The telephone (from the _el. τῆλε, "tēle", "far" and φωνή, "phōnē", "voice") is a
telecommunications device that is used to transmit and receive sound(most commonly speech), usually two people conversing but occasionally three or more. It is one of the most common household appliances in the world today. Most telephones operate through transmission of electric signals over a complex telephone network which allows almost any phone user to communicate with almost anyone.
A traditional landline telephone system, also known as "plain old telephone service" (POTS), commonly handles both signaling and audio information on the same
twisted pairof insulated wires: the telephone line. Although originally designed for voice communication, the system has been adapted for data communication such as Telex, Faxand Internetcommunication. The signaling equipment consists of a bell, beeper, light or other device to alert the user to incoming calls, and number buttons or a rotary dial to enter a telephone numberfor outgoing calls. A twisted pair line is preferred as it is more effective at rejecting electromagnetic interference (EMI) and crosstalkthan an untwisted pair.
calling partywishing to speak to another party will pick up the telephone's handset, thus operating a button switch or "switchhook", which puts the telephone into an active state or " off hook" by connecting the transmitter ( microphone), receiver (speaker) and related audio components to the line. This circuitry has a low resistance (less than 300 Ohms) which causes DC current(48 volts, nominal) from the telephone exchangeto flow through the line. The exchange detects this DC current, attaches a digit receiver circuit to the line, and sends a dial toneto indicate readiness. On a modern telephone, the calling party then presses the number buttons in a sequence corresponding to the telephone number of the called party. The buttons are connected to a tone generator that produces DTMFtones which are sent to the exchange. A rotary dial telephone employs pulse dialing, sending electrical pulses corresponding to the telephone number to the exchange. (Most exchanges are still equipped to handle pulse dialing.) Provided the called party's line is not already active or "busy", the exchange sends an intermittent ringing signal(generally over 100 volts AC) to alert the called party to an incoming call. If the called party's line is active, the exchange sends a busy signalto the calling party. However, if the called party's line is active but has call waitinginstalled, the exchange sends an intermittent audible tone to the called party to indicate an incoming call.
When a landline phone is inactive or "
on hook", its alerting device is connected across the line through a capacitor, which prevents DC current from flowing through the line. The circuitry at the telephone exchange detects the absence of DC current flow and thus that the phone is on hook with only the alerting device electrically connected to the line. When a party initiates a call to this line, the ringing signal transmitted by the telephone exchange activates the alerting device on the line. When the called party picks up the handset, the switchhook disconnects the alerting device and connects the audio circuitry to the line. The resulting low resistance now causes DC current to flow through this line, confirming that the called phone is now active. Both phones being active and connected through the exchange, the parties may now converse as long as both phones remain off hook. When a party "hangs up", placing the handset back on the cradle or hook, DC current ceases to flow in that line, signaling the exchange to disconnect the call.
Calls to parties beyond the local exchange are carried over "trunk" lines which establish connections between exchanges. In modern telephone networks, fiber-optic cable and digital technology are often employed in such connections. Satellite technology may be used for communication over very long distances.
In most telephones, the transmitter and receiver (microphone and speaker) are located in the
handset, although in a speakerphonethese components may be located in the base or in a separate enclosure. Powered by the line, the transmitter produces an electric current whose voltage varies in response to the soundwaves arriving at its diaphragm. The resulting current is transmitted along the telephone line to the local exchange then on to the other phone (via the local exchange or a larger network), where it passes through the coil of the receiver. The varying voltage in the coil produces a corresponding movement of the receiver's diaphragm, reproducing the sound waves present at the transmitter.
Lineman's handsetis a telephone designed for testing the telephone network, and may be attached directly to aerial lines and other infrastructure components.
Credit for inventing the electric telephone remains in dispute. As with other great
inventions such as radio, television, light bulb, and computer, there were several inventors who did pioneer experimental work on voice transmission over a wire and improved on each other's ideas. Innocenzo Manzetti, Antonio Meucci, Johann Philipp Reis, Elisha Gray, Alexander Graham Bell, and Thomas Edison, among others, have all been credited with pioneer work on the telephone.
The early history of the telephone is a confusing morass of claim and counterclaim, which was not clarified by the huge mass of lawsuits which hoped to resolve the patent claims of individuals. The Bell and Edison patents, however, were forensically victorious and commercially decisive.
