Taken from:-
Engineering supplement to the Siemens Magazine (December 1931)


Automatic switching offers a satisfactory solution to the problems involved in providing a private telephone system for offices, factories, large houses or similar premises. When manual switching is employed, it is so often difficult to realise the economic advantages of a telephone system owing to the cost of providing an operator. Further, a continuous day and night service cannot be conveniently given.

For very small installations, of course, intercomununication sets offer an alternative, but the line wiring is costly and the tracing of faults is difficult.

With the progressive conversion of public systems to automatic working. telephone users are quickly becoming accustomed to the use of the dial telephone, and in most cases, we believe, initial prejudice has been converted into a preference for the new method.

With this in mind we have developed, for private installations, small automatic exchanges which incorporate the most modern practice in every feature of their design and will operate reliably under the most exacting conditions.

P.A.X. 36 - Maximum lines = 10
P.A.X. 40 - Maximum lines = 25
P.A.X. 45 - Maximum lines = 50

Further new types with larger capacities are in the course of development.

In every case only two wires are required to connect a standard Neophone to the P.A.X.  All apparatus is covered, and to ensure the utmost reliability all relays are fitted with twin contacts.

Automatic Equipment No. 36
This P.A.X. employs relays only and operates on a voltage range from 9 to 14 volts. Connecting circuits are provided for two simultaneous calls. Two tones are provided. One is similar to the public exchange dialling tone, and is given when the receiver is lifted, and again to denote that the called line is free. The other is similar to the public exchange busy tone, and is given when the called line is busy.

The equipment has been designed for wall mounting, the relays being fitted on a gate which can be swung out on either left or right-hand hinges to allow of access to the wiring, incoming cable and fuses.

The overall dimensions are:-
18.5 ins. high
14 ins. wide
8.5 ins. deep

Automatic Equipment No. 40
This P.A.X. employs relays, and uniselectors (previously known as preselectors) having capacity for 25 lines. The operating voltage range is from 20 to 30 volts. Connecting circuits, comprising a finder switch linked to a connector switch, are equipped for a maximum of 4 simultaneous calls. Dialling, ringing, and busy tones are provided, each being similar to the corresponding public exchange tone.

Ringing is interrupted, but in a manner dissimilar from the public exchange ringing, thereby providing discrimination when a P.A.X. telephone is fitted near a public exchange telephone.

In addition to the normal connecting facilities, the following special services can be provided:-

  • Executive Right of Way

  • Conference

  • Code Call

Executive Right of Way
Certain lines may be provided with a press-key, which enables the caller to make connections with a busy line. When a conversation is in progress and a third connection is made in this way, the conversing parties receive busy tone momentarily as a warning that a third party has entered the connection.

Certain lines may be given the conference facility. A number is allocated to the conference circuit. and all parties to a pre-arranged conference call this number in the normal manner by means of a finder-connector circuit. As each connector reaches the conference number, the calling line is switched directly to the conference circuit and the finder-connector is released and becomes available for other calls. Thus a conference can be held without reducing the number of connecting circuits available for other calls. Any party to the conference may clear without affecting the other connections and may re-enter the conference at will.

Code Call
Facilities are provided for making connection to a code call circuit and for passing the code impulse-train through the finder-connector circuit. The code call apparatus itself is not mounted in the P.A.X., but is provided as a separate unit, when required.

The P.A.X. has been designed for wall mounting, the apparatus being fitted to a gate similar to that employed in P.A.X. No. 36.

The overall dimensions are:-
26.5 ins. high
20 ins. wide
11.5 ins. deep

This equipment is the smallest of this capacity on the market.

A special feature of this P.A.X. is that all relays are mounted in removable relay sets, and no soldering is required at any point when the equipment is being installed or extended. The line circuit has only one relay and the line equipment for 5 lines is mounted in one relay box, the P.A.X. being supplied and extended in multiples of 5 lines. Due to the use of relay sets throughout, storing, packing for transport, and installation have been simplified, amid the cost of extending the equipment has been reduced to a minimum. If an additional line is required and no spare exists, then a relay set sufficient for 5 new lines is jacked in. The P.A.X. then has 4 spare lines available, and the cost of the extension to the equipment is still less than the cost of supplying. fitting. soldering and testing an extension of only one line on an equipment not having this facility. Similarly, in the case of the finder connectors, since the full complement of switches is always equipped, the provision of an additional circuit is simply a matter of 'jacking-in' a relay set.

Automatic Equipment No. 45
This P.A.X. employs relays, and uniselectors having capacity for 50 lines. The main circuit principles are similar to those employed on P.A.X. No. 40, and similar special services are available. In this case also the operating voltage range is from 20 volts to 30 volts. Finder-connector circuits are provided for a maximum of 6 simultaneous calls. The rack for this equipment is of the floor type, and is mounted on a chassis provided with rollers. A back-plate fixed to the wall carries the terminal and fuse panel. A hinged floor plate, normally raised in front of the rack, can be dropped to the floor to provide a runway on which the rack may be drawn forward to permit access to the wiring incoming cable, and fuses. Since the rack is only 19 ins, wide, it is not necessary to pull it forward more than 18 inches to obtain access to any point at the rear. The cover enclosing the equipment is constructed of metal-faced plywood, and is removed completely when the P.A.X. requires attention.

By arranging the equipment to stand against a wall, a considerable economy in floor space has been effected. Normally this plan involves mounting the apparatus on gates, but the necessity for this has been obviated by mounting the rack on a chassis and providing a runway.

In mounting the relays the plan adopted for P.A.X. No. 40 has been followed. The line relay boxes are precisely the same as those used on P.A.X. No. 40 and this equipment also is supplied or extended in multiples of 5 lines. The full complement of switches is always equipped, and an additional finder-connector circuit is provided simply by inserting another relay set.

As in the case of P.A.X. No. 40 no soldering is required when the equipment is being installed or extended.

The overall dimensions of this equipment are:-
5 ft. 5.5 ins. high
1 ft. 7 ins. wide
1 ft. 3.5 ins. deep

This equipment is the smallest of its capacity on the market.

Power Equipment.
All these P.A.X. equipments can, of course, be operated from secondary batteries. hut to meet modern requirements a power unit has been developed to operate them directly from A.C. mains. This unit is called a TRANSRECTER, and is fully described in the following article.

To ensure the most compact arrangement of an installation, the transrecters are designed for mounting immediately above the P.A.X.s, the dimensions of each type being arranged so that the width and depth are the same as those of the P.A.X. served.


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Last revised: December 23, 2022