Rotary dial

The number switch is used in telephones to dial a phone number after the pulse dialing. Visible operating element of the switch number is the dial ( official designation finger hole disc ).

Push-button telephones, which generally operate with the multi-frequency dialing and do not need a number switch, it nevertheless able to reproduce, so that they can be operated at a switch that only supports pulse dialing.

History

In the early days of telephony, it was the user of a softphone not possible to establish a particular telephone connection to another port itself. In order to get a connection, you had the attendant in the central office ( colloquially the " Lady of the Office") to "wake up " (which was actually the official term for this operation ). This was done by means of a hand generator, partly also by pressing a call button. The Conciliation staff was informed orally then its connection request, then this built up the connection by means of flaps cabinets. Since in such a manual switchboard mostly sequentially several people were involved, the connection was long and prone to error due to oral transmission of the desired target.

Only the Almon Strowger of 1889 invented switching system (Automatic Telephone Exchange -. U.S. patent No. 447.918 ) allowed the phone users an independent choice. The Strowger system had a button that had to be pressed as many times as the number for each digit of the number to be dialed. In order to select, for example, the number 432, the caller pressed four times the 100 key, three times the 10 - key and the 1 key twice. The operation was accordingly cumbersome and error -prone. Likewise, the installation cost was high, there was also connected to the existing two wires for voice communication each of these keys on another wire to the switch.

On August 20, 1896 reported AE Keith and the brothers John and Charles J. Erickson, the staff of the " Strowger Automatic Telephone Exchange Company ," were a user- friendly system for a patent. For this system was on 11 January 1898, the U.S. patent no. 597.062 granted. The buttons have been replaced by a dial, the " Strowger finger -wheel sub station dial". This indicated whether the hundreds, tens or ones digits is selected. Two wires served to transmit dial pulses to the central office and were there connected to a respective electromagnet. The hundreds and units digit transmitted on the line, and the tens digit of the other. This device required to operate a local battery ( local battery ), which brought a lot of maintenance with them, as they had to be replaced regularly.

In Germany ( " Hildesheim " called ) was in 1908 by the Imperial Telegraph Administration in the telephone sets of type OB / SA used number switch.

The actual, still used today number switches, which requires no local battery, and also uses the line cores for the voice connection for the transmission of the dialed number has been developed by Siemens & Halske and filed April 29, 1913 as a patent. Various sources mention the Frenchman Antoine Barnay as an inventor, but he had a complete selection system and not the number switch invented itself. This patent he filed on May 18, 1923.

In the past, other controls were used for the number switch, so there was, for example, straight pull switch number and Trommelwählschalter that have not enforced. Also designed by the company SABA shortly after the Second World War, number switch with " finger hollow disk " (see photo) could not prevail. His bakelite dial had hemispherical indentations instead of the usual holes.

In public coin, such as the subscriber mu 55b, special blocking number switches have been used to (eg, primaries, etc. for long distance and international calls, and message services ) to block certain numbers. This had a very complex mechanical structure. When dialing a three-arm lever and contact work has been set in motion, the checked one after the first three digits. Rotary motion and contact guidance of the three arms were similar to that of a voter. By resoldering specific contact links you could "program" the blocked number quasi switch to the locked phone numbers.

Number switches were wearing parts that had to be replaced or overhauled after a longer operating time. Affected by the wear was particularly the governor, the bearing and the contact set. The company crown in Berlin developed in the early 1970s, a maintenance-free, extremely durable type ( number switch 61f ). In this the governor was not decelerated abruptly at the end of the run, he ran out slowly. The control of the nsi contacts was performed by metal balls in a plastic backdrop. The process was very quiet and was done a total of slightly slower than conventional types (approx. 1.4 seconds when dialing a zero). Due to the relatively high manufacturing costs, this model for the equipment of the German Federal Post Office but could not prevail. Over time the transmission have been increasingly made ​​of plastic.

In Germany the era of mechanical switch number went in the 1980s to the end; They have been replaced with electronic parts, the nachbildeten their function. So a key election was made possible in the area of ​​the existing exchanges. Such an assembly is called a push-button block ( TWB).

Older phones with number switches ( such as the classic W48 ) but now enjoy a new boom.

Design and operation

A number switch has a finger hole plate with ten holes, each digit from 1 to 9 and 0 is assigned to each hole. A digit is selected by the user inserts a finger into the hole of the finger hole disc and it by turning it clockwise until it stops ( stop finger ) moves. Inside thereby a rear torsion spring tensioned ( technical term rearing ). Then the finger is pulled out and the rear torsion spring rotates, slowed down by the governor, the finger hole disc back to its original position ( technical term expiration ). In this procedure ( the retraction of the Fingerwählscheibe to its original position ) one of the selected digit corresponding number of interruptions ( pulse) telephone line generated at a rate defined by the governor and thus indicate the number of the exchange by the number switch.

Components

A number switch consists essentially of

• Finger hole disc
• Finger stop
• Dial ( number Wreath)
• Rear torsion spring
• Governor
• Cam
• (Also called pulse disc ) Stromstoßrad
• As well as three contacts.

Depending on how the number switch can happen for transmitting the rotary movement still various gears and friction clutches.

