Step-by-step Telephone

     For automatic operation the first requirement is a way of indicating to the exchange the
     telephone number of the customer to whom you wish to speak.

     Contacts within the dial make and break an electrical circuit which interrupts current flowing
     from a battery in the exchange, through the loop made by to the line to the customer's
     premises and through the phone itself.

    The step-by-step principle was the first automatic system to become practicable for public
    telephone exchanges; the selection of a particular line is based on a one-from-ten selection
    process. For example, Fig. 15 show a simple switch that has ten contacts arranged around a
    semicircular arc or bank, with a rotating contact arm or wiper that can be made to connect the
    inlet to any one of the ten bank contacts outlets as required. The wiper is rotated by a simple
    electro-magnet driving a suitable mechanism, so the arrangement is called an electro-mechanism
    switch.
 

 
 
Fig. 15
Principle of switching
by electro-mechanical
uniselector of
on-from-ten outlets
 
    The wiper rotates in one plane only, so this type of electro-mechanism switch is called a
    uniselector. Clearly the inlet can be connected to any one of ten outlets, but the outlets are
    numbered from 1 to 0, which is normal practice in the step-by-step switching system.
 
    This principle can be extended to enable the inlet to be connected to any one from 100
    outlets by  connecting each of the ten outlets of the first uniselector to the inlet of another
    uniselector, as in   Fig.16. The switching of the inlet to any one of the 100 outlets (numbered
    from 11 to 00) is done in two steps, the first digit being selected on the first uniselector, and the
    second digit being selected on the second uniselector .
Fig. 16
Simple step-by-step
selection of
one-from-a-hundred
outlets

    The same sort of numbering scheme can be  provided (on a step-by-step basis) by a different
    type of electro-mechanism switch called a two-motion selector .The principle is illustrated
    simply in Fig.17.

 
 
 Fig.17
Principle of
one-from-a-hundred
selection by two-motion
selection
 

    The bank of fixed contacts  now contains 10 semi-circular arcs, each having 10 contacts, and
    arranged above each other. The moving contacts or wiper can connected to any one of the
    100 bank contacts by first  moving vertically to the appropriate level, and then rotating
    horizontally to a particular contact on that level .

    One of the basic features of step-by-step exchanges is that each selector or switch is controlled
    by a group of electromagnetic relays which is in effect a small brain, just sufficient to act on
    the digit it receives and to route the call on to another selector which acts on the next digit and
    so on. Each digit dialled takes the caller one step nearer the called  number.
 
 
 
 
 
 
 

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