Editing Rotary dial (section)

==Construction==
[[File:Ericsson bakelittelefon 1931.jpg|thumb|A 1931 [[Ericsson]] rotary dial telephone without lettering on the finger wheel, typical of European telephones. The ''0'' precedes ''1''.]]

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 (number plate), either in whole, or only showing the number assignment for each finger hole. The [[faceplate (housing)|faceplate]] is printed with digits, and often letters, corresponding to each finger hole. Digit 1 is typically set in the upper right quadrant of the dial front, then the numbers progress counterclockwise. A curved device, the ''finger stop'' is positioned in the lower right quadrant. Some dialing mechanism allowed the use of physical locking mechanisms to prevent unauthorized use. The lock could be integral to the telephone itself or a separate device inserted through the finger hole nearest the finger stop to prevent the dial from rotating.

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.

===Principal dial mechanisms in the United States===
In the United States, two principal dial mechanisms arose in the engineering laboratories of the largest manufacturers, that of the [[Western Electric Company]] for the Bell System, and that of the [[Automatic Electric Company]].

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|worm gearing]] 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 [[Clutch#Other designs|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===
[[File:Rotary Dial, Dialing Back with LEDs.ogv|thumb|The back of a rotary dial in operation, with [[LED]]s attached making the contacts' states visible]]

Early dials worked by direct or forward rotating action. The pulses were generated as the dial turned toward the finger stop position.  When the user's hand motion was erratic, it could produce the wrong digit. 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 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.

===Dials located in handsets===
[[Image:TrimlineExposed.jpg|thumb|upright|A 220 [[Trimline telephone|Trimline]] rotary desk phone, showing the innovative rotary dial with moving fingerstop]]

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===
[[File:Phone from 1950s uk - This one is real.JPG|thumb|Phone with letters on its rotary dial (1950s, UK)]]

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===
[[File:Australian table phone 1958.png|thumb|Australian phones had ten letters for the exchange code]]

Before 1960 [[Australia]]n 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 [[Yabby (disambiguation)|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.<ref>[http://www.mgts.ru/partner/company/about/foundation History of Moscow City Telephone Network] {{Webarchive|url=https://archive.today/20140709204720/http://www.mgts.ru/partner/company/about/foundation |date=2014-07-09 }} (in Russian)</ref><ref>[http://www.artlebedev.com/mandership/91/ A short history of telephone numbers], includes a copy of the 1968 flyer about abolishing letters in Moscow telephone numbers from the museum of [[Moscow City Telephone Network]]</ref>

Letter codes were not used in all East European countries.

[[File:New Zealand Rotary Telephone.jpg|thumb|right|Rotary telephone from [[New Zealand]], which used a reversed ordering of the numerals]]

===Emergency calling===
A relic of these differences is found in [[emergency telephone number]]s used in various countries; the United Kingdom selected [[999 (emergency telephone number)|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 (emergency telephone number)|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.