Rotary dial
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The rotary dial is a device mounted on or in a telephone or switchboard that is designed to send interrupted electrical pulses, known as pulse dialing, corresponding to the number dialed. It was invented in 1888 by Almon Strowger. The device was phased out from the 1970s onwards with the onset of Touch Tone™ dialing, which uses a telephone keypad instead of a dial. Some telephone systems no longer recognize pulse dialing by default, so now it would have to be ordered from the telephone company as a special feature, to support older customer equipment.
The dial is circular. In its most common form, the dial is about 78 mm (3 inches) in diameter. Ten finger holes are cut through its outer perimeter. The dial is mounted via a shaft extending from inside the telephone or mounting and sits approximately 6 mm (¼ inch) above a faceplate. The faceplate is set so that through each finger hole, letters and numbers printed on the faceplate may be seen. 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 Operator. However, such letter codes were not used in all countries. Older Australian rotary dial telephones also had letters, but the combinations were often printed in the center plate adjacent to the number. 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 on a clock face, and then the numbers progress upward counterclockwise, with the 0 being at about 5 o'clock. A curved device called a finger stop sits above the dial at the 4 o'clock position. 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 telephone, as the dial returns, electrical contacts wired through the mechanism underneath will open and close six times, thus sending six pulses to the central office.
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 1800s 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 rest position and the governor regulates the dialing pulses at its design rate, usually approximately 10 per second, sometimes as much as 20 pps. The rotary dial governor is subject to wear and aging, and may require periodic cleaning, lubrication and adjustment by a telephone technician. Modern electronic telephones that feature selectable pulse or tone dialing eliminate this maintenance chore.
Some telephones include a much smaller dial inside 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.
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. A relic of this system is found in differing emergency telephone numbers; 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 1-1-1 was selected for the same reason (111 actually pulses 999 to the central office/telephone exchange).
Rotary dial telephones in the U.S. were sometimes equipped with apparatus blanks — a piece of plastic or metal blocking the opening in the telephone's housing — 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.
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