Rotary Dial - Function

Function

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 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. 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 fingerstop.

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), whereas in New Zealand's 111 was selected for the same reason: 111 actually pulses 999 to the central office/telephone exchange.

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 ageing, 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.

Rotary dial telephones in the United States were sometimes equipped with a blankoff plate 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.

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.

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.