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Home Telephone Exchange Service and Systems The Dial
 
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The Dial

A typical dialref.1 consists of the following parts: finger plate, number plate, impulse springs, impulse cam, speed-control governor, shunt springs, and driving mechanism. The dial is operated by inserting the finger in the appropriate hole and turning the dial in a clockwise direction. This winds ' a spring with sufficient tension to return the dial to normal. When the dial is released and returns to normal, a pawl engages in a ratchet which is fastened to a shaft. This shaft is geared to a governor and to an impulse cam which opens and closes the impulse springs shown in Fig. 24 in accordance with the selected digit, and at a regulated speed of approximately 10 interruptions per second.

Figure 24. Circuit of a typical common-battery antisidetone dial telephone set. The transformer windings are on a common core (not shown).

The shunt springs close when the dial is first rotated and remain closed until the dial returns to normal. The shunt is placed on the receiver (Fig. 24) to prevent a clicking noise during dialing, and it is placed on the transmitter to keep transient current impulses from flowing through it.

The basic principles of dial-operated equipment and the dial by which it is controlled have been presented. The details of the five different dial switching systems will now be considered.


Home Telephone Exchange Service and Systems The Dial

Last Update: 2011-05-28