PAGE No. 9

January 1956

The telephones described in this article are designed for short-distance internal lines where the length of cable required is such that the provision of extra line wires for internal battery ringing does not involve high cable cost. Novel circuitry in these sets simplifies operating procedure for intermediate switching, and ensures that the extension has access to the exchange, whether the intermediate telephone is attended or not, provided that the line is not engaged.

The intermediate telephone is for use in a system where two, sometimes three, people share a common exchange line, access to which may be required by any one of them at any time. Such a system may be likened to a very small private exchange where provision must be made for answering incoming calls and switching them from the exchange to the person required. In other words, there must be the equivalent of a small switchboard, i.e., the intermediate telephone.

Small systems of this kind are economic in line rental charges and have been in demand for many years. The earliest examples comprised two ordinary telephones and a separate bell set containing also an extension switch, the bell set being installed near one of the instruments. Later, a special telephone incorporating the extension switch was designed, and was termed the 'intermediate' telephone because of its location between the other instrument in the system - the 'extension' telephone - and the exchange line.

Such a system has various applications, two of which may be cited

(a) for the executive and his assistant in separate offices in the same building, the assistant having the intermediate telephone

(b) for the physician having an intermediate telephone at the surgery and an extension at his private residence.

In application (a) the distance between the two offices, and the cost of the cable between them, is small, and battery signalling offers advantages which are not outweighed by the cost of the extra wires necessary for the signalling circuit. Short-distance systems of this type are known as 'Internal Systems', although they may not, strictly speaking, be internal, since the telephones may be in separate though not widely separated buildings.

In application (b), on the other hand, the distance between surgery and residence may be considerable and may necessitate magneto signalling, which reduces cable cost by using the same wires for speech and ringing.

The Company's earliest intermediate and extension telephones, current in the nineteen-twenties, were all provided with hand generators. They were metal cased instruments supplied with or without a dial, as automatic or manual c.b. working required. Switching was effected by means of a rotary key incorporated in the intermediate telephone.

The more modern version of this telephone, developed in the post-war period, had a moulded plastic case, an 'Alnico' hand generator and four push-button keys for switching. A battery ringing model developed at the same time had a similar case, but two extra push-button keys to permit the selective ringing of two extensions connected in parallel. These two post-war models were described in Bulletin No. 17, July 1948.

Type N1047H Auto Intermediate Telephone Type N1048H Auto Extension Telephone

Fig. 1
The Old Type Telephones which are now superseded

The latest intermediate and extension telephones described below have been designed solely for internal, i.e., battery signalling systems, and are supplied with or without a dial, for connection to different types of exchange line. The Type numbers of the instruments, and of those which they supersede, are as follows:-

Old Model New Model
Auto Intermediate Telephone N1047H N1051A
Auto Extension Telephone N1048H N1052A
C.B. Intermediate telephone N1322H N1323A
C.B. Extension telephone N1327H  N1324A

Physical Features
For purposes of comparison, the old and new auto sets are shown in Figs. 1 and 2 respectively. These show clearly the reduction in the size of the intermediate telephone (now no larger than the extension telephone) and the improvement in the appearance of both instruments. These normally have black moulded cases, but ivory, Chinese red or jade green plastic mouldings are available if preferred. The trigger type automatic dial has a stainless steel finger plate and standard numbering, as illustrated, unless other numbering is requested.

Type N. 1051A Auto Intermediate telephone   Type N.1052A Auto Extension telephone

Fig. 2
The New Instruments

The instruments are tropicalised; viz: they have varnish impregnated coils, cord conductors and connecting wires insulated with p.v.c., the cords having nylon outer braidings, ventilation boles with gauze screens to prevent the ingress of insects, a dial mechanism fitted with a plastic cover, and metal parts protected from the effects of humidity by special finishes.

It will be observed that the new intermediate telephone has four plastic push buttons; the two in the centre are black and are designated 'Ring 1' and 'Ring 2', respectively. The left-hand outer button is white and is labelled 'Extension', while the right-hand button is red and is labelled 'Exchange'. This is the standard provision and marking. Should there be only one extension, the 'Ring 2' button is spare. Below the dial - or the dial dummy on the C.B. set - is a small star indicator which acts as a supervisory signal to show when the exchange line is engaged by the extension; this indicator is more clearly seen in the interior view of the telephone, Figure 3.

