The rotary dial is a device mounted on or in a telephone or switchboard that is designed to send electrical pulses, known as pulse dialing, corresponding to the number dialed. The early form of the rotary dial used lugs on a finger plate instead of holes. Almon Brown Strowger filed the first patent for a rotary dial, U.S. patent#486,909, on December 21, 1891 that was later issued to him on November 29, 1892.[1][2]
The modern version of the rotary dial with holes was first introduced in 1904 but did not enter service in the Bell System until 1919. The rotary dial was gradually supplanted by Dual-tone multi-frequency pushbutton dialing, introduced at the 1962 World's Fair, which uses a keypad instead of a dial. Some telephone systems in the US no longer recognize rotary dialing by default, but will only support push-button phones.
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From as early as 1836, there were various suggestions and inventions of dials for sending telegraph signals. After the first commercial telephone exchange was installed in 1878, the need for an automated, user-controlled method of directing a telephone call became apparent. The rotary dial was invented by Almon Brown Strowger in 1891.[3] There were numerous competing inventions, and 26 patents of dials, push-buttons, and similar mechanisms for signalling which telephone subscriber was wanted by a caller were issued prior to 1891. Most inventions involved costly, intricate mechanisms and required the user to perform complex manipulations.
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-patent designs. The original dial designs were rather cumbersome and development continued during the 1890s and early 1900s hand in hand with the switching technology. In the 1950s, plastic dials supplanted metal ones in most new telephone designs.
Despite their obsolescence, rotary phones 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 pay phones in the 1980s to discourage loitering by drug purchasers, since the dials could not be used to call dealers' pagers.[4]
The dial, also called the finger wheel, is circular. Ten finger holes perforate it in a partial ring near the perimeter. The dial is mounted via a shaft extending from inside the telephone and sits approximately 1⁄4 in (6 mm) above a faceplate. The faceplate is printed with letters and numbers corresponding to—and visible through—each finger hole. 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 having more than 9,999 telephone lines, and additionally given a meaningful mnemonic to facilitate memorization of individual telephone numbers by incorporating their exchange names. For example: "RE7-xxxx" represented "REgent 7-xxxx", 'Regent' being a local exchange name used in Canada, derived from an earlier precursor telephone number, '7xxxx' –with callers actually dialing '73-7xxxx' (737-xxxx).
In the United Kingdom the letter "O" was combined with the digit "0" rather than "6". Older Australian rotary dial telephones also had letters, but the combinations were often printed in the center plate adjacent to the number. The Australian letter to number mapping was: A=1, B=2, F=3, J=4, L=5, M=6, U=7, W=8, X=9, Y=0, so the phone number BX 3701 was in fact 29 3701. However, such letter codes were not used in all countries. The 1 is normally set at approximately 60 degrees clockwise from the uppermost point of the dial, or approximately at the 2 o'clock position, and then the numbers progress counterclockwise, with the 0 being at about 5 o'clock. A curved device called a finger stop sits above the dial at approximately the 4 o'clock position.
Dials outside Canada, the United States, and large cities in Britain (before all-figure dialing) usually did not bear alphabetic characters or an indication of the word "operator" in addition to numbers. Alphabetic designation of exchanges was also used for a short period in the Soviet Union in the 1950s, but by the next decade this practice was largely discontinued.
The physical nature of the dialing mechanism on rotary phones allowed the use of physical locking mechanisms to prevent unauthorized use. The lock could be integral to the phone itself or a separate device inserted through the finger hole nearest the finger stop to prevent the dial rotating.
To dial a number, the user puts a finger in the corresponding finger hole and rotates the dial clockwise until it reaches the finger stop. The user then pulls out the finger, and a spring in the dial returns it to the resting position. For example, if the user dials "6" on a North American phone, electrical contacts wired through the cam mechanism inside the phone will open and close six times as the dial returns to home position, thus sending six pulses to the central office.
Different pulse systems are used, varying from country to country. For example, Sweden uses one pulse to signal the number zero, and 10 pulses to signal the number nine. New Zealand uses ten pulses minus the number desired; so dialling 7 produces three pulses. In Norway, the North American system with the number 1 corresponding to one pulse was used, except from the capital, Oslo, which used the same "inverse" system as in New Zealand. For this reason, the numbers on the dial are shifted in different countries, or even in different areas of one country, to work with their system because of the difference of the number arrangement on the dial. The dial numbering can occur in 4 different formats, with 0 adjacent either to the 1 or the 9 and the numbers running in ascending or descending order with either the 0,1 or 9 being closest to the fingerstop.
A relic of these differences is found in emergency telephone numbers used in various countries; the United Kingdom selected 999 due to the ease of converting call office dials to make free calls ('0' for the Operator was already free), whereas in New Zealand's 111 was selected for the same reason: 111 actually pulses 999 to the central office/telephone exchange.
Early dials worked by direct or forward action. The pulses went out as the dial went around to the finger stop. When the user's hand motion was not smooth, it produced wrong numbers. In the late 19th century the dial was refined to be operated by a recoil spring and centrifugal governor. The user selects the digit to be dialed, rotates the dial to the finger stop, then releases it. The spring causes the dial to return to its home position and the governor regulates the dialing pulses at its design rate, usually approximately 10 per second, and sometimes as many as 20 pulses per second. The rotary dial governor is subject to wear and aging, and may require periodic cleaning, lubrication and adjustment by a telephone technician.
Some telephones include a small dial built into the handset, with a movable finger stop. The user rotates the dial clockwise until the finger stop ceases moving, then releases both. In this setting, there is no section of the rotating dial plate without holes, allowing a smaller dial diameter. This was introduced by Western Electric on the compact Trimline telephone, the first to locate the dial in the handset.
Rotary dial telephones in the United States were sometimes equipped with a blankoff plate in place of a dial. In the Bell System, these telephones were referred to as non-dial. The most common applications for non-dial telephones were on Automatic ringdown circuits or manual service.
Rotary dial telephones in Australia were sometimes equipped, in later years, with touch pad blanks, designed to look like a touch-phone, but providing the rotary dial signalling required by the Australian phone system.
Rotary dial telephones have no redial feature; the complete number has to be dialed for every attempted call. Users could employ pencils or other tools for dialing to avoid finger strain.