1844— Innocenzo Manzettifirst mooted the idea of a “speaking telegraph” (telephone).
26 August 1854— Charles Bourseulpublishes an article in a magazine L'Illustration(Paris) : "Transmission électrique de la parole".
22 August 1865, La Feuille d'Aoste reported “It is rumored that English technicians to whom Mr. Manzetti illustrated his method for transmitting spoken words on the telegraph wire intend to apply said invention in England on several private telegraph lines.”
28 December 1871— Antonio Meuccifiles a patent caveat(n.3335) in the U.S. Patent Office titled "Sound Telegraph", describing communication of voice between two people by wire.
1874— Meucci, after having renewed the caveat for two years, fails to find the money to renew it. The caveat lapses.
6 April 1875— Bell's U.S. Patent 161,739 "Transmitters and Receivers for Electric Telegraphs" is granted. This uses multiple vibrating steel reeds in make-break circuits.
11 February 1876— Gray invents a liquid transmitter for use with a telephone but does not build one.
14 February 1876— Elisha Gray files a patent caveatfor transmitting the human voice through a telegraphic circuit.
14 February 1876— Alexander Bell applies for the patent "Improvements in Telegraphy", for electromagnetic telephones using undulating currents.
19 February 1876— Gray is notified by the U.S. Patent Office of an interference between his caveat and Bell's patent application. Gray decides to abandon his caveat.
7 March 1876— Bell's U.S. patent 174,465 "Improvement in Telegraphy" is granted, covering "the method of, and apparatus for, transmitting vocal or other sounds telegraphically … by causing electrical undulations, similar in form to the vibrations of the air accompanying the said vocal or other sound."
10 March 1876— The first successful telephone transmission of clear speech using a liquid transmitter when Bell spoke into his device, “Mr. Watson, come here, I want to see you.” and Watson heard each word distinctly.
30 January 1877— Bell's U.S. patent 186,787 is granted for an electromagnetic telephone using permanent magnets, iron diaphragms, and a call bell.
27 April 1877— Edison files for a patent on a carbon (graphite) transmitter. The patent 474,230 was granted 3 May 1892, after a 15 year delay because of litigation. Edison was granted patent 222,390 for a carbon granules transmitter in 1879.
Early commercial instruments
Early telephones were technically diverse. Some used a liquid transmitter, some had a metal diaphragm that induced current in an electromagnet wound around a permanent magnet, and some were "dynamic" - their diaphragm vibrated a coil of wire in the field of a permanent magnet or the coil vibrated the diaphragm. This dynamic kind survived in small numbers through the 20th century in military and maritime applications where its ability to create its own electrical power was crucial. Most, however, used the Edison/Berliner carbon transmitter, which was much louder than the other kinds, even though it required an
induction coil, actually acting as an impedance matchingtransformer to make it compatible to the impedance of the line. The Edison patents kept the Bell monopoly viable into the 20th century, by which time the network was more important than the instrument.
Early telephones were locally powered, using either a dynamic transmitter or by the powering of a transmitter with a local battery. One of the jobs of
outside plantpersonnel was to visit each telephone periodically to inspect the battery. During the 20th century, "common battery" operation came to dominate, powered by "talk battery" from the telephone exchangeover the same wires that carried the voice signals.
Early telephones used a single wire for the subscriber's line, with ground return used to complete the circuit (as used in
telegraphs). The earliest dynamic telephones also had only one opening for sound, and the user alternately listened and spoke (rather, shouted) into the same hole. Sometimes the instruments were operated in pairs at each end, making conversation more convenient but were more expensive.
At first, the benefits of an exchange were not exploited. Telephones instead were leased in pairs to the
subscriber, who had to arrange telegraph contractors to construct a line between them, for example between his home and his shop. Users who wanted the ability to speak to several different locations would need to obtain and set up three or four pairs of telephones. Western Union, already using telegraph exchanges, quickly extended the principle to its telephones in New York Cityand San Francisco, and Bell was not slow in appreciating the potential.
Signalling began in an appropriately primitive manner. The user alerted the other end, or the exchange operator, by whistling into the transmitter. Exchange operation soon resulted in telephones being equipped with a bell, first operated over a second wire, and later over the same wire, but with a condenser (
capacitor) in series with the bell coil to allow the ACringer signal through while still blocking DC(keeping the phone "off hook"). Telephones connected to the earliest Strowger automatic exchanges had seven wires, one for the knife switch, one for each telegraph key, one for the bell, one for the push button and two for speaking.
