Telephone 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, 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''.

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. 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 DTMF (dual-tone multi-frequency) push-button dialing, first introduced to the public at the 1962 World's Fair 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)".

History

From as early as 1836 onward, various suggestions and inventions of dials for sending telegraph signals were reported. 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. Addressing the technical shortcomings, Almon Brown Strowger invented a telephone dial in 1891. 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. 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 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. 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 a rotary dial on December 21, 1891, which was awarded on November 29, 1892, as . The early rotary dials used lugs on a finger plate instead of holes, and did not produce a linear sequence of puises, 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. 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.

Function

To dial a number, the user inserts a finger into 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 the wheel to the resting position. During the period of return, the dial operates electrical contacts that break the line current to the exchange a different number of times for each digit on the dial.

The number of pulses sent for each digit depends on the type of dial system used, which have traditionally varied by country, or by the manufacturer of the telephone system. For example, Sweden uses one pulse to signal the number zero, and ten pulses to signal the number nine. New Zealand uses ten pulses minus the number desired; so dialing the digit ''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. The sequencing of the digits on the dial varies accordingly.

Rotary dials have no redial feature; the entire number must be dialed for every attempted call. Additionally, the time it took to dial a number depended on how far the dial had to travel to return to the resting position. A phone number that had digits that required larger numbers of pulses could be tedious to use, by comparison.

Construction

A rotary dial typically features a circular construction. The shaft that actuates the mechanical switching mechanism is driven by the finger wheel, a disk that has ten finger holes aligned close to the circumference. The finger wheel may be transparent or opaque permitting the viewing of the face plate, or number plate below, either in whole, or only showing the number assignment for each finger hole. 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. 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 from rotating.

Principal dial mechanisms in the United States

In the United States, two principal dial mechanisms arose in the development labs of the largest manufacturers, Western Electric for the Bell System, and the Automatic Electric Company.

The Western Electric dial had spur gears to power the governor, so the governor and dial shafts were parallel.

The Automatic Electric governor shaft was parallel to the plane of the dial. Its shaft had worm gearing in which, very atypically, the gear drove the worm. The worm, highly polished, had extreme pitch, its 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 middles of curved springs made from strip stock. When it speeded 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.

Recoil spring

Early dials worked by direct or forward action. The pulses were generated as the dial turned toward the finger stop position. When the user's hand motion was erratic, it produced the wrong numbers. In the late 19th century, the dial was refined to operate automatically by a recoil spring. The user selected the digit to be dialed, rotated the dial to the finger stop, then released it. The spring caused the dial to rotate back to its home position during which time constant speed was maintained with a centrifugal 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 20 pps.

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'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 the telephone receiver during dialing, to prevent audible clicking noise from being heard by the telephone user.

Dials located in handsets

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 design the holes extend around the full circumference of the dial, allowing a reduced diameter. This was introduced by Western Electric on the compact Trimline telephone, the first to locate the dial in the handset. In Spain, such phones 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 the dial ("disco" in Spanish) to be backlit while dialling. 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. In case of line polarity reversal, the LED would not light, but the phone would work anyway. The LED and Zener diode were contained in the same package for ease of assembly in manufacturing.

Britain

In the United Kingdom the letter "O" was combined with the digit "0" rather than "6". In large cities the seven-digit numbers comprised three letters for the exchange name, followed by four numbers.

Australia

Before 1960 Australian rotary dial telephones had each number's corresponding letter printed on a paper disc in the centre of the plate, with space where the subscriber could add the phone number. The paper was protected by a clear plastic disc, held in place by a form of retaining ring which also served to locate the disc radially. 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 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."

Eastern bloc

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.

However, such letter codes were not used in all countries.

Emergency calling

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 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 New Zealand, 111 was selected because New Zealand reverse-numbered dials make each digit "1" send 9 pulses to the central office/telephone exchange (like "9" in Britain), allowing British exchange equipment to be used off the shelf.

Alphanumeric dialing

In addition to the ten digits, the faceplate is often printed with letters corresponding to each position.
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. For example, "RE7-3456" represented "REgent 7-3456".

In 1917, W. G. Blauvelt of AT&T developed the pattern of combination of letters to digits, that became the standard in North America. Large cities like New York would ultimately require a seven-digit number, but some tests in the early 1900s indicated that the short-term memory span of many people could not handle seven digits and many dialing errors due to memory lapse might occur (the documentation for these tests is lost). As large cities would have both manual and automatic exchanges for some years, the numbers for manual or automatic exchanges would have the same format (originally MULberry 3456, with three letters then four numbers), which could be either spoken or dialed. ''A History of Engineering and Science in the Bell System: The Early Years (1875-1925)'' p578 by M. D. Fagen (editor) & Bell Labs technical staff (1975, Bell Telephone Laboratories)

In the late 1940s, telephones were redesigned with the numbers and letters displayed on a ring outside the finger wheel to provide better visibility.

Push-button pulse dialing

Even after the transition to DTMF dialing with push-button keypads in most areas, pulse-dialing telephones continued to be produced for some time, even with keypads for dialing, for use with certain private exchange systems. Some of these can be distinguished visually by the lack of keys with the symbols ''#'' and ''*''. Some telephones may have an option to select pulse dialing or DTMF dialing.

See also

* AIOD leads (automatic identified outward dialing)
* Crossbar switch
* Dial tone
* Direct distance dialing (DDD)
* History of the telephone
* Single-frequency signaling
* Stepping switch
* Title 47 CFR Part 68

References

External links

"The Invention and Development of the Dial Telphone: The Contribution of Three Lindsborg Inventors"
by Emory Lindquist; Spring 1957 issue of '' The Kansas Historical Quarterly''

"How Your Dial Phone Works"
'' Popular Science'', August 1946—detailed article on subject with illustrations

''AT&T Archives Director's Cut - Now You Can Dial''
(YouTube), from the AT&T archives

{{DEFAULTSORT:Rotary Dial
American inventions
Telephony signals