Number switch

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A number switch is used on telephones to dial a number using the pulse dialing method . The visible control element of the number switch is the dial (also called number disc , official name finger hole disc ).

Button telephones usually work with the multi-frequency dialing method and do not have a mechanical number switch. Many pushbutton telephones can emulate it electronically and thus still be operated on an exchange that only supports pulse dialing.

Rotary dial on a telephone desk set . Injection-molded part made of transparent thermoplastic, central fastening screw covered by cardboard washer, printed with emergency numbers and labeled with subscriber number, and clipped cap.


In the early days of telephone technology, it was not possible for the user of a telephone to set up a particular telephone connection to any other connection in the telephone network himself. To get a connection, you had to “wake up” the operator in the telephone exchange ( colloquially the “ Miss from the office ”) (that was actually the official expression for this process). This was done using a crank inductor , sometimes also by pressing a call button. The operator was then verbally informed of his connection request, whereupon they set up the connection using flap cabinets . Since several people were usually involved in such a manual exchange, the connection setup took a long time and was prone to errors due to the verbal transfer of the desired destination.

Only the switching system invented by Almon Strowger in 1889 (Automatic Telephone Exchange - US patent No. 447,918) enabled telephone users to choose independently. The Strowger system had a key for each digit of the number to be dialed, which had to be pressed often according to the number. For example, to dial the number 432, the caller presses the 100 key four times, the 10 key three times and the 1 key twice. The operation was accordingly cumbersome and error-prone. The installation effort was also high, since in addition to the two existing wires for the voice connection, each of these buttons was connected to the exchange via an additional wire.

On August 20, 1896, AE Keith and the brothers John and Charles J. Erickson, who were employees of the Strowger Automatic Telephone Exchange Company, filed a patent for a more user-friendly system. U.S. Patent No. 597,062 was issued January 11, 1898 for this system. The buttons have been replaced by a rotary dial, the Strowger finger-wheel sub station dial . This indicated whether the hundreds, tens or ones digit was chosen. Two lines were used to transmit the dialing pulses to the exchange, where they were each connected to an electromagnet. The hundreds and units digits were transmitted over one line and the tens digit over the other. This facility required a local battery ( local battery ) to operate , which required high maintenance costs as it had to be replaced regularly.

In Germany in 1908 the Reich Telegraph Administration used number switches in the telephone sets of the OB / SA type (called "Hildesheim").

The actual number switch, which is still in use today, does not require a local battery and also uses the wires for voice communication to transmit the dialed number, was developed by Siemens & Halske and a patent was applied for on April 29, 1913. Various sources name the Frenchman Antoine Barnay as the inventor, but he invented a complete dialing system and not the number switch itself. He applied for this patent on May 18, 1923.

SABA number switch with finger recessed disk (1946)

In the past, other operating elements were also used for the number switch, for example there were straight-pull number switches and drum dials , but these have not caught on. The number switch with a “finger-tipped disc” (see photo) designed by the SABA company shortly after the Second World War also failed to gain acceptance. His Bakelite dial had hemispherical indentations instead of the usual holes.

In coin-operated telephones , such as the Tln Mü 55b , special barred number switches were used to block certain telephone numbers (e.g. area codes for long-distance and international calls, announcement services, etc.). These had a very complex mechanical construction. When dialing, a three-armed lever and contact mechanism was set in motion that checked the first three digits one after the other. The rotary movement and contact guidance of the three arms resembled that of a voter . By re-soldering certain contact connections, the blocked number switches could be “programmed” to the blocked phone numbers.

Number switches were wearing parts that had to be replaced or overhauled after a long period of operation. The centrifugal governor , its bearings and the contact set were particularly affected by wear . In the early 1970s, the Krone company in Berlin developed a maintenance-free, particularly durable type (number switch 61f). In this case, the centrifugal governor was not braked abruptly at the end of the process, it slowly ran out. The control of the nsi contacts was carried out by metal balls in a plastic gate, and this construction also created the "space" between dialing processes without additional dialing pulses, so that no nsr contact was necessary as with conventional number switches. The process was very quiet and was a little slower overall than with conventional types (approx. 1.4 seconds when selecting a zero). However, due to the relatively high production costs, this model could not prevail for the devices of the Deutsche Bundespost . Over time, the gearboxes were increasingly made of plastic.

