How a phone worksThe
telephone handles two kinds
of information: signals and
voice, at different times on
the same twisted pair of wires.
The signaling equipment
consists of a bell to alert
the user of incoming calls, and
a dial to enter the phone
number for outgoing calls. A
calling party wishing to
speak to another telephone will
pick up the handset, thus
operating the switchhook, which
puts the telephone into
active state or off hook with a
resistance short
across the wires, causing current to flow.
The telephone
exchange detects the DC current, attaches a
digit receiver,
and sends dial tone to indicate readiness.
The user pushes
the number buttons, which are connected to
a tone generator
inside the dial, which generates DTMF
tones. The exchange
connects the line to the desired line
and alerts that line.
When a phone is inactive,
that is on hook, its bell,
beeper, flasher or other alerting
device is connected
across the line through a capacitor. The
inactive phone
does not short the line, thus the exchange
knows it is on
hook and only the bell is electrically
connected. When
someone calls this phone, the
telephone exchange applies a
high voltage pulsating signal,
which causes the sound
mechanism to ring, beep or otherwise alert
the called
party. When that user picks up the handset, the
switchhook
disconnects the bell, connects the voice parts of
the
telephone, and puts a resistance short on the line,
confirming that the phone has been answered and is active.
Both lines being off hook, the signaling job is complete.
The parties are connected together, and may converse using
the voice parts of their telephones.
The voice parts of the telephone are in the
handset, and
consist of a transmitter (often called
microphone) and a
receiver. The transmitter, powered from the
line, puts out
an electric current which varies in response
to the
acoustic pressure waves produced by the voice. The
resulting variations in electric current are transmitted
along the telephone line to the other phone, where they are
fed into the coil of the receiver, which is a miniature
loudspeaker. The varying electric current in the coil
causes
it to move back and forth, reproducing the acoustic
pressure
waves of the transmitter. Thus, it speaks.
When a party "hangs up", that is puts the handset
on the
cradle, DC current ceases to flow in that line, thus
signaling to the exchange switch to disconnect the
telephone call.
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