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"I'LL GIVE YOU A CALL", we often say to our friends, but we rarely wonder how it is that, only a few seconds after entering a number on a key pad, we can be speaking to our friend many kilometers away -- even on the other side of the world. A telephone is a device that transforms a person's voice into an electrical signal made up of varying electric currents. This signal travels along copper cables to reach its destination. Sometimes, it is changed into pulse of light and sent along thin glass strands called optical fibres. The signal can also be transmitted as radio waves or microwaves.
Every call reaches its destination via a network of communication links. A local exchange can make connections with any telephone in the caller's area. Long-distance connections are made via national or internation exchanges, or even satellites. Cellular exchanges handle the radio signals that carry calls to and evenmobile phones.
How telephones work
Once two telephones are linked via the telephone network, the sounds of the speakers' voices are picked up by microphones in the handsets. Loudspeakers reproduce these sounds, so that each caller can hear what the other is saying.
1905 Candlestick phone: users asked the exchange operator to dial the number they wanted to call.
1890 Crank handle telephone: user turned the crank to contact the operator in order to call, and again to tell the operator that the call was finished.
1930 Candle stick phone with dial: allowed user to call without going through operator.
1920 Wlnut-effect phone: moulded in Bakelite plastic to look like wood.
1930 Coloured phone: new plastics allowed different coloured phone.
1965 Compact table phone: designed for use at home.
mid-1990 Modern phone: this phone stores frequently called numbers in its electronic memory.
1914-18 Metal dial: was used from pre-world war (1914-18)
1930 Coloured plastic dials: -/-
1920 Black dial: Swiss-designed phone.
1963 Lightweight plastic dial: -/-
1960 Alphanumeric dial: designed with both letters and numbers.
1960 Phone engineer's dial: -/-
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Long-distance telephone calls are often sent as microwave signals via satellites orbiting high above the Earth. The satellite boosts (strengthens) the signal and sends it to a reciving satellite station back to Earth.
An exchange is a building containing equipment that recognizes dialled pulse and tones, and sends calls to the correct destination. This prosess, known as switching, is controlled by powerful computers inside the exchange.
Mobile phones allow you the freedom to talk from almost any where, because they are not physically connected to the network. They send and recieve calls as radio-wave signals
Alexander Graham Bell
In 1875, the Scottish-born inventor Alexander Graham Bell (1847-1922) made the first successful transmission of the human voice along an electrical wire.
He patented the telephone the following year, beating his American rival Elsha Gray (1835-1901) by just two hours.