Rotary dial

A traditional North American rotary phone dial. The associative lettering was originally used for dialing named exchanges, but was kept because it facilitated memorization of telephone numbers.

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 (finger stop). 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 that 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.^{[citation needed]} when AT&T abandoned its determined reliance on manual exchanges, and embraced automatic switching.

From the 1960s onward, the rotary dial was gradually supplanted by dual-tone multi-frequency push-button dialing, first introduced to the public at the 1962 World's Fair under the trade name Touch-Tone®. Touch-tone technology primarily used a keypad in form of a rectangular array of push-buttons for dialing.

The LM Ericsson Dialog from the 1960s that remained popular in Sweden up until the 1980s

Western Electric model 500 telephone, leased between 1949 to 1984 to subscribers.

History

1912 Advertisement for dial telephone service in Chicago from the Illinois Tunnel Company, showing Automatic Electric-made phone

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. 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 98-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 lack of modern features, 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. They are also retained for authenticity in historic properties such as the U.S. Route 66 Blue Swallow Motel which date from an era of named exchanges and pulse dialling.

General form

A 1931 Ericsson rotary dial telephone, without associative lettering, typical of European telephones.

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 numbers (and sometimes letters) corresponding to each finger hole. 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.^{[citation needed]} A curved device called a finger stop sits above the dial at approximately the 4 o'clock position. 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.

Letters

Main article: Telephone exchange names

In addition to the numbers, the faceplate is often printed with letters corresponding to 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.^{[citation needed]} 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. When Australia changed to all-numeric telephone numbers, a mnemonic to help people associate letters with numbers was the sentence, "All Big Fish Jump Like Mad Under Water eXcept Yabbies." However, such letter codes were not used in all countries.

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.^{[citation needed]} 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.^{[citation needed]}

Function

Rotary telephone from New Zealand which used a reversed ordering of the numerals.

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 dialing 7 produces three pulses. In Norway, the North American system with the number '1' corresponding to one pulse was used, except for 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 finger stop.

Swedish rotary telephone. Note the reverse numbering order.

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, and the cam which 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 New Zealand 111 was selected for the same reason: on New Zealand reverse-numbered dials each digit '1' sends 9 pulses to the central office/telephone exchange (like '9' in Britain).

The back of a rotary dial in operation, with LEDs attached making the contacts' states visible.

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. Typically, a 10 pulses per second dial is used into a classic Strowger Exchange and a 20 pps dial would be used into a Crossbar or Electronic exchange, where line lengths were reasonable and the receiving register was not limited by classical telephone relays, even with the isthmus armature.

The rotary dial governor is subject to wear and aging, and may require periodic cleaning, lubrication and adjustment by a telephone technician. In the video, the green LED shows the dial impulse pulses and the red LED shows the Dial Off Normal contact function. Dial impulses are explained already, but the Dial Off Normal contact is used to short circuit the telephone transmitter circuit out of the telephone loop, making the pulsing more distinct and also to remove impulsive clicking on the caller's earpiece.

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. In Spain, these ones were manufactured for CTNE (Compañía Telefónica Nacional de España) by Málaga based factory "CITESA", being named as "Góndola" phones by its particular shape. Spanish Góndola sets were fitted from the beginning with a red LED series connected with the line allowing that the dial ("disco" in Spanish) were back-lighted while dialling. For that, the LED itself must be bridged by another diode, I.E. a Zener diode in opposition, to allow the DC to pass even if the line polarity are inverted. In such case, the LED doesn't light, but the phone will work anyway. That second diode was fitted in the same chip and capsule as the LED for an easy and cheap manufacturing.

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.^{[citation needed]}

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. Use of pencils or other objects would rub the numbers and letters off the dials where numbers and letters were viewed through the finger holes. About the 1950s, telephones in the USA and elsewhere were redesigned with the numbers and letters on a ring outside the finger wheel.

See also

• GPO Telephones
• Crossbar switch
• Stepping switch
• Dial tone
• Off-hook
• Single-frequency signaling
• Ringdown
• Ringing signal
• Direct Distance Dialing (DDD)
• Hotline
• Automatic identified outward dialing (AIOD)
• Part 68

External links

• Spring 1957 Issue of The Kansas Historical Quarterly
• "How Your Dial Phone Works", August 1946, Popular Science detailed article on subject with illustrations