In Germany , the era of mechanical number switches came to an end in the 1980s; they were replaced by electronic components that simulated their function. In the area of ​​the existing exchanges, a button selection was made possible. Such an assembly is known as a keypad (TWB).

Older telephones with number switches (such as the classic W48 ) are enjoying increasing popularity again today.

Structure and functionality

Sound sample

A number switch has a finger hole disc with ten holes, each digit from 1 to 9 and the 0 is assigned a hole. A number is selected by the user inserting the index finger into the corresponding hole in the finger hole disc and moving it by turning it clockwise until it stops (finger stop). In this way, a return spring is tensioned on the inside (technical term “ winding” ). Then the finger is pulled out, whereupon the return spring turns the finger hole disc back to its original position at a speed defined by the centrifugal governor (technical term: sequence ). During this process (moving the finger dial back to its original position), the number switch generates a number of interruptions ( pulses ) in the telephone line corresponding to the number selected (0 corresponds to ten pulses) with a period defined by the governor and thus the number of the exchange signals.


Number switch (interior view) type NrS 38 M from Merk Telefonbau from 1959

A number switch essentially consists of

  • Finger hole disc
  • Finger stop
  • Dial (number wreath)
  • Return spring
  • Centrifugal governor
  • Cam disc
  • Impulse wheel (also called pulse wheel)
  • and three electrical contacts.

Depending on the design of the number switch, various gear wheels and slip clutches can be added to transfer the rotary movement .

The governor

The centrifugal governor ensures a standardized duration of the impulses. This keeps the speed of the selector and cam disk constant (at around 43 min −1 , with older number switches 50 min −1 ) and the impulse wheel.

Depending on the design, it is coupled to the pulse disc with a worm gear or a spur gear . In the centrifugal governor, two rotating, spring-pretensioned brake shoes are pressed against a stationary brake drum by the centrifugal force . By changing the spring preload, you can calibrate the speed of the governor and thus the deceleration time of the number switch.

The contacts

Both when pulling up and when the number switch is running, three contacts are actuated via a so-called impulse wheel and a cam disk.

The names of these three contacts are

  • nsa = N ummern- S chalter- A rbeits- (or A bschalte-) Contact
  • nsi = N ummern- S chalter- I mpuls contact
  • NSR = N ummern- S chalter- R eduzier- ( R ücklauf, or R uhe-) Contact
The number switch
contacts (nsa, nsi) in a FeAp 611
nsa contact
The nsa is closed when the number switch is opened by the cam disk and remains that way until the end of the process. It bridges the 'inner' telephone circuit (speech circuit). This ensures that no pulse distortion occurs and the dialing pulses (clicks) do not reach the handset ( telephone receiver ). As a result of this short-circuiting, the voltage that can be measured on the telephone drops from around 8-12 volts to 0 volts, depending on the type of terminal.
nsi contact
The nsi , which is in series with nsa and speech circuit, generates the dialing impulses in that the impulse wheel interrupts this contact. The desired number is created by a regular rhythmic interruption of the line loop (telephone line). This means that the open circuit voltage (approx. 60 volts) is on the phone for a brief moment. It should be noted that the nsi always generates two additional pulses, the so-called idle pulses. For example, if the number 5 is selected, the nsi will produce 5 + 2 = 7 pulses.
nsr contact
The task of the nsr is to disable the two additional idle pulses generated by the nsi on the line loop ( telephone line ). This is done by bridging the nsi with the two idle pulses . This bridging can take place at the beginning or at the end of the pulse series, depending on the design of the number switch. Bridging at the beginning has the advantage that the centrifugal governor has more time to reach its target rotational speed. The nsr is closed by the cam disk when it is running. This ensures that there is a sufficiently large pause between two dialed digits. The pause between the selected digits is at least the time of two impulses, i.e. 200 ms. This pause is called inter-digit pause ( IDP ), rarely also called space . This is to prevent z. B. two digits "1" dialed in quick succession are recognized by the exchange as digit "2". In addition, this impulse pause provides sufficient free time to control the mechanical group selectors ( lever-operated dials , precious metal motorized dials ) in the switching technology . The total expiry time including space is 1.20 seconds. Older number switches of the type N24 or N30, which were built into telephones such as the W28 or the first model 36 , did not yet have this compulsory break. It was not until 1938 that Siemens & Halske used number switch types with nsr (type NrS 38) with the introduction of the W38 telephone . The 38 types also had faster running centrifugal governors than their predecessors, which made the process quieter and ensured that the nominal speed was reached earlier.