Rural and other telephones that were not on a common battery exchange had a magneto or hand-cranked generator to produce a high voltage alternating signal to ring the bells of other telephones on the line and to alert the operator.
In the 1890s a new smaller style of telephone was introduced, packaged in three parts. The transmitter stood on a stand, known as a "candlestick" for its shape. When not in use, the receiver hung on a hook with a switch in it, known as a "switchhook." Previous telephones required the user to operate a separate switch to connect either the voice or the bell. With the new kind, the user was less likely to leave the phone "off the hook". In phones connected to magneto exchanges, the bell, induction coil, battery and magneto were in a separate "bell box." In phones connected to common battery exchanges, the bell box was installed under a desk, or other out of the way place, since it did not need a battery or magneto.
Cradle designs were also used at this time, having a handle with the receiver and transmitter attached, separate from the cradle base that housed the magneto crank and other parts. They were larger than the "candlestick" and more popular.
Disadvantages of single wire operation such as
crosstalkand hum from nearby AC power wires had already led to the use of twisted pairs and, for long distance telephones, four-wire circuits. Users at the beginning of the 20th century did not place long distancecalls from their own telephones but made an appointment to use a special sound proofed long distance telephone booth furnished with the latest technology.
What turned out to be the most popular and longest lasting physical style of telephone was introduced in the early 20th century, including Bell's Model 102. A carbon granule transmitter and electromagnetic receiver were united in a single molded plastic handle, which when not in use sat in a cradle in the base unit. The [http://www.porticus.org/bell/images/we-102.jpgcircuit diagram] of the Model 102 shows the direct connection of the receiver to the line, while the transmitter was induction coupled, with energy supplied by a local battery. The coupling transformer, battery, and ringer were in a separate enclosure. The dial switch in the base interrupted the line current by repeatedly but very briefly disconnecting the line 1-10 times for each digit, and the hook switch (in the center of the circuit diagram) permanently disconnected the line and the transmitter battery while the handset was on the cradle.
After the 1930s, the base also enclosed the bell and induction coil, obviating the old separate bell box. Power was supplied to each subscriber line by central office batteries instead of a local battery, which required periodic service. For the next half century, the network behind the telephone became progressively larger and much more efficient, but after the dial was added the instrument itself changed little until touch tone replaced the dial in the 1960s.
Public Switched Telephone Network(PSTN) has gradually evolved towards digital telephony which has improved the capacity and quality of the network. End-to-end analog telephone networks were first modified in the early 1960s by upgrading transmission networks with T1 carrier systems. Later technologies such as SONETand fiber optictransmission methods further advanced digital transmission. Although analog carrier systems existed, digital transmission made it possible to significantly increase the number of channels multiplexed on a single transmission medium. While today the end instrument remains analog, the analog signals reaching the aggregation point ( Serving Area Interface(SAI) or the central office(CO) ) are typically converted to digital signals. Digital loop carriers (DLC) are often used, placing the digital network ever closer to the customer premises, relegating the analog local loopto legacy status.
Internet Protocol (IP) telephony (also known as
Internettelephony) is a service based on Voice over IP(VoIP), a disruptive technologythat is rapidly gaining ground against traditional telephone network technologies. In Japanand South Koreaup to 10% of subscribers, as of January 2005, have switched to this digital telephone service. A January 2005 Newsweekarticle suggested that Internet telephony may be "the next big thing." [http://msnbc.msn.com/id/6831938/site/newsweek/] As of 2006 many VoIP companies offer service to consumers and businesses.
IP telephony uses a
broadbandInternet connection and IP Phones to transmit conversations as data packets. In addition to replacing POTS(plain old telephone service), IP telephony is also competing with mobile phone networks by offering free or lower cost connections via WiFihotspots. VoIP is also used on private wireless networks which may or may not have a connection to the outside telephone network.
IP telephony technology transforms many non-telephone electronics devices into
unified communications devices which simulate telephone usage, such as adding telephone-like features to portable game devices, digital picture frames, or handheld GPS receivers, typically by incorporating a voice engine. When used on a personal computer, an IP telephone is referred to as a soft phone.
