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22
attracting the diaphragm further inward. When the current is weakened, the
attraction on the receiver's diaphragm is weakened too, and it moves outward. The
vibrations of the receiver's diaphragm match those of the original sound The
movements of the diaphragm set up waves in the air immediately surrounding it,
and the sound of the caller's voice passes to the ear of the person being called.
Telephone Exchanges. When the telephone service began, all calls were
connected manually by an operator at a local exchange who had in front of him or
her a switchboard consisting of a bank of jacks. Plugs connecting the telephone of
each subscriber to the exchange could be inserted into the jacks and coupled up so
that one subscriber could talk to another.
An automatic process of call connection, known as automatic switching, was first
introduced in 1889. The telephone user called up the number required by pushing
each of a series of buttons a certain number of times. At the exchange the electrical
contacts leading to other users' telephones were arranged in rows on a cylinder.
When a number was called, an arm on the central shaft of the cylinder was moved
step by step in accordance with the digits of the number until it found the contact
of the called subscriber. It then moved across and made the connection, causing the
called subscriber's phone to ring.
Improvements to this system were made, including the introduction of a rotary
dial for selecting the numbers, and the first fully automatic exchange using
mechanical equipment was opened in Omaha, Nebraska, in 1921. A major
development in automatic switching was brought about by the introduction of
electronic tubes, which were later replaced by transistors. These pieces of
equipment could be arranged in circuits that could be either "on" or "off' and could
be set up to work in a similar way to the old mechanical selectors. Further
advances were made in the second half of the 20th century, and by the late 1980s
fully computerized automatic switching was commonplace. The telephone handset
too underwent many changes in design. Rotary dials were improved and eventually
replaced by "push-button" keypads. Telephones fitted with suitable silicon chips
have an internal memory. They can remember frequently dialed calls, can display
the number of somebody who is calling, and in some cases can be used as small
computers. "Cordless" telephones, which are portable radio-telephones, arc also
available. For more information about radio-telephones, see the next section of this
article.
Lines of Communication
Up to the 1960s, telegraph and telephone messages were carried from transmitter
to receiver along insulated copper wire cables. The great spread of telegraph
services in the second half of the 19th century led to the need to communicate over
longer and longer distances. Overhead wires supported on poles and requiring little
protective covering could be used for connecting two places inland. However, it
was more of a problem to connect two places separated by sea. Cables had to be
laid under water, they had to be protected by insulation, and they had to be strong
enough to withstand high tension (that is, a strong pulling force) and immense
pressure from the seawater above them. The first undersea cables were laid in the
1850s and in 1858 the first transatlantic cable joined Ireland with Newfoundland.
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