AUTOMATIC SYSTEMS

Manual exchanges are worked on the common battery system, i.e., one large battery is provided at the exchange for the supply of talking and signalling current, instead of the older practice in which batteries were provided at each subscriber's office.

There have been many automatic systems in general use and, like their immediate predecessors of the manual type, they are all worked upon the common battery principle.  In practice there are to methods of signalling the exchange - by a dial or press buttons which emulate a dial and by press buttons sending tones.

The Step by Step system (also known as Strowger) was the one most widely used in the UK between 1920 to around 1992 and developed into a standard design.  Step by Step systems are always signalled to by the use of loop disconnect (LD) pulses performed by a dial mechanism or press button emulation.

These dialling operations consist of the manipulation of a dial which is part of the subscriber's apparatus.  Numbers can be seen appearing behind a series of holes in a movable finger plate. In order to make a call the subscriber lifts the receiver from the switch-hook and inserts his forefinger into one of the holes and pulls the finger plate round until the progress of the finger is arrested by a stop.  The disc is then released and, whilst being driven back to its original position by a main spring located within the dial, disconnects the subscriber's line a certain number of times corresponding to the digit shown on the number plate near the hole into which the forefinger was inserted.  This operation is repeated for each digit of the number called.  The dial is arranged so that the disconnections occur on the return journey of the dial, not on its forward journey, in order that the system may he rendered reasonably independent of any peculiarity in dialling on the part of the calling subscriber, such as hesitation in the middle of a digit.  In order to avoid trouble which would occur if the dialling circuit included the variable resistance of the transmitter, and also to avoid annoyance to the subscriber
from clicks in the receiver, it is usual to switch the speaking apparatus out of circuit during dialling.

The requirements to be satisfied by the automatic system are:-

1. When the subscriber lifts his receiver he must be provided at the exchange with a circuit to the automatic apparatus which will later respond to the impulses he dials.  Dial tone is sent to line to show that dialling can commence.
2. When the subscriber has sent impulses he must be connected to the line whose number he has dialled.
3. When the called line is found and is disengaged the called subscriber's bell must be rung and ringing tone sent to the caller.
4. When the called subscriber answers the telephone the ringing current must be disconnected and talking current supplied.
5. While the conversation is in progress the lines connected together must be rendered engaged to other calls and safeguarded against interference.
6. When the calling subscriber replaces his receiver both subscribers must be left free to make or receive calls and the apparatus restored to normal so as to be available for use on further calls.
7. When one subscriber calls another whose line is engaged the calling subscriber must receive a distinctive engaged signal to acquaint him with the fact.

Step by step exchanges have been superseded by electronic and other mechanical exchanges.  Mechanical exchanges in the UK were generally Crossbar exchanges which accepted dial pulses and have now been replaced by electronic exchanges.  Electronic exchanges on the other hand will accept dial pulses and tones.

Last revised: October 30, 2022