By the end of 2006, there were a total of nearly 4 billion mobile and fixed line subscribers and over 1 billion Internet users worldwide. This included 1.27 billion fixed line subscribers and 2.68 billion mobile subscribers. [http://www.itu.int/newsroom/press_releases/2007/20.html]
Telephone operating companies
In some countries, many telephone operating companies (commonly abbreviated to "telco" in American English) are in competition to provide telephone services. Some of them are included in the following list. However, the list only includes facilities based providers and not companies which lease services from facilities based providers in order to serve their customers.
* [http://patft1.uspto.gov/netacgi/nph-Parser?patentnumber=0174465 US 174,465] -- "Telegraphy" (Bell's first telephone patent) -- Alexander Graham Bell
* [http://patft1.uspto.gov/netacgi/nph-Parser?patentnumber=0186787 US 186,787] -- "Electric Telegraphy" (permanent magnet receiver) -- Alexander Graham Bell
* [http://patft1.uspto.gov/netacgi/nph-Parser?patentnumber=0474230 US 474,230] -- "Speaking Telegraph" (graphite transmitter) -- Thomas Edison
* [http://patft1.uspto.gov/netacgi/nph-Parser?patentnumber=0203016 US 203,016] -- "Speaking Telephone" (carbon button transmitter) -- Thomas Edison
* [http://patft1.uspto.gov/netacgi/nph-Parser?patentnumber=0222390 US 222,390] -- "Carbon Telephone" (carbon granules transmitter) -- Thomas Edison
* [http://patft1.uspto.gov/netacgi/nph-Parser?patentnumber=0485311 US 485,311] -- "Telephone" (solid back carbon transmitter) -- Anthony C. White (Bell engineer) This design was used until 1925 and installed phones were used until the 1940s.
* [http://patft1.uspto.gov/netacgi/nph-Parser?patentnumber=3449750 US 3,449,750] -- "Duplex Radio Communication and Signalling Appartus" -- G. H. Sweigert
* [http://patft1.uspto.gov/netacgi/nph-Parser?patentnumber=3663762 US 3,663,762] -- "Cellular Mobile Communication System" -- Amos Edward Joel (Bell Labs)
* [http://patft1.uspto.gov/netacgi/nph-Parser?patentnumber=3906166 US 3,906,166] -- "Radio Telephone System" (DynaTAC cell phone) -- Martin Cooper et al. (Motorola)
Elisha Gray and Alexander Bell telephone controversy
History of the telephone
Invention of the telephone
Plain old telephone service
Public switched telephone network
Telephone related articles
Timeline of the telephone
Tip and ring(Wiring terminology)
* Coe, Lewis (1995), "The Telephone and Its Several Inventors: A History", McFarland, North Carolina, 1995. ISBN 0-7864-0138-9
* Evenson, A. Edward (2000), "The Telephone Patent Conspiracy of 1876: The Elisha Gray - Alexander Bell Controversy", McFarland, North Carolina, 2000. ISBN 0-7864-0883-9
* Baker, Burton H. (2000), "The Gray Matter: The Forgotten Story of the Telephone", Telepress, St. Joseph, MI, 2000. ISBN 0-615-11329-X
* Huurdeman, Anton A. (2003), "The Worldwide History of Telecommunications", IEEE Press and J. Wiley & Sons, 2003. ISBN 0-471-20505-2
* Josephson, Matthew (1992), "Edison: A Biography", Wiley, 1992. ISBN 0-471-54806-5
* Bruce, Robert V. (1990), "Alexander Graham Bell and the Conquest of Solitude", Cornell University Press, Ithaca, 1990.
* Kenneth P. Todd, [http://www.porticus.org/bell/capsule_bell_system.html "A Capsule History of the Bell System"]
* [http://www.1911encyclopedia.org/Telephone 1911 Britannica "Telephone" article]
* [http://www.telephones.se/ Vintage Telephones]
* [http://www.mobile.am/ Mobile Phones] - Specifications in a number of languages
* [http://www.theengineer.co.uk/Articles/294346/July+1877+The+Telephone.htm The Telephone, 1877]
* [http://www.dougrice.plus.com/dougnapTheory/index.htm Modelling Telephone hybrids as 2 x 2 matices ]
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