|An automatic telephone system is one in which the calling
party is enabled, without the aid of a telephonist, to complete a call
through remotely controlled switches. As distinct from the automatic
system of telephony the switching systems which require operators to
manipulate plugs and cords are called manual 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
The requirements to be satisfied by the automatic system are:-
Step by step exchanges have been superseded by electronic and other mechanical exchanges.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