Transcript Communicating by Wire

Communicating by Wire
The Telephone
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The telephone was one of the
many devices that was invented by
accident.
Alexander Graham Bell was a
speech teacher who was working
on ways to send several telegraph
messages at the same time over
one wire.
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He knew that if a musical note of a
certain frequency was played near a
group of tuning forks, only the tuning
fork tuned to that frequency would
vibrate.
Bell thought that he could send several
musical notes over the same wire,
several tuning forks at the other end
could sort out the notes.
Although Bell was never successful in
making this system work, it is the same
principle used in many electronic
communication circuits today.
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In 1875, Bell’s assistant, Thomas Watson,
was hanging metal strips to make musical
notes while Bell was in another room
listening to a receiver.
One of the metal strips accidentally got
stuck and acted as a diaphragm, picking
up the jingling noise of the other strips.
Bell heard the jingling noises and realized
what had happened.
How a Telephone Works
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The principle of the diaphragm is basic
to the operation of the telephone.
Simple telephones use a thin metal disk
as a diaphragm. When you speak into
the mouthpiece of a modern telephone,
the diaphragm vibrates.
It pushes against a container holding
carbon granules.
As the granules are packed more tightly
by the diaphragm’s pushing on them,
there electrical resistance goes down.
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As the diaphragm relaxes the pressure
on the granules, the electrical
resistance goes up.
If a voltage is applied to the container
holding the carbon granules, then the
varying resistance changes the current
flowing through the circuit.
A varying electrical current that exactly
represents speech is thus generated.
The telephone receiver earpiece has a
diaphragm made out of a metal that is
attracted to a small electromagnet.
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The current flowing through the
electromagnet is the varying current
produced by the mouthpiece of the
other telephone.
As the current varies, the attraction of
the electromagnet for the diaphragm
also varies.
This causes the diaphragm to vibrate in
exactly the same way the mouthpiece
diaphragm first did.
The vibrating diaphragm moves the air
turning the varying electric current back
into sound.
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The difference between the
telegraph and the telephone is that
the telegraph operates using on off
pulses of electric current, while the
telephone uses a smoothly varying
continues current.
Telegraph signals could only be
sent and received by someone who
had been trained to understand
the Morse Code.
A telephone could be used by
anyone without formal training.
The Telephone System
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The growth of the widespread telephone
use was slow at first.
The rate increased as the telephone
network grew.
At first, telephones were not as useful
as they are today because there were
only a few others that could be called.
(How useful would a telephone be if it
could only call two other people)
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Telephone networks were started
only within towns or small areas.
Communication with other distant
towns was not possible.
As the number of telephones grew,
networks grew to connect more
people over wider areas.
This made having a telephone
more desirable, further increasing
the number of telephones.
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By 1900, there were about one million
telephones in use in the United States.
When Bell died in 1922, all 13 million
telephones in Canada and the United
States were shut off for one minute in
tribute.
In the United States today, there are
more than 200 million telephones.
Any phone can be used to communicate
with any part of the world almost
instantly.
There are about 1.3 billion telephone
conversations each day in the United
States.