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The Western Electric model 500 rotary dial telephone was a pulse-dialing instrument.

Pulse dialling is a signaling technology in telecommunications in which a direct current local loop circuit is interrupted according to a defined coding system for each signal transmitted, usually a digit. This lends the method the often used name loop disconnect dialing. In the most common variant of pulse dialing, decadic dialing, each of the ten arabic numerals are encoded in a sequence of up to ten pulses. The most common version decodes the digits 1 through 9, as one to nine pulses, respectively, and the digit 0 as ten pulses. Historically, the most common device to produce such pulse trains is the rotary dial of the telephone, lending the technology another name, rotary dialing.

The pulse repetition rate was historically determined based on the response time needed for electromechanical switching systems to operate reliably. Most telephone systems used the nominal rate of ten pulses per second, but operator dialing within and between central offices often used pulse rates up to twenty per second.

Early automatic exchanges[edit]

Automatic telephone exchange systems were developed in the late 19th and early 20th century. For identification, telephone subscribers were assigned a telephone number unique to each circuit. Various methods evolved to signal the desired destination telephone number for a telephone call directly dialed by the subscriber. An automatic switch-hook was designed by Hilborne Roosevelt.[1]

The first commercial automatic telephone exchange, designed by Almon Brown Strowger, opened in La Porte, Indiana on 3 November 1892, and used two telegraph-type keys on the telephone, which had to be operated the correct number of times to control the vertical and horizontal relay magnets in the exchange. But the use of separate keys with separate conductors to the exchange was not practical. The most common signaling system became a system of using direct-current pulse trains generated in the telephone sets of subscribers by interrupting the single-pair wire loop of the telephone circuit.

Rotary dial[edit]

Strowger also filed the first patent for a rotary dial in 1891. The first dials worked by direct, forward action. The pulses were sent as the user rotated the dial to the finger stop starting at a different position for each digit transmitted. Operating the dial error-free required smooth rotary motion of the finger wheel by the user, but was found as too unreliable. This mechanism was soon refined to include a recoil spring and a centrifugal governor to control the recoil speed. The user selected a digit to be dialed by inserting a finger into the corresponding hole and rotated the dial to the finger stop. When released from this position, the dial pulsing contacts were opened and closed repeatedly, thus interrupting the loop current in a pattern on the return to the home position. The exchange switch decoded the pattern for each digit thus transmitted by stepping relays or by accumulation in digit registers.

Pulse rate and coding[edit]

When electromechanical switching system were still in use, the current pulses generated by the rotary dial on the local loop operated electrical relays in the switches at the central office. The mechanical nature of these relays and the loop capacitance, affecting pulse shape, generally limited the speed of operation, the pulsing rate, to ten pulses per second.

The specifications of the Bell System in the US required service personnel to adjust dials in customer stations to a precision of 9.5 to 10.5 pulses per second (pps), but the tolerance of the switching equipment was generally between 8 and 11 pps.[2] The British (BPO, later Post Office Telecommunications) standard for Strowger exchanges was 10 impulses per second (allowable range 7 to 12) and a 66% break ratio (allowable range 63% to 72%)[3]

In most countries one pulse is used for the digit 1, two pulses for 2, and so on, with ten pulses for the digit 0; this makes the code unary, excepting the digit 0. Exceptions to this are: Sweden (example dial), with one click for 0, two clicks for 1, and so on; and New Zealand with ten clicks for 0, nine clicks for 1, etc. Oslo, the capital city of Norway, used the New Zealand system, but the rest of the country did not. Systems that used this encoding of the 10 digits in a sequence of up to 10 pulses, are sometimes known as decadic dialing systems.

Some later switching systems used digit registers which doubled the allowable pulse rate to 20 pulses per second, and the inter-digital pause could be reduced as the switch selection did not have to be completed during the pause. These included some Crossbar systems, the later version (7A2) of the Rotary system, and the earlier 1970s stored program control exchanges.

In some telephones, the pulses may be heard in the receiver as clicking sounds. However, in general, such effects were undesirable and telephone designers suppressed them by mechanical means with off-normal switches on the dial, or greatly attenuated them by electrical means with a varistor connected across the receiver.

