how it work the telephone

A CST PUBLICATION
HOW
IT
WORKS
THE
.L
J
1""
I
I

~
,
BY
NAVKALA
ROY
DESIGNED
AND
ILLUSTRATED
BY
SUBIR
ROY

How
it
works
.
__
.

THE
CHANGING
SHAPE
OF
SOUND
I
I-
1875
-
The
first
telephone
instrument
made
by
Alexander
Graham
Bell in
1875
I
1879
1880 1880
1905
The
telephone
t-
today
I
The
candlestick

telephone
of
1905
The
Gower-Bell
telephone
of
the
early
1880's
with
two
listening
tubes
This
device required
the
user

to
speak
into
the
box
with
the
receiver to
his
ear (1880)
I

An
Edison receiver (1879)
I
R.
Watson, come here, I want to see
you," shouted
an
angry Bell.
Watson jumped out of his chair. There was
no one
in
the room. Yet he'd heard a voice.
It was a familiar voice and it was loud and
clear. Then suddenly it hit him. The
telephone. It had come alive at last. The
miracle had happened. .
Watson rushed to Bell's room, breathless
with joy. "I could hear you. It works," he said.
That was March 10, 1876. More than a
hundred years ago.
From ship to shore; from air to land; from
car to car; from just about anywhere to
anywhere today you can speak to someone
by just dialling a number.
In
fact, you have
the world at your finger-tips. And when
Astronaut Rakesh Sharma calls up Mrs. Indira
Gandhi from space you just take it
in

your
stride.
So
dramatic has been the development
of the telephone. And only forty years before
the telephone was invented, man was patting
himself
on
his back for having perfected the
methods of communication.
That was when the electric telegraph was
used.
It
was
in
1838 that the American,
Samuel Morse, patented his single wire
telegraph. His design used the famous Morse
code
in
which combinations of short and long
signals - dots and dashes - indicate letters.
Messages were sent
at
up to ten words a
minute with a hand-operated key and were
received as marks made by a pen
on
a paper
tape. These signals had to

be
decoded
and
written out by hand.
In
1855 Professor David Hughes invented
a printing telegraph. The operator sent
messages from a keyboard, each key of
which represented a letter. The machine
turned the letters into electric signals
automatically and, at the other end, another
machine printed the message.
These were major breakthroughs
in
the
field of communication, but still not the same
as 'talking' to someone,
and
nowhere near
having a cosy chat with someone.
o
It
was at this time that Alexander Graham
Bell, the young professor of speech, began
his experiments with electricity. Often
he would visit the mills and factories located
near his house and observe how the
machines were operated. Once he called on
Charles Wheatstone, the inventor of the
.magnetic needle telegraph. So impressed

was
he
by this mail that
he
determined to
follow
in
his footsteps.
Bell was keen to develop a telegraph·
system that would allow multiple transmission
of messages at once. He felt that this could
be
achieved by transmitting each message
on a separate, specially tuned steel strip,
or
reed. Each reed would vibrate a different
number
of
times per second and so produce
a different musical note.
It was while one such experiment was
being carried out, on June
2,
1875, that a
receiving reed, which was being watched
closely by his assistant, Thomas Watson,
in
another room, failed to vibrate. Watson
thought the reed was stuck and pulled at it.
When he did that. a similar receiving reed

vibrated
in
Bell's room.
"What's this!" said Bell astonished, but
realized almost immediately that he had hit
upon something great. He had discovered
that a tiny electric current caused
by
one
vibrating reed was powerful enough to cause
another reed to vibrate audibly. He also
realized that instead of a single note the reed
had reproduced several notes. Human
speech, as Bell knew only too well, is also
made up of a mixture of sounds of different
frequencies and Bell believed that he could
use this system to transmit the human voice.
Lo
and behold, a month later, Bell produced
a pair of simple telephones.
Bell had made a deep study
on
sounds as
he had always wanted to help deaf and dumb
children. He, therefore, knew that a stretched
membrane would be more suitable for sound

'"

. Bell demonstrating

the first telephone
I
/

Number, please
As news spread, a keener interest was
created
in
the telephone, though it was
restricted to small areas until the 1890's.
Individual subscribers were connected to
each other by exchanges that were controlled
by operators.
When somebody wished
to
make a call all
he did was lift the receiver and wait for the
operator's response.
"Number, please," the operator would say
and connect you to the number you wanted.
In
fact, so personal was everything those
days that
on
some exchanges all you did
•
.