Since the switched circuit contains inductive components, the number switch contacts are subject to wear as a result of sparks and material migration. That is why additional components for spark extinguishing were built into telephones as early as the 1920s. In the course of the years there were further technical improvements - for example a so-called "reverse rotation lock" was installed (type NrS 38 R). This prevented the impulse wheel from moving backwards when it was being pulled up, thereby causing interference.

The last generation of number switches (developed in the GDR, including and used in the Variant 74) from around the mid-1970s with all-plastic gears no longer had an nsr . There the time delay of 0.2 seconds between the selected digits has now been achieved mechanically by means of plastic molded parts. As a result, the nsi only generated a maximum of 10 pulses.

Pulse diagram of the nsi at the digit 0

Pulse ratio

The smooth running of the number switch is of decisive importance for the correct selection of the desired telephone number.
The pulse ratio, i.e. the ratio of opening to closing, of the
nsi contact should be in a ratio of 1.6: 1 in Germany and the expiry time for 10 pulses (selection of the number 0) should be 1 second.

This means that the time for a pulse (expiry time) is 100 ms. This results in a pulse ratio of 62 ms opening time and 38 ms closing time of the nsi contact for one pulse .

The default tolerated expiry times of 90 ms to 110 ms and pulse ratios of 1.3: 1 to 1.9: 1 Some electronic pulse dialing -enabled phone systems (PBXs), however, can be much more critical to respond to incorrect expiration times and false pulse conditions within the tolerance limits as a Main connection of the exchange. Then the wrong number is dialed and it is advisable to set the number switch as precisely as possible. For it will pulse recorder or reed frequency meter used.

Some modern digital exchanges also allow 20 pulses per second.

National characteristics

The numbers on the dial are arranged counterclockwise in almost all countries: 1, 2, 3, 4, 5, 6, 7, 8, 9, 0. The arrangement is based on Recommendation ITU-T E.161 of the International Telecommunication Union " Arrangement of digits, letters and symbols on telephones and other devices that can be used for gaining access to a telephone network " standardized. In New Zealand the order is reversed. In Sweden , zero comes first, followed by 1 to 9.

In Germany and Austria has been from the interwar period on the dial itself of each digit by a letter (A, B, C, D, E, F, G, H, J, K, or in the case of the Vienna dial I, F, A, B, R, U, M, L, Y, Z) embossed. So you could choose combinations between numbers and letters as a telephone number. One could call it the forerunner of the choice of letters .

In Denmark , the number switch generated a total of eleven pulses, the last of which was short -circuited by the nsr contact. The total expiry time is 1.10 seconds when fully wound. The gearbox and the centrifugal governor also ran during the winding process, thus avoiding the idling jolt when changing between the lift and the drain.

In Australia the pulse ratio - the ratio of opening to closing - is 2: 1. In most countries, however, it is standardized at 1.6: 1.

Due to the earlier different standards of the individual national supervisory authorities for telecommunications technology, this ratio differs for number switches. Despite these deviations, the functionality of a number switch manufactured abroad is usually also available in another country. Only the approval requirements of the respective country were not met, which prevented an official use of this component in other countries.


