How Everything Works
How Everything Works How Everything Works

Question 749

How do rotary telephones work? — JG, DeSoto, Kansas
As your finger turns the dial of the telephone, you wind a spring and store energy in that spring. When you remove your finger, the spring unwinds and its stored energy drives the dialing mechanism. This mechanism consists of a cogged wheel and a switch, as well as a centrifugal governor. As the dial unwinds, the cogged wheel turns and it's cogs close and open a switch one time for each number on the dial. For example, if you dial a "6", the switch closes briefly 6 times. For a "0", the switch closes 10 times. Each time the switch closes during this action, it "hangs up" the telephone briefly. The switching system at the telephone company recognizes these brief hang-ups as signals for establishing the connection. The centrifugal governor controls the rate at which the dial unwinds and makes sure that the pulses coming from the telephone occur at a uniform rate.

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