•
•

•
•
.
.
reproduction than a reed. He finally decided
to use
an
iron diaphragm. On March 10, 1876,
when
he
accidently discovered that his phone
worked, he was delirious with joy.
It
was the first time
in
the world that people
could talk to each other over long distances
and
feel that they had almost met the person.
After all there can
be
no
substitute for a
human voice.
Bell was keen to promote the idea of this
new device and travelled extensively
in
the
United States and Europe to spread the word.
He

even demonstrated how one could talk to
someone under water.
But most people pooh-poohed the idea.
In
London, a post office official said it would
never catch on because there were sufficient
messenger boys.
Finally on January 24, 1878, Bell carried
out a demonstration for Queen Victoria at
Osborne House,
on
the Isle of Wight. So
impressed was the Queen that she asked
Bell to supply her with telephones
Immediately.
•
An
1879
hand-operated
switchboard
was lift the receiver and ask for the person
you wished to talk to. Only one had to shout
in
order to be understood by the other person.
Early models resembled a box camera with
a round projection
at
one end. This served
as the transmitter and receiver. So anyone
making a call had to

be
extremely careful
while moving his ear and mouth. Bruised lips
and ears were not
an
uncommon sight.
In
fact, one model carried the notice: "Do not
listen with your mouth and talk with your ear!"
•
•
Do
Not
Li~ifN
With
YWR
Mouth
A~d
1ciJk
with
•
OUI"
Bo.Y'!
•
~
0
As
Bell's transmitters had poor sensitivity,
calls were limited to a few miles.
It

was at
this time that Thomas Alva Edison, the
famous American inventor, stepped in.
Edison was the next best thing that
happened to the telephone. He produced a
telephone with a separate mouthpiece and
a much superior transmitter with a carbon
component. When spoken into,it changed the
sound of the voice into a varying electrical
signal which was converted back into speech
by the ear-piece at the other end.
By
the beginning of the 1900's, the
telephone had grown
in
popularity, especially
in
the United States. Some exchanges were
so large that there were long lines of
operators seated at switch boards made up
of hundreds of plugs and sockets.
India, believe
it
or not, was one of the first
countries
in
the world to have a telephone
exchange. And Calcutta was where
it
all

started.
In
1881, barely five years after Bell made
his discovery, a 50-line exchange was set up
in
Calcutta. Then came the automatic
telephone exchange with 700 lines, which
was established
in
Shimla
in
1913. But
it
was
only after
1951
that the Indian telephone
service made rapid progress. Subscriber
Trunk Dialling (STD), first introduced between
Kanpur and Lucknow
in
1960, now operates
on
practically every route
in
India and many
outside the country too.
'Tele' literally means 'at a distance'
and
'phone' is

an
instrument using sound. Thus
'telephone' would imply 'an instrument that
carries sound from a distance.'
Today telephone users
in
most parts of the
world can dial 80% of the world's subscribers
directly. Telephone 'hot-lines' keep world
leaders
in
contact with each other to avoid
the accidental outbreak of a nuclear war.
Even
on
the battlefield it
is
now possible to
link soldiers to the international telephone
network and a person from the most isolated
oil platform
in
the sea can make calls
throughout the world.
Your parents can hold international
business meetings by merely going to a
closed circuit television studio and talking to
executives
in
similar studios

in
other countries
while the television pictures and the sound
are being carried over the telephone network.
The telephone network has also been able
to link computers
in
many countries to vast
information networks. It can transmit
television programmes such as the Olympic
Games to more than a 100 countries. It can
be
used to turn a television set into a terminal
connected to a computer, providing vast
amounts of information through videotex.
In
the future
this
could form the basis of
an
electronic mail service with people sending
private messages from one television set to
another via the telephone network. It would
be
cheaper and much faster than
conventional post.
With so much happening around us it is
hard to believe that once upon a time
messages were sent
by

using a line of
bonfires on hill-tops, by beating drums or
tying notes to carrier pigeons speci.ally trained
to fly home quickly from a distance.
Few of us realize how complex
and
ingenious
is
the mechanism that is set
in
motion the moment one dials a telephone
number. How does your voice get carried
through miles
and
miles of wire? How is it
that
you
can hear even kids crying
in
the
background, doors slamming and music
playing through the wires of the telephone?
Without the telephone today, business
and
social life would
be
seriously disrupted. This
was demonstrated in 1979 when a strike by
telephone workers halted the telephone sy-
stem

