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    A CSTPUBLICATION HOW IT WORKSTHE

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    .L ..J 1""I

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    BY NAVKALA ROYDESIGNED AND ILLUSTRATED BYSUBIR ROY

    --------- How itworks

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    THECHANGINGSHAPEOFSOUND

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    1875- The first telephone instrument madeby Alexander Graham Bel l in 1875

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    1879 1880 1880 1905 The telephone t-today

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    The candlesticktelephone of 1905

    The Gower-Bell telephone oft he ear ly 18 80 's with tw ol istening tubes

    This device required the user ....to speak into the box with thereceiver to his ear (1880)I

    An Edison receiver (1879)I

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    R. Watson, come here, I want to seeyou," shouted an angry Bell.Watson jumped out of his chair. There wasno one in the room. Yet he'd heard a voice.It was a familiar voice and it was loud andclear. Then suddenly it hit him. Thetelephone. It had come alive at last. Themiracle had happened. .

    Watson rushed to Bell's room, breathlesswith joy. "I could hear you. It works," he said.That was March 10, 1876. More than ahundred years ago.From ship to shore; from air to land; fromcar to car; from just about anywhere toanywhere today you can speak to someoneby just dialling a number. In fact, you havethe world at your finger-tips. And whenAstronaut Rakesh Sharma calls up Mrs. IndiraGandhi from space you just take it in yourstride. So dramatic has been the developmentof the telephone. And only forty years beforethe 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 inventeda 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 sameas 'talking' to someone, and nowhere nearhaving a cosy chat with someone.

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    It was at this time that Alexander GrahamBell, the young professor of speech, beganhis experiments with electricity. Oftenhe would visit the mills and factories locatednear his house and observe how themachines were operated. Once he called onCharles Wheatstone, the inventor of the. magnetic needle telegraph. So impressedwas he by this mail that he determined tofollow in his footsteps.Bell was keen to develop a telegraphsystem that would allow multiple transmissionof messages at once. He felt that this couldbe achieved by transmitting each messageon a separate, specially tuned steel strip, orreed. Each reed would vibrate a differentnumber of times per second and so producea different musical note.It was while one such experiment wasbeing carried out, on June 2, 1875, that areceiving 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 produceda pair of simple telephones.Bell had made a deep study on sounds ashe 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 demonstratingthe first telephone

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    ......

    Number, pleaseAs 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 controlledby 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 decidedto use an iron diaphragm. On March 10, 1876,when he accidently discovered that his phoneworked, he was delirious with joy.It was the first time in the world that peoplecould talk to each other over long distancesand feel that they had almost met the person.After all there can be no substitute for ahuman voice.Bell was keen to promote the idea of thisnew device and travelled extensively in theUnitedStates and Europe to spread the word.He even demonstrated how one could talk tosomeone under water.But most people pooh-poohed the idea. InLondon, a post office official said it wouldnever catch on because there were sufficientmessenger boys.

    Finally on January 24, 1878, Bell carriedout a demonstration for Queen Victoria atOsborne House, on the Isle of Wight. Soimpressed was the Queen that she askedBell to supply her with telephonesImmediately.

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    An 1879 hand-operated switchboardwas lift the receiver and ask for the personyou wished to talk to. Only one had to shoutin order to be understood by the other person.

    Early models resembled a box camera witha round projection at one end. This servedas 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!"

    Do Not L i ~ i f NWith YWRMouth A ~ 1ciJkwith

    OUI" Bo.Y'!

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    As Bell's transmitters had poor sensitivity,calls were limited to a few miles. It was atthis time that Thomas Alva Edison, thefamous American inventor, stepped in.Edison was the next best thing thathappened to the telephone. He produced atelephone with a separate mouthpiece anda much superior transmitter with a carboncomponent. When spoken into,it changed thesound of the voice into a varying electricalsignal which was converted back into speechby the ear-piece at the other end.By the beginning of the 1900's, thetelephone had grown in popularity, especiallyin the United States. Some exchanges wereso large that there were long lines ofoperators seated at switch boards made upof 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 upin 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 operateson 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 thatcarries sound from a distance.'

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    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 callsthroughout the world.Your parents can hold internationalbusiness 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 ableto link computers in many countries to vastinformation networks. It can transmittelevision programmes such as the OlympicGames to more than a 100 countries. It can

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    be used to turn a television set into a terminalconnected to a computer, providing vastamounts of information through videotex.In the future this could form the basis ofan electronic mail service with people sendingprivate messages from one television set toanother via the telephone network. It wouldbe cheaper and much faster thanconventional post.With so much happening around us it ishard to believe that once upon a timemessages were sent by using a line ofbonfires on hill-tops, by beating drums ortying notes to carrier pigeons speci.ally trainedto 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 andsocial life would be seriously disrupted. Thiswas demonstrated in 1979 when a strike bytelephone workers halted the telephone system in Ireland for several weeks. Millionsof pounds worth of orders were lost becausecompanies could not reply quickly to requestsand their business was won by competitors.

