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Consumer Electronics by S. P. Bali

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UNISELECTORS

For automatic operation the first requirement is a way of indicating the telephone number of the customer to whom you wish to speak. The rotary dial with ten finger holes is now nearly a century old in basic concept but is still in wide use. 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 the line to the customer's premises and through the phone itself. If for example, you dial 6 the dial breaks circuit 6 times with each break lasting a pre-determined time, usually about 1/20th of a second. Relays in the exchange respond to these break signals.

 

 

Fig. 38.5 Principle of switching by electromechanical uniselector of one-from-ten ...

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