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Rotary dial

A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone number to a telephone exchange.

On the rotary dial, the digits are arranged in a circular layout so that a finger wheel may be rotated with one finger from the position of each digit to a fixed stop position, implemented by the finger stop, which is a mechanical barrier to prevent further rotation. When released at the finger stop, the wheel returns to its home position by spring action at a speed regulated by a governor device. During this return rotation, the dial interrupts the direct electrical current of the telephone line (local loop) a specific number of times for each digit and thereby generates electrical pulses which the telephone exchange decodes into each dialed digit. Each of the ten digits are encoded in sequences of up to ten pulses. For this reason, the method is sometimes called decadic dialling.

The first patent for a rotary dial was filed by Almon Brown Strowger on December 21, 1891, which was awarded to him on November 29, 1892 as U.S. Patent 486,909. The early forms of a rotary dial used lugs on a finger plate instead of holes and the pulse train was generated without the control of spring action or a governor on the forward movement of the wheel, which proved to be difficult to operate correctly. The commonly known appearance of the rotary dial with finger holes was first introduced in 1904 but did not enter service in the Bell System in the United States until 1919, when AT&T abandoned its determined reliance on manual exchanges, and embraced automatic switching.