Essay/Term paper: The telephone

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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.

THE HANDSET

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.

THE TRANSMITTER

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
speech.

THE RECEIVER

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
speech.

TELEPHONE NETWORKS

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!

TELEPHONE HOOK SWITCH

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.

THE PHONE RINGER

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 that made the bell select one frequency.

For example, if you have 5 telephones on one party line. If a call came
through for line 1, the central board would send 10 Hz (this is a guess) signal
to the party line. Line 1 would ring and all the others would remain quiet. If
the call was for line 5, the central board would send a 50 Hz (this is also a
guess) signal so that only line 5 would ring.

The phone rings by applying voltage where needed, a resonant circuit is
made. The xx Hz signal would make a magnetic field around a device called the
hammer. The hammer is attracted and then repelled by the constant changing of
the magnetic field. If two gongs were placed on either side of the hammer, the
hammer would strike each gong successively.

Other phones can use a one gong system. This system is like the two gong
system, but more compact. Due to the compact in size, this ringer is perfect for
small wall phones or such.

THE ROTARY DIAL (PULSE)

A rotary dial creates equally spaced make -- and -- break pulses
according to how far the plastic dialing plate goes. A good description of a
dialing plate is like this: it has regularly spaced holes to dial with and a
metal object called the finger stop. That makes the number you want to go to as
easy as 1-2-3. Each hole in the wheel represents one number 1 through to 10.

By using some small gears and a device that times the velocity of the
return of the finger wheel after you have dialed, the internal switches are
opened and closed at a rate of 1 pulse per second. The number of pulses created
is determined by how far the finger wheel has gone around before being stopped
by the finger stop. Let's say that you dial the number 5, that means the
internal switches open and close 5 times before the finger stop stops it.

During the dial, a second set of switches remain closed and stay that
way for the entire time that you are dialing. The purpose for this second set of
switches is to keep the telephone receiver short for the whole dialing period.
If this switch was not there, you would hear loud and frustrating clicks in the
receiver.

TONE DIALING

There is also an alternative to the pulse dial, that is the tone dial.
Phones that use tone dialing are made with a piece of machinery that makes tones
on the phone line. These tones are transformed by the central board into numbers.


The act of putting an audio signal on the telephone line as a dialing
utility is called the DTMF (dual tone multi -- frequency) dialing. It is called
this because the tone dial makes a combination of two tones. These audio signals
are made by a mixture of both high and low frequencies. When a button on the
dial pad is pushed, vertical and horizontal tones are combined to make a signal.
It is this newly made tone that is sent down the central board and then
transformed back to the number.

TELEPHONE CORDS

Older telephone lines were made of fork shaped piece of metal attached
to wires with a tool called the crimper. When installed, these wires were
screwed into the terminal box on the wall. This is really a pain in the rear end
because if you are going to fix the phone, you have to unscrew the box, then all
the screws. This process could last for hours at a time.

To make this job a lot easier, coiled cords and modular lines were
invented. To take out the handset or telephone, all you have to do is to unplug
the modular connector from its match and that is it. Modular cords can be bought
nearly in any electronics store.

There are three kinds of cords. One is the full modular cord. There are
small modular clips on both ends of the cord. The second is the one mentioned in
the first paragraph, this is called the spade -- lug cord. The third one is
called the 1/4 modular, this cord has one modular connector on one side and the
old fashioned spade -- lug end on the other. These 1/4 cords are not very common.


BIBLIOGRAPHY

BOOK: THE TALKING TELEPHONE AUTHOR: STEVE SOKOLOWSKI PUBLISHER: TAB BOOKS NOV.
1991

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