The Telephone

About 100 years ago, Alexander Graham Bell invented the telephone by
accident with his assistant Mr. Watson. Over many years, the modern version of
the telephone makes the one that Bell invented look like a piece of junk.
Developments in tone dialing, call tracing, music on hold, and electronic
ringers have greatly changed the telephone.

This marvelous invention allows us to communicate with the entire globe
24 hours a day just by punching in a simple telephone number. It is the most
used piece of electronic apparatus in the world. It is probably one of the most
easy to use electronics available too. All you have to do is pick up the
receiver, listen for the tone, and then select a number using either tone or
pulsing dial.

A telephone can be separated into two main categories: there is the tone
(touch tone) or the older rotary dial (pulse) telephones. Then you can divide
those into other categories such as business line (multi -- line) or home line
(single line). You can also have many other types of phones: there are those
that hang on the wall, on the desk, etc.


No matter what kind of telephone you own, there has to be some device
that allows you to talk to and listen to. This device is called the handset. The
handset is usually made out of plastic and inside it are two main components:
the transmitter and the receiver.


It is the job of the transmitter to turn the air pressure created by
your sound waves to electrical signals so they can be sent to the other
telephone. The waves hit a thin skin called the diaphragm that is physically
connected to a reservoir of carbon granules. When the pressure hits the
diaphragm, it shakes up the carbon granules. Then the carbon expands and
contracts, depending on what force is exerted. At two points on the outer shell
of the reservoir of the carbon are two outlets of electricity from the talk
battery. By applying voltage, a current is made and is passed along the lines to
the waiting telephone. At the other end the current is transformed back to


The receiver turns an ever varying current back to speech. A permanently
magnetized soft iron core is covered in many turns of very fine wire. Through
the wire, the electrical current is applied. The currents attract and repel an
iron diaphragm. By the vibrating actions the diaphragm does, a different
pressure is created and these pressures are translated by ear into intelligible


If you have ever opened up a phone (do not try this at home, you might
screw it up) you will probably see a PC (printed circuit) board. The board
contains the needed electronics for the phone to work properly. In older models
of a working telephone, this board may look like an electronic box. This board
is called the telephone network.

The telephone network\'s function is to provide all the necessary
components and termination points (screw on or push on terminals). The
components and the termination points connect and match the impedance of a
handset (transmitter and receiver) to a two -- wire telephone circuit.

Every component in the telephone has to be connected to the PC board.
Usually, the board is the most reliable component inside the phone. All the
delicate components are securely sealed by a metal enclosure. The PC board is a
very fragile object and can be broken easily. If you look closely, you can see
wires poking out of the board. The wires are soldered to the terminal legs. If
you break one of those wires, man are you dead!


Every time you talk over a line, you always need to disconnect. The most
simple thing to do is to let the handset sit down. While sitting down, the
handset can give force to a spring loaded operating arm, which is connected to a
number of switch contacts. When this happens, the phone disconnects.


Once a call has been dialed through, the telephone of the person being
called must be given some kind of signal to let him/her know that he/she has
been called. This is when the telephone rings. This type of signal is generated
using an alternating current somewhere between 90 to 220 V with a frequency of
30 Hz.

But what if you have 5 or 6 phones connected on a party line? How can
you signal one telephone to ring? The answer is by frequency selection. Older
telephones had a different capacitor and ringer coil impedance values. It was
these small differences