Fig. 3
Intermediate Set with Case off

All the interior apparatus is so arranged on the removable metal baseplate as to afford free access to the parts for maintenance. Adjustments to the buzzer mounted beside the indicator, or to the push-button and cradle-switch springsets at the back end of the plate, can readily he made; cord and wire terminations are also conveniently placed

The buzzer is operated only from the extension telephone, an incoming call from the exchange ringing an a.c. bell in the moulded wall case seen in Fig. 2. This bell set also contains twelve screw terminals for the connection of the line wires and, if required, an extension bell. A flexible cord connects the telephone to the bell set, which is supplied with the instrument under the one Type number.

The extension telephone closely resembles the intermediate telephone in external appearance but has only two push-button keys, one designated 'Ring', the other 'Bell On-Off'. The latter key has an attachment by which it may be pressed and turned clockwise to lock in the 'Bell On' position. This action enables the extension to receive incoming exchange signals, a facility usually needed only when the intermediate telephone, which normally receives such calls, is unattended. The Bell key may be restored to the unoperated ('Off') position by pressing and turning it anti-clockwise.

Fig. 4
Extension Set with Case off

Fig. 4 is an interior view of the extension telephone, which contains both the bell for exchange signals and the buzzer for signalling calls from the intermediate telephone. The leads from the latter are brought in via a cord from a moulded desk terminal block which also contains terminals for connecting an extension bell if required.

Power Supply and Length of Line
Current for ringing and speech circuits is obtained from a 6-volt battery of ordinary dry cells which can he bought locally and is, therefore, not normally supplied with the telephones.

The system will operate on any auto. or manual c.b. exchange line of up to 1,000 ohms loop resistance and does not introduce any earthed or unbalanced line conditions, nor is there any appreciable insertion loss when the star indicator on the intermediate telephone is in circuit.

As has already been indicated, the system is designed for conditions in which the lines between intermediate and extension telephones are relatively short, the maximum line length being determined by economic rather than technical considerations.

The various connections established when the appropriate push-button keys on the intermediate telephone are in the specified positions are shown diagrammatically in Fig. 6, there being four conditions of switching. These constitute Numbers 1 to 4 of the following facilities provided by the system:-

  1. Intermediate to exchange.
  2. Intermediate to extension.
  3. Information call (intermediate to extension with exchange held).
  4. Through connection of exchange to extension on replacement of the intermediate handset, but with incoming exchange calls normally answered by intermediate.
  5. Extension to exchange conversation can be made secret, or non-secret, from intermediate by terminal strappings.
  6. Indication is given at intermediate when the exchange line is engaged by extension.
  7. Provision is made for two extensions, with selective ringing from intermediate
  8. Terminals for connecting an extension bell for the reception of exchange ringing are available at the intermediate and the extension telephones
  9. Extension can always make outgoing calls to exchange, whether intermediate is attended or not.
  10. Extension can, when required, receive exchange ringing with out switching at intermediate.
  11. Extension cannot overhear intermediate to exchange conversation.
Fig. 5
Extension Telephone Circuit

Circuit Notes
The design of the circuits shown in Figs. 5 and 7 reveals a new approach to the problem of providing the facilities usually associated with intermediate extension working; simpler operation with fewer keys is the result. Notable improvements have been effected, firstly, in the method of transfer to the 'through' or 'extension to exchange' connection, secondly, in the way in which the information call, i.e. 'intermediate to extension with exchange held', is obtained. With the new circuitry, all switching at the intermediate telephone can be effected with two keys, whereas four keys were formerly necessary. Switching procedure is thereby simplified and operational reliability improved.

The 'through' connection (Facility No. 4) is set up automatically when the handset of the intermediate telephone is replaced. As a result, the extension is given a through connection to the exchange whenever the intermediate telephone is not on the line. This eliminates a difficulty commonly experienced in earlier systems, in which the extension could be left without service if the intermediate attendant neglected to operate the necessary keys to set up the through condition before leaving the telephone.

