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 used a telephone keypad instead of a dial. Many phone lines no longer recognize pulse dialing, and it often has to be ordered separately from a phone company.
The dial is circular. In its most commonly used design, the dial was about 78 mm (3 inches) in diameter. 10 finger holes were cut through its outer perimeter. The dial is mounted via a shaft extending from inside the phone 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. The illustration shows a UK dial. Older Australian rotary dial phones also had them, but the letter combinations were often printed in the centre 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 clockface, 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. As the dial returns, electrical contacts wired through the mechanism underneath will open and close 6 times, thus sending 6 pulses to the receiving end. A centrifugal governor regulates the speed at which the dial returns under the force of the spring.
The governor and return spring were late 19th Century refinements. Previous dials worked by direct or forward action rather than by recoil. 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.
Some telephone sets include a much smaller dial inside the handset, with a movable finger stop that the user is to press against, while rotating the dial clockwise, until the finger stop itself moves no longer, then release both. In this setting, there is no section of the rotating dial plate that does not have holes. 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 1 pulse to signal the number zero, and 10 pulses to signal the number nine. New Zealand uses ten pulses minus the number desired; so to dial 7, the hook must be pressed three times. 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 9-9-9 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 !).
In the U.S., rotary dial phones were sometimes equipped with apparatus blanks—a piece of plastic or metal blocking the opening in the telephone's housing–in place of having a dial. In the Bell System, these telephones were referred to as non-dial. The most common applications for non-dial phones were on Automatic ringdown circuits or manual service.
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