In the 1970s, a small telephone lock was occasionally inserted into a finger hole and locked as a protruding barrier to restrict unauthorized use.

This barrier was often installed in hole 3 and thus only enabled the emergency calls 112, 122 and 133 (fire brigade, police) to be dialed; The lock in hole 9 was more liberal, as a result of which the traffic elimination number "0" required for long-distance calls was blocked, but many local connections - without a "0" - could still be reached. In private branch exchanges in which the "0" is used to get an outside line, only "internal calls" were possible.

The mechanical lock could be bypassed by replacing the finger hole disc (pointed tweezers loosen the 2-slot nut). Experienced users were also able to reproduce the number switch function by simply knocking on the finger side simply by learning to operate the hook switch in a rhythmically appropriate manner .

With the function of abbreviated dialing numbers beginning with “9” between numerous Austrian cities from 1973 to 1996, the blocking position required there changed to “8”. The lock could be parked neutrally at the most distant position "0". The keypad telephones that followed often had a lock integrated into the control panel.

Mostly the index finger or the longer middle finger was chosen. With dirty or wet fingers, in order to protect long fingernails, to reach a little further at the desk or to minimize forearm movement when making a lot of calls, people also liked to use a pen held the other way as a finger replacement.

Up until the 1980s there were special number switches with twelve instead of ten finger holes to select special services within the Federal Post Office (BRD).

In 1908, in the Hildesheim office, which had been converted to self-dialing, a wall telephone set with a dial (type ZB SA - central battery self-connection) was used in Germany, with the dial being rotated by a maximum of 180 ° (for zero) with the increased radius at the finger hole arch.

Telephone sets with rotary dials were introduced in Austria in 1910. Before that, there had been lever devices for four digits to be set with levers, the selection of which was transmitted by a turn of the crank.

For a short time, roughly estimated around 1970, there were police patrol cars in Austria with a radio set with a control unit in the middle in front of the passenger seat of the VW Beetle , which is common throughout . A standard telephone receiver, which was common at the time, lay across the direction of travel and near the windshield in recesses on the device and thus just above a rotary dial, which could only be operated freely after it was lifted. At that time there was (rather) no mobile network for telephony.

There were - rarely - narrow desk telephones with a dial at a slight angle and a receiver placed lengthways above it in a double recess. The rotary dial was not freely accessible until it was lifted.

Transition to push-button phones

When the first telephones with a keypad were released, they also used pulse dialing (IWV). H. the behavior of a number switch was electronically simulated. When the electromechanical rotary dials and motorized dials were later replaced by electronic systems on the switching side , the situation arose where two electronic systems were communicating with each other, but both simulated electromechanical behavior. That only changed with the introduction of digital switching using multi-frequency dialing (MFV). Nevertheless, for reasons of downward compatibility, most of the digital exchanges of German network operators (e.g. Deutsche Telekom ) and most of the telephone systems and analog telephone adapters continue to use pulse dialing, so that many rotary telephones can still be operated today.


  • Gert Kaszynski, Jürgen Schönhoff: Telephone devices . 2nd edition, Verlag Technik, Berlin 1991, ISBN 3-341-00822-5 .
  • Günther Mergelsberg: The telephone and its development. 2 volumes, collectors and interest group for the historical telecommunications e. V., Bad Homburg 1996, OCLC 174321014 / OCLC 174417497 .

Web links

Wiktionary: rotary dial  - explanations of meanings, word origins, synonyms, translations
Commons : Number Switches  - collection of pictures, videos and audio files

Individual evidence

  1. patent specification , US patent No. 447.918
  2. patent specification , US patent No. 597,062
  3. The Hacker Bible Part 1, p. 111: October 8, 2007
  4. First automatic local exchange in Germany with a central battery, Association Message Technology once and now., Accessed on March 28th 2014
  5. First telephone in Austria with a rotary dial, Association of Communications Technology once and now, accessed on March 28, 2014
This version was added to the list of articles worth reading on October 25, 2005 .