No. 30 PAGE No. 15


The intermediate telephone enables a subscriber having two premises but only one public exchange line to have access to the line from both premises, and intercommunication between them. The telephone incorporates an extension switch for effecting the desired connections and is installed at one of the two stations, an ordinary extension instrument being used as the other. The magneto telephone described is of the moulded type and comparable in size with the ordinary table telephone.

AN Intermediate Telephone combines the functions of an ordinary subscriber’s instrument with those of an extension switch. It is normally used with one extension telephone when it is desired that two stations (a considerable distance apart) should have inter-communication and access to the same exchange line, the intermediate telephone being located at the point most convenient for controlling exchange calls. For instance, it might be installed in a subscriber’s office where an assistant is normally in attendance, and be connected by an extension line to an ordinary telephone at the subscriber’s residence. With such an arrangement, exchange calls could be made from either point, intercommunication between the office and residence is made possible, and incoming exchange calls would normally be answered by the assistant and switched through to the residence only when desired.

In a previous issue were described two types of intermediate instruments for working to either an automatic or a manual C.B. exchange; one type was for external working, i.e. where the two stations are a considerable distance apart and are connected by a two-wire circuit, the other for internal (short distance) working where battery ringing over a four-wire extension line is economically possible.

This article describes a magneto intermediate telephone designed for external working with an extension telephone of orthodox type, such as would normally be supplied to a subscriber in the magneto exchange area concerned.

The style of the intermediate or “main” instrument, as it is often termed, is based upon that of the Type N.2121 moulded magneto table set. It is known as Type N.2140A and is made in the usual range of colours, i.e. ivory, Chinese red, jade green or the standard black, the associated moulded desk terminal block being coloured to match. The external fittings are chromium plated.

A view of the telephone is given in Fig. 1. There are three push-button keys for effecting the various switching operations, and a star indicator which actuates as a “calling” and a “ring-off” signal. The indicator is restored to the un-operated condition by pressing the button below it. The internal components are mounted on the removable metal baseplate of the telephone, as shown in Figs. 2 and 3. All parts are so arranged that they are easily accessible for maintenance.

Fig 2
Interior view, Front aspect
Fig 3
Interior view, Rear aspect

In addition to the bell in the telephone, an all-metal a.c. bell, suitable for mounting on a wall, is supplied. The manner in which the bells are connected or disconnected by the various switching operations is shown in the explanatory diagram Fig. 4. In this diagram, the internal ringer is Bell 1, and the external ringer Bell 2.

The standard designations on the label below the three push-buttons on the telephone are, reading from left to right:




The letters in brackets are used in Fig. 4 to indicate which buttons are operated, and in Fig. 5 - a schematic diagram of the telephone circuit - to designate the key springs.


Fig 4
Schematic of connections set up by operation of the push buttons

The normal connection for the intermediate (main) telephone is Main-to-Exchange, in order that incoming exchange calls shall be answered by the main station ; in the following description of facilities this connection is therefore considered first.

(a) Main-to-Exchange

This is the receiving condition the internal bell and the indicator are across the extension line, and the external bell is across the exchange line. The main station can initiate or receive exchange calls without further switching. An extension call operates the internal bell and indicator, the latter signifying that a button must be pressed to answer.

(b) Main-to-Extension

The internal bell and the indicator are across the exchange line, whilst the main

telephone and Bell 2 are on the extension line. The operation of the indicator now signifies that a button should be pressed to answer exchange calls.

(c) Exchange-to-Extension

The internal bell and the indicator are across the exchange line, and Bell 2 is disconnected. In this condition the main telephone cannot be used to overhear, or take part in, the conversation, unless both the keys Main to Extension and Main to Exchange are pressed simultaneously, as described under (d).

The indicator operates on incoming exchange calls, and also as a clearing signal when the extension rings off, signifying in either case that the main telephone can be switched to the normal position. Should the handset be left off the main telephone, audibility of the incoming or outgoing ringing signal will not be diminished, as a capacitor is included in the telephone circuit.

(d) Exchange-to-Extension with Main in parallel

This is the non-secret condition. When both of the keys Main to Extension and Main to Exchange are pressed simultaneously, the extension and exchange lines arc connected to the main telephone circuit, thus permitting the attendant to listen to, or take part in, the conversation. This facility can be eliminated, and complete secrecy given, by strapping terminals inside the main telephone.

In the following description of the circuit functions, it should be borne in mind that KT, the centre key on the telephone, has no contacts but serves to restore either of the other keys to normal when they are previously operated. KT is non-locking; therefore, when it has been depressed and released all keys are normal and the facility given is that indicated for KT on the label, i.e. exchange-to-extension.

Fig 5
The telephone circuit

(1) Exchange-to-Extension
In this condition the exchange line is connected to the extension line via KE and KX, with the indicator and Bell 1 across the inner contacts. Bell 2 does not function at this stage, as calls from either the exchange or the extension are signalled on Bell 1. In this condition the indicator will act as a “ring-off” signal to inform the attendant that the call is concluded and that the key KX should be operated to restore the connection to the normal Main-to-Exchange condition.

(2) Main-to-Exchange
With KX springs operated, the extension line, Bell 1 and indicator, are all disconnected from the exchange line which is now through to the main telephone circuit. Bell 2 and the hand generator are connected across the line via the cradle-switch contact; exchange calls can thus be answered or originated without further switching.

A ring from the extension will operate the indicator and Bell 1. As the switch is not in the through condition, this will signify a call and not a ring-off; moreover, it serves to remind the operator to press the button to answer. In this case the button pressed would be KE, giving the following facility:-

(3) Main-to-Extension
KE operated releases KX (if operated). KE disconnects the extension line from Bell 1 and indicator at the break contacts, and connects the main telephone and Bell 2 to the extension line. KX, restoring, disconnects the exchange line from the main telephone circuit and connects the line to Bell 1 and indicator for the reception of incoming exchange signals. Operation of the indicator again signifies that a button should be pressed to answer. Since button IKE has already been pressed, no further operation is required to answer calls from the extension, so the ringing is received on Bell 2.

(4) Exchange-to-Extension with Main in parallel
In condition (1), above, that is, with all keys normal, the main station is excluded from the line. However, as there is nothing to prevent keys KE and KX being operated together, the circuit is arranged to utilize this condition if required, that is, to permit the main station to listen to, or take part in, exchange-to-extension conversation. At the same time, provision is made for this facility to be excluded and complete secrecy ensured.

Key springs KE and KX shown in the centre of Fig. 5, and the alternative straps between auxiliary terminals 3 - 4 or 5 - 6, are concerned with the secrecy feature.

If keys KE and KX both are operated, with terminals 3 - 4 strapped, the main telephone circuit is connected to the exchange and extension lines via the cradle-switch springs, and the attendant may take part in the conversation. The indicator and Bell 1 are disconnected at the back contacts of KE and KX, while Bell 2 is connected in parallel with the through connection.
With terminals 5 - 6 strapped and 3 - 4 open, the main telephone circuit is always disconnected from the line during a through connection, irrespective of whether all keys are normal, as in condition (1), or whether keys KE and KX are operated together.

Application is unrestricted. Instruments for use in tropical climates have ventilating apertures which are gauze screened to prevent the entry of insects, while special cords and varnish impregnated coils are used to withstand conditions of high humidity. The standard and tropical models have p.v.c. insulated connecting wires.


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Last revised: July 18, 2003