The discovery
of the telephone
Telephone Instrument for communicating by
voice over long distances were invented by US inventor
Alexander Graham Bell 1876. The transmitter (mouthpiece) consists of a carbon
micro-phone, with a diaphragm that vibrates when a person speaks into it. The
diaphragm vibrations compress grains of carbon to a greater or lesser extent,
altering their resistance to an electric current passing through them. This
sets up variable electrical signals, which travel along the telephone lines to
the receiver of the person being called. There they cause the magnetism of an
electromagnet to vary, making a diaphragm above the electromagnet vibrate and
give out sound waves, which mirror those that entered the mouthpiece
originally.
The Microphone Primary is a component in a
sound-reproducing system, whereby the mechanical energy of sound waves is
converted into electrical signals by means of a transducer. One of the simplest
is the telephone receiver mouthpiece, invented by Scottish-US inventor Alexander
Graham Bell in 1876; other types of microphone are used with broadcasting and
sound-film apparatus.
Telephones have a carbon microphone, which reproduces only a narrow
range of frequencies. For live music, a moving-coil microphone is often used. In
it, a diaphragm that vibrates with sound waves moves a coil through a magnetic
field, thus generating an electric current. The ribbon microphone combines the
diaphragm and coil. The condenser microphone is most commonly used in recording
and works by a capacitor.
Telecommunications and Communications over a
distance is generally made by electronic means.
Long-distance voice communication was pioneered 1876 by Scottish
scientist Alexander Graham Bell when he invented the telephone. Today it is
possible to communicate with most countries by telephone cable, or by satellite
or microwave link, with over 100,000 simultaneous conversations and several
television channels being carried by the latest satellites. Integrated-Services
Digital Network (ISDN) makes videophones and high-quality fax possible; the
world's first large-scale center of ISDN began operating in Japan 1988.
ISDN is a system that transmits voice and image data on a single transmission
line by changing them into digital signals. The chief method of relaying
long-distance calls on land is microwave radio transmission.
The first mechanical telecomunications
systems were the semaphore and heliograph (using flashes of sunlight), invented
in the mid-19th century, but the forerunner of the present telecommunications
age was the electric telegraph. The earliest practicable telegraph instrument
was invented by William Cooke and Charles Wheatstone in Britain 1837
and used by railroad companies. In the US,
Samuel Morse invented a signaling code, Morse code ,
which is still used, and a recording telegraph, first used commercially between
England and France 1851.
Following German physicist Heinrich Hertz's discoveries using electromagnetic
waves, Italian inventor Guglielmo Marconi pioneered a `wireless' telegraph,
ancestor of the radio. He established wireless communication between England and France 1899 and across the Atlantic
1901. The modern telegraph uses teleprinters to send coded messages along
telecommunications lines. Telegraphs are keyboard-operated machines that
transmit a five-unit Baudot code. The receiving teleprinter automatically
prints the received message.
The drawback to long-distance voice
communication via microwave radio transmission is that the transmissions follow
a straight line from tower to tower, so that over the sea the system becomes
impracticable. A solution was put forward 1945 by the science-fiction writer
Arthur C Clarke, when he proposed a system of communications satellites in an
orbit 35,900 km/22,300 mi above the equator, where they would circle the Earth
in exactly 24 hours, and thus appear fixed in the sky. Such a system is now in
operation internationally, by Intelsat . The
satellites are called geostationary satellites (syncoms). The first to be
successfully launched, by Delta rocket from Cape Canaveral,
was Syncom 2 in July 1963. Many such satellites are now in use, concentrated
over heavy traffic areas such as the Atlantic,
Indian, and Pacific oceans. Telegraphy, telephony, and television transmissions
are carried simultaneously by high-frequency radio waves. They are beamed to
the satellites from large dish antennae or Earth stations, which connect with
international networks. Recent advances include the use of fiber-optic cables
consisting of fine fiberglass for telephone lines instead of the usual copper
cables. The telecommunications signals are transmitted along the fibers on
pulses of laser light.
telecommunications Satellite dish. Geostationary communications
satellites over the Earth's equator which orbit in 24 hours permit connections
between all points on the Earth's surface to be made using such dishes.
Satellite dishes are commonly used by European households to receive television
channels broadcast by satellite.
