasing appearance.
|Switching Telephone - MT 4150
The switching telephone comprises a handset instrument, with black plastic case and
un-tarnishable metal parts, surmounting a detachable base which accommodates the four-position switch, pushbuttons, buzzer, engaged signal and suppression unit.
Four-position Switch - The switch is robust in construction and foolproof in operation. Its four positions are arranged in logical sequence for easy
operation and are shown in the picture to the right.
Pushbuttons - The pushbuttons (G.E.C. Patent No. 493,649) are self-locating in their holes in the case, and are readily detachable from the springsets.
One or two pushes may protrude through the case, as required, without alteration of wiring. When only one push is in use, the second can be neatly stored within the base in a clip formed by one of the contact springs.
Engaged Signal - A transparent aperture in the base displays an engaged signal whenever a conversation is in progress from exchange to extension.
|Extension Telephone - MT 4301
The extension instrument utilises the same case mouldings as the switching telephone, and is entirely self-contained, with the bell (exchange calls) and buzzer (local calls) mounted within the base.
A pushbutton is provided for calling the switching telephone.
The telephones incorporate the following features:-
- Sidetone suppression by means of the well-known Gecophone anti-sidetone circuit.
- Handset cradles are robust and practically unbreakable in normal use. For tropical service an aperture between cradle-lip and case, and a series of holes in the top of the case, permit circulation of air through the interior to minimise
condensation (see picture to the right).
- A closely-coiled wire inside the cradle lip, and protecting gauze over the sound outlet holes prevent ingress of insects.
- A suppression unit is fitted to the buzzers to eliminate interference with nearby radio apparatus.
- Materials and finishes are specially chosen for long service life under tropical conditions.
For signalling exchange calls at the switching telephone, suitable bells are those of the G.E.C. Cat. No.
BE 1100 series (standard) and BE 1200 series (tropical).
Automatic or Manual
The telephones illustrated are for use on automatic systems. Alternative instruments are available for C.B. manual systems, the dial being replaced by a moulded dial dummy. In other respects the C.B. telephones are the
same as the automatic instruments and offer the same facilities.
Magneto calling over long Extension lines
Where line wires between switching and extension telephones have to run over a pole route, the convenience of buzzer calling may be sacrificed in favour of magneto calling with the attendant advantage of two line wires
instead of four between main and extension. A generator fitted in a separate box, together with a combined bell and capacitor, can then be installed at the
main station, and a generator and capacitor at the extension position.
(Numbers in brackets are the later 1955 catalogue numbers)
Switching telephone - 11lbs / 5kgs
Extension Telephone - 7lbs / 3.2kgs
Taken from the GEC Catalogue MTL 2