how it work the telephone

CHANGING SHAPE OF SOUND I I -1875 - The first telephone instrument made by Alexander Graham Bell in 1875 The Gower-Bell telephone of the early 1880's with two listening tubes This devic

A CST PUBLICATION

HOW IT WORKS

THE

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BY NAVKALA ROY DESIGNED AND ILLUSTRATED BY SUBIR ROY

CHANGING

SHAPE

OF SOUND

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-1875

- The first telephone instrument made

by Alexander Graham Bell in 1875

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)

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An Edison receiver (1879)

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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

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 themethods of communication

That was when the electric telegraph wasused It was in 1838 that the American,Samuel Morse, patented his single wiretelegraph His design used the famous Morsecode in which combinations of short and longsignals - dots and dashes - indicate letters.Messages were sent at up to ten words aminute with a hand-operated key and werereceived as marks made by a pen on a papertape These signals had to be decoded andwritten out by hand

In 1855 Professor David Hughes invented

a printing telegraph The operator sentmessages from a keyboard, each key ofwhich represented a letter The machineturned the letters into electric signalsautomatically and, at the other end, anothermachine printed the message

These were major breakthroughs in thefield of communication, but still not the same

as 'talking' to someone, and nowhere nearhaving a cosy chat with someone

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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, inanother room, failed to vibrate Watsonthought the reed was stuck and pulled at it.When he did that a similar receiving reedvibrated in Bell's room

"What's this!" said Bell astonished, butrealized almost immediately that he had hitupon something great He had discoveredthat a tiny electric current caused by onevibrating reed was powerful enough to causeanother reed to vibrate audibly He alsorealized that instead of a single note the reedhad reproduced several notes Humanspeech, as Bell knew only too well, is alsomade up of a mixture of sounds of differentfrequencies and Bell believed that he coulduse 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 dumbchildren He, therefore, knew that a stretchedmembrane would be more suitable for sound

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. Bell demonstrating the first telephone

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Number, please

As news spread, a keener interest wascreated in the telephone, though it wasrestricted to small areas until the 1890's.Individual subscribers were connected toeach 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 theoperator's response

"Number, please," the operator would sayand connect you to the number you wanted

In fact, so personal was everything thosedays 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

a round projection at one end This served

as the transmitter and receiver So anyonemaking a call had to be extremely carefulwhile moving his ear and mouth Bruised lipsand ears were not an uncommon sight Infact, one model carried the notice: "Do notlisten with your mouth and talk with your ear!"

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 firstcountries in the world to have a telephoneexchange And Calcutta was where it allstarted

In 1881, barely five years after Bell madehis discovery, a 50-line exchange was set up

in Calcutta Then came the automatictelephone exchange with 700 lines, whichwas established in Shimla in 1913 But it wasonly after 1951 that the Indian telephoneservice made rapid progress SubscriberTrunk Dialling (STD), first introduced betweenKanpur and Lucknow in 1960, now operates

on practically every route in India and manyoutside 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 theworld can dial 80% of the world's subscribersdirectly Telephone 'hot-lines' keep worldleaders in contact with each other to avoidthe accidental outbreak of a nuclear war.Even on the battlefield it is now possible tolink soldiers to the international telephonenetwork and a person from the most isolatedoil platform in the sea can make calls

throughout the world

Your parents can hold international

business meetings by merely going to aclosed circuit television studio and talking toexecutives in similar studios in other countrieswhile the television pictures and the soundare being carried over the telephone network.The telephone network has also been able

to link computers in many countries to vastinformation networks It can transmit

television programmes such as the OlympicGames 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 andingenious is the mechanism that is set inmotion the moment one dials a telephonenumber How does your voice get carriedthrough miles and miles of wire? How is itthat you can hear even kids crying in thebackground, doors slamming and musicplaying 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 forinstance, a sheet of metal and see whathappens when we hit it The force of the blowmakes the metal tremble The to and fromotion so caused is called vibration A guitarstring vibrates when we pluck it As the metalsheet vibrates it pushes the air forward andbackward quickly, so that little ripples orwaves are made; which travel away from themetal in all directions

These waves in the air are so tiny that youcannot feel them, yet they are strong enough

to make another sheet of metal vibrate when

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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

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

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

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

The ice-cream cup telephone

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

vibrate_ The thread carries the vibration bylittle tugs and makes the other diaphragmvibrate in exactly the same way That makesnew waves of the same kind in the other cupand so the other person hears what you say.You'll be able to hear each other as long asthe thread is kept tight

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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 strengthand 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 longdistances, there are special stations alongthe way These are called "repeaters" Theyhave equipment similar to amplifiers in a radioset This equipment helps to boost the currentcarried by the wires

The transatlantic telephone cable, forinstance, 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 ourtelephones have only one set of wires At thesame time think how difficult it would be ifyou were to have a different wire for everyhouse you wanted to talk to To avoid thiseach telephone has its own set of wiresrunning to a telephone exchange or centraloffice

When you pick up the receiver and dial thenumber you want, your phone is immediatelyconnected to the exchange.lt is then that thenext step of the journey is determined If you

At the exchange

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