telephone sends and receives sound, especially the human voice, over wires in electric
When you dial a phone number, this causes
interruptions in the phones electric current. These interruptions create a pattern
representing numbers. Special electric switches called relays count those interruptions,
or pulses. When you dial a sequence of numbers, the automatic relays ring the person
To talk to the person, this is what happens. A
telephones transmitter is basically a microphone. It has loosely packed grains of
carbon in it. When someone talks in the telephone, the microphone vibrates a diaphragm in
the transmitter, causing the carbon grains to be squeezed and released. This motion
charges the electric current flow in the electric circuit.
Electric current is transmitted over wires to another
telephone, and the diaphragm in that telephone vibrates in response to changes in the
electric circuits magnetic field. So when the person youre calling picks up
the phone, he or she can hear your voice.
Long-distance telephone calls can also use radio waves or microwaves. Microwaves can be sent to an orbiting
communications satellite and relayed back to the earth.
Microwaves are electromagnetic radiation in the radio
frequency range of 1,000 to 300,000 megahertz (wavelengths of 300 to one millimeter, or
about 12 to about .04 inches). Microwaves are used in microwave ovens, of course, but in
radar and communications links as well.
Computers can communicate over
telephone circuits via a modem.
The cellular telephone doesnt need wires, so you can
take it almost anywhere. A cellular phone is a small radio transmitter. The phones
signals are too weak to be heard at long distances. The phone sends and receives microwave
signals to a base transmitter and receiving station. The receiving station retransmits the
signals to the place youre calling, connecting to a conventional (with wires)
A cellular telephone is called that because a geographic
region serving the cellular system is divided into areas called cells. Each cell has a
central base station and two sets of assigned transmission frequencies. The base station
uses one set of frequencies. The other frequency is used by mobile telephones.
To prevent radio interference, each cell uses frequencies
different from cells around it. (Cells far enough away from each other can use the same
frequencies, since signals wont travel far enough to cause interference.) When a
mobile telephone leaves one call and enters another, the telephone call is transferred
from one base station to the one in the new cell. Special circuits in the base station
detect the cellular phone signals strength. Cells next to other cells can also
detect a phones signal strength. So when the signal in one cell drops to where the
call may break up because the signal is too weak, the next cell with the strongest signal
A computerized switching system changes transmission
frequencies. This station switching continues automatically until you take your phone out
Each cellular phone is unique. Each has its own code or
serial number so base stations know whose phone is where. That same computer system that
switches transmission frequencies keeps track of all code numbers and where the phones are
within the cell, as well as who people call and how much they will be charged to make
Since cellular phones send microwave radio signals to make
a call, anyone with the right receiver can listen in to your call. Although cellular
telephones are very handy because they are mobile, cellular telephone calls are not
For more information on telephones and phone networks,
link to HowStuffWorks.com/telephone.htm.
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