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    In order to understand this let's firstunderstand sound. Air helps sound to travel.If there was no air we would not hear anysound. You can prove this by placing a bellunder a glass bowl and ringing it. You'll beable to hear it clearly. Now if you draw theair out with a suction pump, the sound of thebell will disappear. This is because there isnothing to carry the sound.Air is something quite real, even thoughwe cannot see it. Just as ripples are madein 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 enoughto make another sheet of metal vibrate when

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    ,..: / ).. " /I I , ', , - - - - t _.- .. . . "--.:.:.

    Smoke s ignals . . . carner p igeons horses. man has used allthese to fulfil a vital need-eommunlcatlon.. so rapId has been theprogress that today. thanks t o the telephone. we literally have theworld at our fingertips

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    .-.l " /"

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    they hit it. This you can tryout by hangingtwo sheets of metal of the same size andshape facing one another. Hit the first andmake it ring. Then put your hand on it tostop the vibrations. If you listen carefully, youwill hear the second sheet of metal vibrating.

    This is how we hear a noise. The first sheetis like the vocal cords in our throats, whichwe move while speaking and set vibrating.The second sheet is like the tight ear-drumin our ears. When the air waves strike ourear-drums we hear a sound.A telephone works basically on the sameprinciples - the carrier of the sound beingelectricity. When you want to talk to a friendyou lift the receiver and dial a number. Yourline is then connected to your friend's at thetelephone exchange. Electricity flows throughthe wires. The sound-waves of your voicemake the electric current stronger and weakeras the case may be. In your friend's phonethe current is changed back into soundwaves.

    We know about sound and sound-waves.Now let us follow these sound-waves as theygo into a telephone.Inside the telephone mouthpiece is a thincircular piece of metal- the diaphragm. Thisvibrates whenever any sound hits it. Behindit is a container which contains tiny grains of

    1. Carbon grains loose. Litt le flow of current

    2. Carbon grains tight when flow of current increases

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    specially prepared carbon The lead of yourpencil is also a form of carbon. Through thiscontainer of carbon and through the wires ofthe telephone flows an electric current.The moment sound-waves hit the thin pieceof metal it bends a little - so little that youcannot notice it. When it bends (or vibrates)it squashes up the carbon. The grains thusget tightly packed. When there is no pressureon the diaphragm the grains are let looseagain. This too you would not be able todetect with the naked eye.What Edison discovered was that anelectric current will pass through grains ofcarbon more easily when they are tightlypacked than when they are loose.So, when you speak, your voice causesvibrations in the metal disc. These vibrationscompress the carbon grains according to theintensity of your voice, which, in turn, causesdifferent amounts of electricity to passthrough the telephone wires, until it reachesthe receiver of another phone.

    Tran8lYlitter

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    Receiver

    1. Diaphragm 2. Carbon granules 3. Diaphragm4. ElectromagnetIn the receiver of a telephone is a thinmetal disc or diaphragm, just as in themouthpiece. But the receiver does not havethe little container of carbon grains. Insteadthere is an electro magnet, that is, a magnetworked by the electricity coming through thewires. The more the electricity coming

    through, the more powerful the magnetbecomes. This magnet pulls on the thin metaldisc in the receiver and makes it vibrate.These vibrations will sound exactly like yourvoice talking into the mouthpiece at the otherend, a long long way away.

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    The ice-cream cup telephone

    Do' ou eTo understand this better, you can makeyourself a very simple telephone. Whenyou've eaten your favourite ice-cream don'tthrow the cup away. In fact, get hold of twoice-cream cups. Pierce a hole in the centreof each. Take a long piece of fine thread andpass each end through the two cups. Knot itwell so that it doesn't come out. And that'sit. As long as the thread is stretched out tight,you have your telephone. One person talksinto one ice-cream cup the bottom of whichfunctions as a diaphragm and the otherperson puts his ear to the other cup. When

    you speak into the cup you make t ~ b ~ t t o mvibrate_ 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 hearswhat 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 mustbe wires. These wires are special as theyhave to pass over different lands, under waterand over mountains too. Their job is to carrythe electric currents from phone to phone.If you call up someone who is just a fewmiles away from you, this is what happens.You speak into the mouthpiece and theelectrons or particles of electricity in themouthpiece start bouncing against the otherelectrons that form the electrical current inthe wire. The current varies according to thevibrations of your voice and in a fraction ofa second the person you've called can hearyou. So fast does electricity travel that beforeyou can snap your fingers the current passingthrough a telephone wire can flash all roundthe world.When the electrons at one end of the linestart to pass the current along, they are verystrong. 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 telephonecable is the Commonwealth Pacific Cable(COMPAC) which runs for more than 14,480kms. from Australia via Auckland, New Zealand and the Hawaiian Islands to Port Alberi,Canada. It cost about 35,000,000 and wasinaugurated 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