in
Ireland for several weeks. Millions
of pounds worth of orders were lost because
companies could not reply quickly to requests
and their business was won
by
competitors.
In
order to understand this let's first
understand sound. Air helps sound to travel.
If there was
no
air we would not hear any
sound. You can prove this
by
placing a bell
under a glass bowl and ringing it. You'll be
able to hear it clearly. Now if you draw the
air out with a suction pump, the sound of the
bell will disappear. This
is
because there is
nothing to carry the sound.
Air is something quite real, even though
we cannot see
it.
Just
as
ripples are made
in

water, they are made
in
the air too and
How
sound travels
are called sound-waves. Let us take for
instance, a sheet of metal and see what
happens when we hit
it.
The force
of
the blow
makes the metal tremble. The to and fro
motion
so
caused
is
called vibration. A guitar
string vibrates when we pluck
it.
As
the metal
sheet vibrates it pushes the air forward and
backward quickly, so that little ripples or
waves are made; which travel away from the
metal
in
all directions.
These waves
in

the air are
so
tiny that
you
cannot feel them, yet they are strong enough
to make another sheet of metal vibrate when
, :
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I
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_.
-


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~
Smoke signals .•
carner
pigeons horses.
man
has used all
these to
fulfil
a vital
need-eommunlcatlon

so rapId

has
been
the
progress
that
today. thanks
to
the
telephone.
we
literally
have
the
world
at our
fingertips
l
"
/"
<2
•

'';
f
./
~
C
r ""
c.

they hit
it.
This
you
can
tryout
by
hanging
two sheets of metal of the same size
and
shape facing one another. Hit the first and
make it ring. Then put your hand
on
it
to
stop the vibrations.
If
you listen carefully, you
will hear the second sheet of metal vibrating.
This
is
how we hear a noise. The first sheet
is
like the vocal cords
in
our throats, which
we move while speaking and set vibrating.
The second sheet
is
like the tight ear-drum

in
our ears. When the air waves strike our
ear-drums we hear a sound.
A telephone works basically on the same
principles - the carrier of the sound being
electricity. When you want to talk to a friend
you lift the receiver and dial a number. Your
line
is
then connected to your friend's at the
telephone exchange. Electricity flows through
the wires. The sound-waves of your voice
make the electric current stronger and weaker
as the case may
be.
In
your friend's phone
the current
is
changed back into sound-
waves.
We know about sound and sound-waves.
Now let
us
follow these sound-waves as they
go
into a telephone.
Inside the telephone mouthpiece
is
a thin

circular piece of
metal-
the diaphragm. This
vibrates whenever any sound hits
it.
Behind
it
is
a container which contains tiny grains of
1.
Carbon grains loose. Little
flow
of
current
2. Carbon grains
tight
when
flow
of
current
increases
specially prepared carbon The lead of your
pencil
is
also a form of carbon. Through this
container of carbon and through the wires of
the telephone flows
an
electric current.
The moment sound-waves hit the thin piece

of metal it bends a little - so little that you
cannot notice
it.
When
it
bends (or vibrates)
it
squashes up the carbon. The grains thus
get tightly packed. When there
is
no
pressure
on
the diaphragm the grains are let loose
again. This too you would not
be
able to
detect with the naked eye.
What Edison discovered was that
an
electric current will pass through grains of
carbon more easily when they are tightly
packed than when they are loose.
So,
when you speak, your voice causes
vibrations
in
the metal disc. These vibrations
compress the carbon grains according to the
intensity of your voice, which,

in
turn, causes
different amounts of electricity to pass
through the telephone wires, until it reaches
the receiver of another phone.
Tran8lYlitter
3
Receiver
1.
Diaphragm 2. Carbon granules 3. Diaphragm
4. Electromagnet
In
the receiver of a telephone is a thin
metal disc or diaphragm, just as
in
the
mouthpiece. But the receiver does not have
the little container of carbon grains. Instead
there is
an
electro magnet, that
is,
a magnet
worked by the electricity coming through the
wires. The more the electricity coming
through, the more powerful the magnet
becomes. This magnet pulls on the thin metal
disc
in
the receiver and makes it vibrate.