Switch-hook dialing[edit]

British (BPO) Type 232 phone of 1932

As pulse dialing is achieved by interruption of the local loop, it was in principle possible to dial a telephone number by rapidly tapping, i.e. depressing, the switch hook the corresponding number of times for each digit at approximately ten taps per second. However, many telephone makers implemented a slow switch hook release to prevent rapid switching.[citation needed]

In the United Kingdom, it used to be possible to make calls from coin-box phones (payphones) by tapping the switch hook without depositing coins. A person caught tapping could be charged with 'abstracting electricity' from the General Post Office and several cases were prosecuted under this offence.[citation needed]

In popular culture, tapping was used in the film Red Dragon as a way for prisoner Hannibal Lecter to dial out on a phone with no dialing mechanism. This method was also used by the character 'Phantom Phreak' to call 'Acid Burn' when taken to prison in the film Hackers.


It was recognized as early as the 1940s that faster, more accurate dialing could be done with push buttons, but this was too unreliable in customer trials until transistors transformed the industry. In 1963, the Bell System introduced to the public its dual-tone multi-frequency (DTMF) technology under the name Touch-Tone, which was a trademark in the U.S. until 1984.[4] The Touch-Tone system used push-button telephones. In the decades after 1963, pulse dialing was gradually phased out as the primary dialing method to the central office, but many systems still support rotary telephones today. Some keypad telephones have a switch for the selection of tone or pulse dialing.

Mobile telephones and most voice-over-IP systems use out-of-band signaling and do not send any digits until the entire number has been keyed by the user. Many VoIP systems are based on the Session Initiation Protocol (SIP), which uses a form of Uniform Resource Identifiers (URI) for addressing, instead of digits alone.

See also[edit]


  1. ^ Wikisource-logo.svg Isa Carrington Cabell (1900). "Roosevelt, Nicholas I.". Appletons' Cyclopædia of American Biography. 
  2. ^ AT&T Specification No. 4566, February 1926, p.113
  3. ^ J. Atkinson, Telephony Volume 1, p.142 (1948, Pitman, London)
  4. ^ The Trademark Electronic Search System on the U.S. Patent and Trademark Office web site shows the trademark with serial number 72109459, registered 1962-09-04 and canceled 1984-03-13.

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Pulse_dialing — Please support Wikipedia.
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45 news items


Tue, 24 Nov 2015 07:01:50 -0800

Particularly in contrast with a British design of the same era. Amazingly, and a testament to international standardization, I was still able to make a call with this phone. 6000 miles away from its intended point of use, and 55 years later the pulse ...

Toronto Sun

Toronto Sun
Fri, 06 Nov 2015 09:03:45 -0800

Well unless you are still pulse dialing with a retro phone, your phone is another home for a computer. It's actually what brought me to today's topic. Most of us have a lot to protect in our homes. Whether it's our family, possessions, or just the home ...

No Jitter

No Jitter
Mon, 20 Apr 2015 04:58:32 -0700

Rotary phones used something called pulse dialing. You put your finger in a numbered hole in a "finger wheel," pulled that wheel back to the "finger stop," and let go. During the return rotation, the electrical current of the telephone line would be ...

No Jitter

No Jitter
Tue, 26 May 2015 06:15:18 -0700

For reasons dating back to rotary telephones and pulse dialing, the number 1 has no letters. A single pulse could be a disturbance or noise spike on the line and they didn't want the exchange that served 1x to be inundated with erroneous calls. Despite ...


Mon, 07 Jul 2014 05:16:40 -0700

This was particularly important in old-style rotary/pulse-dialing phones, which were still popular when the 9-1-1 system was first implemented. (The touch-tone phone wasn't first widely introduced until 1963 and took a few decades to completely ...

Wall Street Journal

Wall Street Journal
Tue, 13 Jan 2015 14:51:58 -0800

It is time for the refrigerator to pull its weight. For decades it has kept food chilled, and spit out ice cubes. Now, it is a computer, stereo, telephone, television, water filter and sparkling water dispenser. This fall, it will also make you a cup ...

EDN.com (blog)

EDN.com (blog)
Mon, 18 Nov 2013 08:37:23 -0800

Rotary dial telephones used pulse dialing, in which a direct current local loop circuit was rapidly connected and disconnected according to a defined coding system for each digit. Operators were required to make long distance calls. With touch-tone ...


Thu, 25 Sep 2014 10:33:11 -0700

9/25/14 1:45pm. or atdp2125551234 for those of us who had PULSE dialing and weren't going to pay the additional $2/mo to convert to tone dialing... (remember that? When the phone company charged you extra for TONE dialing instead of PULSE dialing?!).

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