No. 17 PAGE No. 44

Push-Button Intermediate and Extension Telephones
JULY 1948

The merit of this Company’s intermediate and extension telephones, by the use of which two stations share one line to the public exchange has long been proved, nevertheless the Company’s policy has ever been progressive and recent development work on these instruments has been undertaken to make them more consistent with modern practice.

Fig. 1
Intermediate Telephone, External System
Fig. 2
Extension Telephone, External System

Two systems have been devised, one termed “external working“, for use where the extension and intermediate stations are some distance apart, and the other “internal working“, for conditions where the stations are relatively close, as, for instance, when they are in the same building. The only difference between the systems is that the external sets operate over two wires, with magneto signalling, while the internal sets are fitted with push buttons for battery signalling between extension and intermediate, necessitating two additional wires between these stations. Basic facilities, switching and circuit operation are identical in each case, therefore, the following description of the external type instruments should suffice, with the relative illustrations, to make both systems clearly understood.

Figs. 1 to 4 show the new telephones fitted with P.O. pattern dials for exchange calling but dials of other types can he fitted. Equivalent sets for manual CB working are available. The intermediate telephone resembles the House Exchange instrument and similarly utilizes pushbuttons for switching purposes, while the extension telephone is the standard CB/Auto table set with a plinth to provide extra space for a generator. The moulded bakelite cases are dust and insect proof and components and wiring are suitably finished to withstand tropical conditions.

Fig. 3
Intermediate Telephone, Internal System
Fig. 4
Extension Telephone, Internal System

Improvements on previous methods in the design of the new circuits have been made to provide the basic facilities of

British Post Office “Plan No. 7“, as represented schematically in Fig. 5. An additional facility is offered, when required, by which audible indication of the termination of extension-to-exchange conversations is given at the intermediate station.

The group of four push-buttons used for setting up the desired connections can be seen in the top panel of the intermediate telephones in Figs. 1 and 3. These buttons are inter-acting, so that the operation of one automatically releases any other previously depressed.

They are designated as follows:-

Fig. 5—Schematic of Connections set up by the Operation of Switching Buttons

Visual indication that an extension-to-exchange call is in progress is given at the intermediate station by a flag device fitted to a P.O. 3000-type relay inside the set and displaying “engaged” through a window just below the switching keys, when the relay is operated.

The intermediate instrument contains a bell which is actuated by ringing current from the exchange, while a similar bell in a
bakelite wall case which also functions as a terminating point for the outside cables and is connected to the instrument by a flexible cord, is used for incoming signals from the extension. This definite association of each bell with its particular line is an improvement on previous practice in which the rotary key switched the bells from one line to the other.

The extension instrument is self contained, the bell being included in the set, provision is also made for connecting a remote extra bell if required. Alnico magnet generators are used in both instruments, and provide ample ringing current. Other components also, are of standard pattern. The disposition of the apparatus inside the intermediate instrument may be seen in Fig. 6.

Fig. 6
Interior View of Telephone in Fig. 1

The system will work with any standard subscriber’s line circuit on automatic or C B exchanges and no line earthing or unbalanced line condition is used at any stage of operation.

The 50-ohm indicator-relay which is in the line during extension-to-exchange conversations, requires 30 m.a. minimum current to operate, is shunted by a condenser and has a negligible insertion loss of less than one decibel.

The maximum loop resistance between exchange and extension with which satisfactory working is assured depends upon the exchange battery voltage and the resistance of the feeding coils in the transmission bridge, e.g., allowing 30 m.a. for the indicator-relay, a 50-volt system with a 200/200-ohm feeding bridge will work effectively with an 800-ohm line loop, the position of the intermediate station in the line being immaterial.

A battery of from 6 to 9 volts is required at the intermediate point for speech current for extension-to-intermediate conversations.

The following brief description of the circuit conditions set up when the respective switching buttons are depressed, will enable the reader to readily associate the facilities depicted by Fig. 5 with the operation of the intermediate telephone circuit reproduced in Fig. 7:-

Intermediate to Exchange (Button No. 1)
KX contacts operated. KX1 and 2 connect intermediate to exchange line and KX3 completes the dial circuit, Bell in wall case is in circuit for reception of signals from extension.

Intermediate to Extension (Button No. 2)
No key contacts operated. The required condition of connecting intermediate to extension is set up by this button restoring all operated key contacts. Exchange ringing operates the bell inside the telephone.

Intermediate to Extension, Exchange Held (Button No. 3)
KHI operating, connects holding coil to exchange line, thus bridging bell and condenser. Cradle-switch contact in hold circuit opens when hand-set is replaced, as a safe-guard against intermediate omitting to press button No. 4, to complete the connection, or No. 1, to restore the exchange line to normal.

Extension to Exchange (Button No. 4)
KEX contacts operated. KEX1 and 2 connect extension to exchange, with the bell in the intermediate telephone in circuit for the reception of ringing from either end of the line. In this condition a call from exchange will also ring the bell at the extension. KEX3 and 4 are associated with terminals which can be strapped to make extension-to-exchange conversation secret or non-secret from intermediate. Indicator-relay, RI, operates when hand-set is lifted from extension instrument and remains operated during dialling, contact RI1 opening bell circuit to obviate “tinkling“. The additional contact of RI, the buzzer and the small switch for breaking the buzzer circuit, shown by broken lines in Fig. 7, comprise the circuit of the audible signal indicating the completion of an extension-to-exchange call.

Fig. 7
Circuit of Intermediate Instrument (External System)

As a safeguard against the simultaneous depression of more than one button, preference is given to No. 4 so that, unless this is actually released, the operation of additional buttons will be without effect; also, with button No. 4 operated, there can be no interference with an extension-to-exchange conversion by the manipulation of the dial or generator at the intermediate point. No mis-operation of the buttons will prevent exchange signals from being received if the hand-set is on the cradle.

Fig. 8
Circuit of Extension Instrument
(External System)
Fig. 9
Circuit of Intermediate Instrument (Internal System)

The diagram of the extension instrument is shown in Fig. 8. The generator of this otherwise standard CB/auto circuit is in 

Fig. 10
Circuit of Extension Instrument
(Internal System)

series with a condenser when the microtelephone is on the cradle, in order to avoid placing a calling condition on the exchange line when extension calls the intermediate and button No. 4 is operated.

The relative circuits for internal working sets are shown in Figs. 9 and 10 and it will be observed that the intermediate telephone is provided with twin ringing buttons, enabling two extensions to be connected in parallel. It is recommended that when two extensions are working, the bell for the reception of exchange ringing should be connected in only one of the extension telephones.

The intermediate telephone contains the A.C. bell for exchange signals and also a DC buzzer operated from the first extension station. For the second extension, a 100-ohm DC bell is mounted away from the intermediate instrument so that there can be no confusion in determining which party is calling.

The extension telephone has a push button for calling and a magneto bell. The battery bell or buzzer for signals from the intermediate is fitted externally near by.


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Last revised: June 21, 2003