These vibrations will sound exactly like your
voice talking into the mouthpiece at the other
end, a long long way away.
The
ice-cream
cup
telephone
Do'
ou
e
To understand this better, you can make
yourself a very simple telephone. When
you've eaten your favourite ice-cream don't
throw the cup away.
In
fact, get hold of two
ice-cream cups. Pierce a hole
in
the centre
of each. Take a long piece of fine thread and
pass each end through the two cups. Knot it
well so that it doesn't come out. And that's
it.
As
long
as
the thread
is
stretched out tight,
you have your telephone. One person talks

into one ice-cream cup the bottom of which
functions
as
a diaphragm and the other
person puts his ear
to
the other cup. When
you speak into the cup you make
t~e
b~ttom
vibrate_
The thread carries the vibration by
little tugs and makes the other diaphragm
vibrate
in
exactly the same way. That makes
new waves of the same kind
in
the other cup
and so the other person hears what you say.
You'll be able to hear each other
as
long as
the thread is kept tight.
•
•
•
•
Wherever there are telephones there must
be wires. These wires are special as they

have to pass over different lands, under water
and over mountains too. Their job
is
to carry
the electric currents from phone to phone.
If you call up someone who is just a few
miles away from you, this
is
what happens.
You speak into the mouthpiece and the
electrons or particles
of
electricity
in
the
mouthpiece start bouncing against the other
electrons that form the electrical current
in
the wire. The current varies according to the
vibrations of your voice and
in
a fraction of
a second the person you've called can hear
you.
So
fast does electricity travel that before
you can snap your fingers the current passing
through a telephone wire can flash all round
the world.
When the electrons

at
one end of the line
start to pass the current along, they are very
strong. If the current were to travel a long
distance it would gradually reduce in strength
and would not
be
strong enough to work the
The world's longest submarine telephone
cable is the Commonwealth Pacific Cable
(COMPAC) which runs for more than 14,480
kms. from Australia via Auckland, New Zea-
land and the Hawaiian Islands to Port Alberi,
Canada. It cost about £35,000,000 and was
inaugurated
on
December
2,
1963.
magnet at the receiving end. Therefore,
where telephone wires extend over long
distances, there are special stations along
the way. These are called "repeaters". They
have equipment similar to amplifiers
in
a radio
set. This equipment helps
to
boost the current
carried

by
the wires.
The transatlantic telephone cable, for
instance, carries 120 two-way telephone
circuits. For this, special undersea repeater
equipment was designed. Every
35
miles
along the cable there
is
a bulge that contains
the equipment to boost the signals. These
undersea repeaters are sealed
in
containers
that withstand pressures upto 8000 pounds
a square inch so that they cannot
be
crushed
by
the tremendous weight of water above
them.
And, what happens, if something goes
wrong with these cables? It's precisely for
this reason that
an
accurate map has to
be
made of where the cable
is,

so that
it
can
easily
be
located when repairs are needed.
You may wonder how we are able to talk
to so many different people when our
telephones have only one set of wires. At the
same time think how difficult it would be if
you were to have a different wire for every
house you wanted to talk to.
To
avoid this
each telephone has its own set of wires
running to a telephone exchange or central
office.
When
you
pick up the receiver and dial the
number
you
want, your phone
is
immediately
connected to the exchange.lt is then that the
next step of the journey is determined.
If
you
At

the
exchange
00
The first telephone exchange which was
opened
in
January 1878,
in
New Haven,
Connecticut served only
21
customers, and
had precisely eight individual telephone lines
which were shared by two or more customers.
It was around this time that someone declared
that phones shou
Id
be
answered with a brisk
"Ahoy! Ahoy!" But thank God for Edison who
is supposed to have been the first man to
say "hello" into the phone. Almost overnight
telephone girls became known
as
"hello
girls".
As
there was nothing to indicate the
end of a conversation, the operator had to
listen to the call from time to time. What a

time the operators must have had!
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want to speak to someone
in
the
neighbourhood, the switchboard at the
exchange lets the current from your
p~one
flow
to
the wires of the phone you are calling.
If the person you are calling
is
very far
away, the exchange
in
your area switches
your line onto a line that leads to other
exchanges. Sometimes the call may have
to
pass through several
exchan~es,

if you are
ringing someone abroad.
In
a dial phone all
this
is
done automatically and, therefore, a
call may take only a few seconds to
materialize.
In
1889 Almon Brown Strowger devised an
automatic telephone exchange which
eliminated the human operator.
He
called it
the 'girl-less', 'cuss-less',
(out-of-order-Iess~
'wait-less' telephone. His associates later
devised a rotary dial which produced
electrical pulses according
to
each number
dialled. The pulses travelled down the
telephone line to the exchange and
automatically connected the caller to the
number
he
wanted.
Strowger exchanges, while automatic, were
slow and clumsy. For long distance calls