The governor

A standardized duration of the pulses is ensured by the governor, which ensures a uniform rotational speed of the number switch at the end, by the speed of the dial to about 43 revolutions per minute adjusts ( in older number switches 50 min -1). Driven by the reversing spring -wound, he controls the speed of the cam disc and Stromstoßrads. Depending on the design it is coupled via a worm shaft or a gear with the Stromstoßrad. The governor two biased by a spring brake shoes are pressed by centrifugal force against a small brake drum. By changing the spring tension can be finely adjust the speed of the controller and thus correct the expiration time of the switch number.

The contacts

Both in rearing and at the expiration of the number three switch contacts on a so-called Stromstoßrad and a cam operated.

Are the names of these three contacts

• Nsa = numbers switch work (or power down ) Contact
• Nsi = number switch pulse contact
• Nsr = number switch Reducing (or rest) Contact

(without nsr )

Nsa contact

Nsi Contact

Nsr - Contact

As in the switched circuit inductive components are included, the number of switch contacts are subject to wear due to arcing and material migration. Therefore, as early as the 1920s, additional components were installed for spark suppression in the telephones. Over the years there were further technical improvements - for example, a so-called " reverse rotation lock " installed (type NRS 38 R). This prevented the Stromstoßrad could move along when pulling backwards, thereby causing interference.

The last generation of number switches from about the end of the 1970s with all-plastic gear had no nsr more. Here, the time delay of 0.2 seconds between the dialed digits have now achieved mechanically by plastic moldings. The nsi generated by a maximum of only 10 pulses.

Pulse ratio

For a correct choice of the desired telephone number is the steady running of the number switch crucial. The pulse ratio, ie the ratio of opening to closing, the nsi - contact is in Germany in the ratio 1.6: 1 done and the expiration time for 10 pulses ( extension number, dial 0) should be 1 second.

This is the time for one pulse ( expiration time ) 100 ms. Thus, for a pulse, a pulse ratio of 62 ms opening time and closing time of 38 ms nsi contact.

The default tolerated expiration times of 90 ms to 110 ms and pulse ratios of 1.3 1 to 1:9 1 Some electronic IWV -enabled telephone systems ( PBXs ), however, can be much more critical react to incorrect expiration times and false pulse ratios within the tolerance limits as a main of the exchange. Then arises false choice and it is recommended to set the number switches as accurately as possible. For a pulse recorder are available.

Some modern digital exchanges also allow 20 pulses per second.

National Features

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

In Germany and Austria as early as the interwar period on the dial itself to each digit was accordingly 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 ) is impressed. So you could choose combinations of numbers and letters as a phone number. It could be a precursor to the letter, respectively.

In Denmark, the number switches generated a total of 11 pulses, the last of which was short-circuited by the nsr contact. The total elapsed time is 1.10 seconds at full lift. The gear and the governor went with also when pulling, thereby the idle jerk Exchange Elevator / procedure was avoided.

Ie the ratio of opening to closing - - In Australia, the duty ratio is 2: 1 In most countries, the duty ratio of the nsi - contact is but the ratio 1.6: 1 standardized.

Due to the previous different normalizations of the various national regulatory bodies for telecommunications differs this ratio number switches from one another. Despite these differences, the operability of a foreign-made number switch is usually given in another country. Only the admission requirements of the country were not met, which prevented an official use of this component in other countries.

Others

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

This lock was often mounted in the hole 3 and thus made it possible practically only the dialing of emergency 112, 122 and 133 ( fire, police ), liberal, the lock was in the hole 9, whereby the necessary long distance prefix " 0" was closed, but many local connections - those without "0" - were still accessible. In private exchanges, where is "0" for an outside line, were only " internal calls " possible.

Bypass to let the mechanical lock by replacing the finger-hole disc ( tweezers solves the 2 -slot nut). Experienced users were able to track the number Switch by "tapping " with the fingers side solely by learned, rhythmically matching pressing the switchhook.

With the function of with "9" starting speed dials between numerous cities in Austria from 1973 to 1996, the there needed for this locking position changed to "8". At the farthest position " 0" could the lock be parked neutral. Successive emergence of modern phones and keys then had a lock on the top right front integrated as standard.

Most was selected with the right index or the middle finger extended. With dirty or wet fingers to spare long fingernails, at the desk to reach a little further, or to minimize the forearm movement during frequent phone you like to also chose a reversed -held stylus as a finger substitute.

To select bundespost internal (BRD ) special services existed until the 1970s special number switch with 12 instead of 10 finger holes.

1909 a telephone wall apparatus with dial ( Type ZB SA - Central battery self connection ) was used in Germany, in which the disc with the enlarged finger hole radius of curvature for the election by a maximum of 180 ° ( for zero ) was filmed.

Telephones with rotary dials were introduced in Austria in 1910. Before that there had been lever apparatus for 4 adjusted with levers digits whose election was transferred by a crank revolution.

Transition to key telephones

When the first phones were released with keyboard, these are still in use also pulse dialing (DP), ie the behavior of a number switch was simulated electronically. When later on the exchange side electromechanical Hebdrehwähler and motor rotary selector have been replaced by electronic systems, was the situation that two electronic systems communicating with each other, but both this simulated an electro-mechanical behavior. This changed with the introduction of digital switching using multi-frequency (DTMF ). Nevertheless dominate for reasons of backward compatibility, most digital exchanges German network operator (eg German Telekom ) and most PBXs continue the PD so that dial telephones can be operated even today.