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A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into electronic signals that are transmitted via cables and other communication channels to another telephone which reproduces the sound to the receiving user. The term is derived from Greek: (tle, far) and (phn, voice), together meaning distant voice. A common short form of the term is phone, which came into use early in the telephone's history.[1]

The essential elements of a telephone are a microphone (transmitter) to speak into and an earphone (receiver) which reproduces the voice at a distant location.[3] The receiver and transmitter are usually built into a handset which is held up to the ear and mouth during conversation. The transmitter converts the sound waves to electrical signals which are sent through the telecommunication system to the receiving telephone, which converts the signals into audible sound in the receiver or sometimes a loudspeaker. Telephones permit transmission in both directions simultaneously.

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Most telephones also contain an alerting feature, such as a ringer or a visual indicator, to announce an incoming telephone call. Telephone calls are initiated most commonly with a keypad or dial, affixed to the telephone, to enter a telephone number, which is the address of the call recipient's telephone in the telecommunication system, but other methods existed in the early history of the telephone.

The first telephones were directly connected to each other from one customer's office or residence to another customer's location. Being impractical beyond just a few customers, these systems were quickly replaced by manually operated centrally located switchboards. These exchanges were soon connected together, eventually forming an automated, worldwide public switched telephone network. For greater mobility, various radio systems were developed for transmission between mobile stations on ships and automobiles in the mid-20th century. Hand-held mobile phones were introduced for personal service starting in 1973. In later decades, their analog cellular system evolved into digital networks with greater capability and lower cost.

Convergence in communication services has provided a broad spectrum of capabilities in cell phones, including mobile computing, giving rise to the smartphone, the dominant type of telephone in the world today.

Before the development of the electric telephone, the term telephone was applied to other inventions, and not all early researchers of the electrical device used the term. Perhaps the earliest use of the word for a communications system was the telephon created by Johann Sigismund Gottfried Huth [de] in 1796. Huth proposed an alternative to the optical telegraph of Claude Chappe in which the operators in the signaling towers would shout to each other by means of what he called "speaking tubes", but would now be called giant megaphones.[5] A communication device for sailing vessels, called telephone, was invented by Captain John Taylor in 1844. This instrument used four air horns to communicate with vessels in foggy weather.[6][7]

The term telephone was adopted into the vocabulary of many languages. It is derived from the Greek: , tle, "far" and , phn, "voice", together meaning "distant voice".

Credit for the invention of the electric telephone is frequently disputed. As with other influential inventions such as radio, television, the light bulb, and the computer, several inventors pioneered experimental work on voice transmission over a wire and improved on each other's ideas. New controversies over the issue still arise from time to time. Charles Bourseul, Antonio Meucci, Johann Philipp Reis, Alexander Graham Bell, and Elisha Gray, amongst others, have all been credited with the invention of the telephone.[8][9]

Alexander Graham Bell was the first to be awarded a patent for the electric telephone by the United States Patent and Trademark Office (USPTO) in March 1876.[10] Before Bell's patent, the telephone transmitted sound in a way that was similar to the telegraph. This method used vibrations and circuits to send electrical pulses, but was missing key features. Bell found that this method produced a sound through intermittent currents, but in order for the telephone to work a fluctuating current reproduced sounds the best. The fluctuating currents became the basis for the working telephone, creating Bell's patent.[11] That first patent by Bell was the master patent of the telephone, from which other patents for electric telephone devices and features flowed.[12]

In 1876, shortly after Bell's patent application, Hungarian engineer Tivadar Pusks proposed the telephone switch, which allowed for the formation of telephone exchanges, and eventually networks.[13]

In the United Kingdom, the blower is used as a slang term for a telephone. The term came from navy slang for a speaking tube.[14] In the U.S., a somewhat dated slang term refers to the telephone as "the horn", as in "I couldn't get him on the horn", or "I'll be off the horn in a moment."[15]

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 plant personnel was to visit each telephone periodically to inspect the battery. During the 20th century, telephones powered from the telephone exchange over the same wires that carried the voice signals became common.

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 port opening for sound, with the user alternately listening and speaking (or rather, shouting) into the same hole. Sometimes the instruments were operated in pairs at each end, making conversation more convenient but also more expensive.

At first, the benefits of a telephone exchange were not exploited. Instead, telephones were leased in pairs to a subscriber, who had to arrange for a telegraph contractor to construct a line between them, for example, between a home and a 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 City and 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 in a ringer box, 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 AC ringer signal through while still blocking DC (keeping the phone "on hook"). Telephones connected to the earliest Strowger switch 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. Large wall telephones in the early 20th century usually incorporated the bell, and separate bell boxes for desk phones dwindled away in the middle of the century.

Rural and other telephones that were not on a common battery exchange had a magneto 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. Some local farming communities that were not connected to the main networks set up barbed wire telephone lines that exploited the existing system of field fences to transmit the signal.

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 or "ringer box".[16] In phones connected to common battery exchanges, the ringer box was installed under a desk, or other out-of-the-way place, since it did not need a battery or magneto.

Disadvantages of single-wire operation such as crosstalk and 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-distance calls from their own telephones but made an appointment and were connected with the assistance of a telephone operator.[17]

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 202-type desk set. A carbon granule transmitter and electromagnetic receiver were united in a single molded plastic handle, which when not in use was secured in a cradle in the base unit. The circuit diagram of the model 202 shows the direct connection of the transmitter to the line, while the receiver was inductively coupled. In local battery configurations, when the local loop was too long to provide sufficient current from the exchange, the transmitter was powered by a local battery and inductively coupled, while the receiver was included in the local loop.[18] The coupling transformer and the ringer were mounted in a separate enclosure, called the subscriber set. The dial switch in the base interrupted the line current by repeatedly but very briefly disconnecting the line one to ten times for each digit, and the hook switch (in the center of the circuit diagram) disconnected the line and the transmitter battery while the handset was on the cradle. ff782bc1db