operators were still required. Significant
improvements were made with the
development of
an
electro-mechanical
exchange known
as
crossbar.
Let us see how the automatic telephone
works. When you lift the receiver the switch
that connects your phone to the telephone
exchange
is
activated. Your phone
is
then
connected
to
a line switch, also called a
'hunter'. This
is
an
automatic rotating switch
which will search along a row of contacts
until it finds a disengaged path to the
automatic selector mechanism. The moment
this happens you hear the familiar 'trrr' of the
dial tone. This
is
a signal for you to start

dialling.
Let's suppose that the number
you
want
is
634520. The first two digits, that is '63'
represent the exchange. When you turn
number 6 to the dial stop you are only winding
up
a spring
in
the dial mechanism. The
moment you let go, the dial returns under the
tension of the spring and transmits six
electrical 'kicks' along the line. The number
of 'kicks' correspond
to
the number dialled,
except
in
the case of zero. When dialling
zero, ten impulses are transmitted. You then
dial number 3 and the process is repeated.
At the exchange the chain of impulses
transmitted by dialling '63'
is
fed
to
an
apparatus known as a 'director'. This

recognizes from the impulses which
exchange
is
required and its 'memory' tells
it
that
in
order to get from one exchange to
0 0
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0
0
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K.)
0
0
0
0
0
0 0
0
0
10
0
0
0
0 0

0
0
0
0
0
0 0
0
0
0 0
0
10
0
0
0
0 0
0
0
0
0
0
0
p
0
0
0
0 0
0
0
0
0

0
0
0 0
0
0
0
0
u
u
0
0
0
0
0
0
I.
0
0
0
0
Ie
0
0
0
0
IC
D
0
0
0

0
:0
0 0
0
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0
0
0
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1 2 3 4 5
~
7 8 9
10
o
I-
An
automatic
exchange
o
9
8
7
6
5
4
3
2

1
the other the call must
be
routed through
several other exchanges.
All
this
is
done
in
a matter of seconds and
before
you
know it
you
are connected to the
1. Exchange
2.
Satellite
3.
Exchange 4. Earth
'63' exchange and you can dial the rest of
the numbers.
The switches that locate the other numbers
are known
as
'selectors'. They move
in
two
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In
some countnes the telephone system
provides children with bedtime stories. It ta-
kes
a message for you if you are ill and
gives
you
the latest news
too.
directions and are activated by
electro-magnets which follow the pulses
transmitted by the telephone dial. They move
vertically and
in
a sweeping circular
movement.
When number 4 is dialled, the contact arm
on the selector moves up 4 lines and then
sweeps round the row of contacts on line 4
until it finds a route that will connect it to the
next, number
2,
and finally
O.
The whole
process, that is, dialling the exchange and
then the rest of the numbers, takes hardly

any time. Your telephone line is connected
to the line of the person you are calling almost
instantly.
Now
an
automatic calling system, known
as Subscriber Trunk Dialling (STD), operates
in
nearly all the major cities of the world.
It
is slightly more complicated than the local
dialling system as
it
needs electronic
equipment which operates
in
much the same
way
as
an electronic brain. Not only
is
STD
able to pick the particular route for a long
distance call, but it can choose
an
alternative
path if the first
is
busy. Additionally
it

works
out the cost of a call and transfers that
information to a meter which ticks up the
amount to
be
charged to the subscriber's bill.
The system of charging for calls is
interesting. A meter connected to the
subscriber's line
is
arranged to mark up one
unit of charge each time it receives six pulses
from the exchange equipment. The rate at
which it ticks up the money depends on the
speed of the pulses reaching
it.
The charge
for a call depends on two factors, time
and
distance.
So,
for long-distance calls the speed
at
which the charging-up pulses are fed to
the meter increases and for short-distance
calls it decreases.
The latest
in
electronic components is the
'Command Dialer' a - programmable phone

that
can
recognize words.
If
you
want to make a call with the Com-
mand Dialer, all you do is utter two words.
The first tells the machine whose number to
Relief, fear, joy, anxiety, shock! A telephone
ring can convey all these and more. It is no
wonder then, that the
great
poet, Tagore,
was inspired to write this about the telephone:
"Kauto aujaanare janaile tumi,
Kauto ghore deele thhain,
Door ke koreele nikat bandhu,
Paur ke koreele bhai. "
(You have
made me known to friends
whom I knew not. You have given me an "
entry into homes not my own. You have
brought the distant near and turned strangers
into brothers of mine.)
look up while the second tells it
to
go ahead
and dial. And this could
be
anywhere

in
the
world, provided the same person speaks
each time, because the Command Dialer is
a one-man-machine and it will not respond
to different voices.