Rotary dial
When released at the finger stop, the wheel returns to its home position driven by the spring at a speed regulated by a governor device.
During this return rotation, an electrical switch interrupts the direct current (DC) of the telephone line (local loop) the specific number of times associated with each digit and thereby generates electrical pulses which the telephone exchange decodes into each dialed digit.
Thus, each of the ten digits is encoded in sequences to correspond to the number of pulses; thus, the method is sometimes called decadic dialing.
The typical average baud rate is 10 bits per second, though the system will usually accept from about 9 through 13 pulses per second, a requirement due to variations in the rotary dial mechanism governor speed.
The first patent for a rotary dial was granted to Almon Brown Strowger on November 29, 1892, but the commonly known form with holes in the finger wheel was not introduced until about 1904.
[1] From the 1960s onward, the rotary dial was gradually supplanted by push-button telephones, first introduced to the public at the 1962 World's Fair under the trade name Touch-Tone (DTMF).
[2] Before 1891, numerous competing inventions, and 26 patents for dials, push-buttons, and similar mechanisms, specified methods of signalling a destination telephone station that a subscriber wanted to call.
A workable, albeit error-prone, system was invented by the Automatic Electric Company using three push-buttons on the telephone.
[4][5] The early rotary dials used lugs on a finger plate instead of holes, and did not produce a linear sequence of pulses, but interrupted two independent circuits for control of relays in the exchange switch.
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.
Despite their lack of modern features, rotary dials occasionally find special uses, particularly in industrial equipment.
For instance, the anti-drug Fairlawn Coalition of the Anacostia section of Washington, D.C., persuaded the phone company to reinstall rotary-dial pay phones in the 1980s to discourage loitering by drug purchasers, since they lacked a telephone keypad to leave messages on dealers' pagers.
[6] They are also retained for authenticity in historic properties such as the U.S. Route 66 Blue Swallow Motel, which date back to the era of named exchanges and pulse dialing.
Digit 1 is typically set in the upper right quadrant of the dial front, then the numbers progress counterclockwise.
The spring caused the dial to rotate back to its home position during which time constant speed was maintained with a centrifugal governor.
The rotary dial governor is subject to wear and aging, and may require periodic cleaning, lubrication and adjustment by a technician.
They may implement a shunt across the transmitter circuit and induction coil to maximize the pulsing signal of the dial by eliminating all internal impedances of the telephone set.
For that, the LED was bridged by an anti-parallel Zener diode, to allow the DC to pass even if the line polarity were reversed.
When Australia around 1960 changed to all-numeric telephone dials, a mnemonic to help people associate letters with numbers was the sentence, "All Big Fish Jump Like Mad Under Water eXcept Yabbies."
Alphabetic designation of exchanges with Cyrillic letters (А, Б, В, Г, Д, Е, Ж, И, К, Л for each of the digits from 1 through 0 respectively) was also used for a short period in the Soviet Union in the 1950s and 1960s, but by the next decade this practice was largely discontinued.
"0" for the Operator was already free, and the cam that removed the shunt on the line when the dial was rotated to the "0" position could be altered to include the adjacent digit "9" (and "8" if required) so that calls to "0" and "999" could be made without inserting coins.
In North America, traditional dials have letter codes displayed with the numbers under the finger holes in the following pattern: 1, 2 ABC, 3 DEF, 4 GHI, 5 JKL, 6 MNO, 7 PRS, 8 TUV, 9 WXY, and 0 (sometimes Z) Operator.
Letters were associated with the dial numbers to represent telephone exchange names in communities that required multiple central offices.
[10] In the late 1940s, telephones were redesigned with the numbers and letters displayed on a ring outside the finger wheel to provide better visibility.
