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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 as a succession of individual digits.
On the rotary dial, the digits are arranged in a circular layout, with one finger hole in the finger wheel for each digit. For dialing a digit, the wheel is rotated against spring tension with one finger positioned in the corresponding hole, pulling the wheel with the finger to a stop position given by a mechanical barrier, the finger stop. 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. Pulse count dialing is a digital addressing system which uses decimal pulse count modulation. 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 an automatic telephone exchange was granted to Almon Brown Strowger on November 29, 1892, but the commonly known rotary dial with holes in the finger wheel was not introduced until about 1907. While used in telephone systems of the independent telephone companies, rotary dial service in the Bell System in the United States was not common until the early 1920s.
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). Touch-tone technology primarily used a telephone keypad in the form of a rectangular array of push-buttons. Although no longer in common use, the rotary dial's legacy remains in the verb "to dial (a telephone number)".
The first commercial installation of a telephone dial accompanied the first commercial installation of a 99-line automatic telephone exchange in La Porte, Indiana, in 1892, which was based on the 1891 Strowger designs. The original dials required complex operational sequences. A workable, albeit error-prone, system was invented by the Automatic Electric Company using three push-buttons on the telephone. These buttons represented the hundreds, tens, and single units of a telephone number. When calling the subscriber number 163, for example, the user had to push the hundreds button once, followed by six presses of the tens button, and three presses of the units button.Smith, A. B.; Campbell, W. L., Automatic Telephony, New York, McGraw-Hill, 1921, p. 38. In 1896, this system was supplanted by an automatic contact-making machine, or calling device. Further development continued during the 1890s and the early 1900s in conjunction with improvements in switching technology.
Almon Brown Strowger was the first to file a patent for an automatic telephone exchange on December 21, 1891, which was awarded on November 29, 1892, as . The company later developed a rotary dial with lugs on a finger plate instead of holes that 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. For instance, the anti-drug Fairlawn Coalition of the Anacostia section of Washington, D.C., persuaded the phone company to reinstall rotary-dial in the 1980s to discourage loitering by drug purchasers, since they lacked a telephone keypad to leave messages on dealers' . 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.
The number of pulses sent for each digit is defined by the type of dial system in use. The encoding has traditionally varied by country, or by the manufacturer of the telephone system. For example, Sweden used one pulse to signal the number zero, and ten pulses to signal the number nine. In Finland, where many telephones were of Swedish manufacture, one pulse signaled digit 1, and ten pulses 0. New Zealand used ten pulses minus the number desired; so dialing the digit 7 produces three pulses. In Norway, the North American system with the digit 1 corresponding to one pulse was used, except for the capital city, Oslo, which used the same "inverse" system as in New Zealand. The sequencing of the digits on the dial varies accordingly.
The rotational return speed of the dial to its rest position is controlled by the governor, a mechanical regulating device, that assures a constant electrical pulsing rate of the dial.
The Western Electric dial had spur gears to power the governor, so the axis of the governor was parallel to the dial shaft. The Automatic Electric governor shaft was parallel to the plane of the dial at a right angle to the dial shaft. The governor shaft had worm gear in which, very atypically, the gear drove the worm. The worm, highly polished, had extreme pitch, with teeth at about 45° to its axis. This was the same as the gearing for the speed-limiting fan in traditional music boxes. The Western Electric governor was a cup surrounding spring-loaded pivoted weights with friction pads. The Automatic Electric governor had weights on the middle of curved springs made from strip stock. When it sped up after the dial was released, the weights moved outward, pulling the ends of their springs together. Springs were fixed to a collar on the shaft at one end and to the hub of a sliding brake disc at the other end. At speed, the brake disc contacted a friction pad. This governor was similar to that in spring-driven windup phonograph turntables of the early 20th century.
Both types had wrap-spring clutches for driving their governors. When winding the dial-return spring, these clutches disconnected to let the dial turn quickly. When the dial was released, the clutch spring wrapped tightly to drive the governor.
While winding the dial, a spring-centered pawl in the Western Electric dial wiggled off-center when driven by the cam on the dial shaft. Teeth on that cam were spaced apart by the same angle as dial hole spacing. During winding, the pawl moved off-center away from the normally-closed pulsing contacts. When the dial was released, the cam teeth moved the pawl the other way to open and release the dial contacts. In the Automatic Electric dial, the pulsing cam and governor were driven by a wrap-spring clutch as the dial returned. When winding, that clutch disconnected both cam and governor.
Dials at user stations typically produced pulses at the rate of ten pulses per second (PPS), while dials on operator consoles on crossbar or electronic exchanges often pulsed at 18 PPS.
The rotary dial governor is subject to wear and aging, and may require periodic cleaning, lubrication and adjustment by a technician. In the video, the green LED shows the dial impulse pulses and the red LED shows the dial's off-normal contact function.
Off-normal contacts typically serve two additional functions. 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. Another function is to short-circuit or interrupt the telephone receiver during dialing, to prevent audible clicking noise from being heard by the telephone user.
Letter codes were not used in all East European countries.
In 1917, W. G. Blauvelt of AT&T developed the combination pattern of letters assigned to each digit, which became the standard in North America. Large cities like New York City would ultimately require a seven-digit telephone number. Some tests in the early 1900s indicated that the short-term memory span of many people was insufficient for seven digits, causing dialing errors (the documentation for these tests is lost). As large cities had both manual and automatic exchanges for many years, the numbers for manual or automatic exchanges used the same format, which could be either spoken or dialed.
In the late 1940s, telephones were redesigned with the numbers and letters displayed on a ring outside the finger wheel to provide better visibility.
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