The other improvement, i.e., in the 'information call' (Facility No. 3), has been achieved by arranging for the Extension key to interlock with the Exchange key, so that when the latter is pressed for intermediate-exchange conversation the attendant need only press the Extension button to be able both to hold the exchange line and to speak to the extension, with the two keys locked in the operated position.

Operating Procedure
The operating procedure for the intermediate attendant is governed by two simple rules, namely:-

  • For Exchange press the red button.
  • For Extension press the white button.

These rules apply in all circumstances, as will be seen in the description of the procedure. Here, then, is another advantage of the new system over its predecessors, since it was formerly necessary for the attendant to remember two ways of calling the extension, one with the exchange held and another without.

The following description makes no reference to dialling or similar operations which are common telephone practice.

Fig. 6
Connections Set Up by Operation of Switching Buttons

Intermediate Telephone
Exchange ringing is always signalled on the bell in the wall case. The star indicator should be observed to verify that the line is free (indicator  unoperated) before the handset is removed to commence operations:-

  1. To call exchange : press red button.
  2. To call extension : press white button, then the appropriate black 'ring' button.
  3. Information call to extension during intermediate-to-exchange conversation: with red button already down, press white button and appropriate 'ring' button.
  4. To transfer exchange call to extension: replace handset when extension is ready to accept call.
  5. To return to exchange line after an information call: press red button to release white button.
  6. To take part in a through conversation: after operation 4 above, take up handset.
  7. Intermediate unattended: before leaving telephone unattended, inform extension so that calls may be received at extension telephone.

Extension Telephone

  1. To call exchange: follow normal telephone procedure.
  2. To call intermediate:
    (a) If exchange is auto, take up handset, press 'Ring' button and await reply.
    (b) If exchange is c.b. manual, press 'Ring' button and await responding buzz from intermediate before taking up handset, thereby avoiding needless calling of exchange operator.
  3. To receive exchange calls when intermediate is unattended: press and turn 'Bell' button to 'On' position. To restore, turn button in reverse direction.
Fig. 7
Intermediate Telephone Circuit

Circuit Explanatory
The following notes on circuit operations during switching procedure refer to the diagram, Fig. 7.

  1. Intermediate to Exchange. When the red button is pressed, contacts KX1 and KX2 disconnect the extension and connect the intermediate telephone circuit to the exchange, KX1 short-circuiting capacitor C3. KX3 prepares a circuit for holding the exchange line during information calls.
  2. Intermediate to Extension. When the white button is pressed, contacts KE1 and KE2 disconnect the exchange line, leaving the exchange bell connected to signal incoming calls. KE5 and KE6 connect the speaking circuit via the 6-volt battery to the extension line. The ring button, KR1 or KR2 applies negative battery to the buzzer in the extension telephone.
  3. Information Call. With the red and white buttons pressed, condition (B) obtains, but the exchange line is held via R1, KE3 and KX3. The bell remains across the line in case the exchange wishes to break down the call in favour of a trunk call.
  4. Transfer. When the handset is replaced, the buttons restore to normal. During this time the cradle-switch contacts CS2 maintain a noise suppression circuit to reduce the click heard by the waiting exchange subscriber.
  5. Return to Exchange Line. This is achieved by further pressing the already operated red button, in order to release the white one and thus restore the circuit to condition (A). The release is effected by a cam associated with the red button.
  6. Intermediate Joins 'Through' Conversation. With the appropriate terminals strapped for non-secret working, the intermediate can lift the handset and take part in an extension to exchange conversation, the 6-volt battery providing microphone current via the buzzer coils, KE4 and KX4, to enable the attendant to speak on the exchange line via capacitor C3. This arrangement does not reduce the transmission from the extension telephone as would a simple parallel circuit.
  7. Secrecy. The intermediate can be prevented from overhearing a through conversation by a change of terminal strappings, as stated in Fig. 7, Note 1.

With the development of the instruments herein described, the Company can now offer a comprehensive range of modern-style intermediate and extension telephones. The range comprises not only the respective auto and manual c.b. types for external and internal working which have been mentioned, but also an intermediate telephone designed solely for magneto exchange lines. This is a moulded set equipped with a hand generator, an indicator and three push-button keys. Its Type number is N.2140A. The associated extension uses an ordinary magneto telephone.

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Last revised: October 15, 2019