AUTO is short for automatic.  Almon Strowger, an undertaker from Kansas (USA), invented a basic electromechanical exchange for automatically switching telephone calls.  He did this because he thought that his business rivals wife, who worked on the towns manual telephone exchange, was diverting his calls to her husband.  Strowgers system automatically routed calls without the assistance of an operator.  The telephone instrument has to have a dial with this automated system and the exchanges are generically known as `Strowger' exchanges.  The GPO standardised their exchanges and components parts were similar.  The UK still used step-by-step Strowger systems until 1990.

The GPO Strowger system
The equipment is - Uniselector - First selector - second selector - third selector

Strowger systems are not the only type of automatic exchange in existence.  Many others were made based on a variety of components ranging from relays to wafer switches.

The BPO introduced Crossbar and Electronic exchanges in the late 1960's - early 1970's and these were superseded by System X and System Y (both digital).

These and other public exchange systems are too complex a subject to describe in this document and you are advised to read Telephony by Atkinson, or browse to an exchange related web site, for a complete technical description.

GPO Exchange Codes
These are the codes, used in the List of Exchanges, to indicate the type of equipment in use.

Type of Exchange 1930 1936
Automatic, Non-Director H ND
Automatic, Director I D
Rural/Unit Automatic J U


Outgoing Calls

A calling subscriber gains access to the automatic equipment at the exchange by lifting the receiver and dialling as directed by the instruction card.  When the receiver is lifted from the telephone gravity switch, the gravity switch rises and the A and B wires of the exchange circuit are looped through the subscriber's induction coil primary winding and transmitter, in parallel with the bell and condenser which are connected permanently across the A and B wires of the circuit.  The loop offers a continuous circuit to the central battery at the exchange in consequence of which current flows round the loop, passing en route through apparatus at the exchange, the operation of which places the switching apparatus at the subscriber's disposal for obtaining a required connexion by the operation of his dial.  Assuming the called subscriber's line to be disengaged and that communication is established, the two subscribers are forthwith enabled to converse in virtue of the speaking circuit arrangements which, so far as the calling subscriber is concerned, depend on the continued flow of current from the central battery through the exchange switching apparatus and round the subscriber's loop via his transmitter.  On the termination of a call, the depression of the telephone gravity switch, by the replacement of the subscriber's receiver on the gravity switch, removes the loop connecting the A and B wires and restores the normal connexion between them via the bell and condenser.  The current previously supplied to the circuit by the central battery at the exchange consequently ceases to flow and normal conditions are restored at the exchange by the release of the switching apparatus.

In the picture above, going off hook makes the Uniselector operate and this finds a free first selector.  The first selector returns dial tone and accepts the first digit.  The second selector accepts the second digit and connects to a third selector.  The third selector accepts the third and fourth digits and then rings the called telephone or gives an engaged tone if the telephone is in use.

Incoming Calls

The the exchange rings the subscriber by generator and the rings are received on the magneto bell associated with the subscriber's telephone set.  In response, the subscriber lifts the receiver from the telephone gravity switch, thus completing the speaking circuit, replies and at the conclusion of conversation, signals the termination of the call to the exchange by replacing the handset on the gravity switch hook, the functions performed by the subscriber's apparatus at each stage being the same as those performed m connexion with an outgoing call.


History of the Automatic Telephone System

Try visiting a PABX 3 page - it's all Strowger, although only a large customer system.


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Last revised: July 12, 2024