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    circuits. For this, special undersea repeaterequipment was designed. Every 35 milesalong the cable there is a bulge that containsthe equipment to boost the signals. Theseundersea repeaters are sealed in containersthat withstand pressures upto 8000 poundsa square inch so that they cannot be crushedby the tremendous weight of water abovethem.And, what happens, if something goeswrong with these cables? It's precisely forthis reason that an accurate map has to bemade of where the cable is, so that it caneasily be located when repairs are needed.

    You may wonder how we are able to talkto 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

    0 0

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    The first telephone exchange which wasopened in January 1878, in New Haven,Connecticut served only 21 customers, andhad precisely eight individual telephone lineswhich were shared by two or more customers.It was around this time that someone declaredthat phones shouId be answered with a brisk"Ahoy! Ahoy!" But thank God for Edison whois supposed to have been the first man tosay "hello" into the phone. Almost overnighttelephone girls became known as "hellogirls". As there was nothing to indicate theend of a conversation, the operator had tolisten to the call from time to time. What atime the operators must have had!

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    want to speak to someone in theneighbourhood, the switchboard at theexchange lets the current from your p ~ o n eflow to the wires of the phone you are calling.If the person you are calling is very faraway, the exchange in your area switchesyour line onto a line that leads to otherexchanges. Sometimes the call may have topass through several e x c h a n ~ e s , if you areringing someone abroad. In a dial phone allthis is done automatically and, therefore, a

    call may take only a few seconds tomaterialize.

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    In 1889 Almon Brown Strowger devised anautomatic telephone exchange whicheliminated the human operator. He called itthe 'girl-less', 'cuss-less', ( o u t - o f - o r d e r - I e s s ~'wait-less' telephone. His associates laterdevised a rotary dial which producedelectrical pulses according to each numberdialled. The pulses travelled down thetelephone line to the exchange andautomatically connected the caller to thenumber he wanted.Strowger exchanges, while automatic, wereslow and clumsy. For long distance callsoperators were still required. Significantimprovements were made with thedevelopment of an electro-mechanicalexchange known as crossbar.

    Let us see how the automatic telephoneworks. When you lift the receiver the switchthat connects your phone to the telephoneexchange is activated. Your phone is thenconnected to a line switch, also called a'hunter'. This is an automatic rotating switchwhich will search along a row of contactsuntil it finds a disengaged path to theautomatic selector mechanism. The momentthis happens you hear the familiar 'trrr' of thedial tone. This is a signal for you to startdialling.Let's suppose that the number you want is634520. The first two digits, that is '63'represent the exchange. When you turnnumber 6 to the dial stop you are only windingup a spring in the dial mechanism. Themoment you let go, the dial returns under thetension of the spring and transmits sixelectrical 'kicks' along the line. The numberof 'kicks' correspond to the number dialled,

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    except in the case of zero. When diallingzero, ten impulses are transmitted. You thendial number 3 and the process is repeated.At the exchange the chain of impulsestransmitted by dialling '63' is fed to anapparatus known as a 'director'. Thisrecognizes from the impulses whichexchange is required and its 'memory' tellsit that in order to get from one exchange to

    0 0 () Ie) 0 0 C) IC) () K.)0 0 0 0 0 0 0 0 0 100 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 10 0 00 0 0 0 0 0 0 0 0 p0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 u u 00 0 0 0 0 I. 0 0 0 0Ie 0 0 0 0 ICD0 0 0 0:0 0 0 0 0 ( 0 0 0 0.-/1 2 3 4 5 7 8 9 10o I- An automatic exchange

    o987654321

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    the other the call must be routed throughseveral other exchanges.All this is done in a matter of seconds andbefore you know it you are connected to the1. Exchange 2. Satellite 3. Exchange 4. Earth

    '63' exchange and you can dial the rest ofthe numbers.The switches that locate the other numbersare known as 'selectors'. They move in two

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    tTl) ita 3001 I

    In some countnes the telephone systemprovides children with bedtime stories. It takes a message for you if you are ill andgives you the latest news too.

    directions and are activated byelectro-magnets which follow the pulsestransmitted by the telephone dial. They movevertically and in a sweeping circularmovement.When number 4 is dialled, the contact armon the selector moves up 4 lines and thensweeps round the row of contacts on line 4until it finds a route that will connect it to thenext, number 2, and finally O. The wholeprocess, that is, dialling the exchange andthen the rest of the numbers, takes hardlyany time. Your telephone line is connectedto the line of the person you are calling almostinstantly.

    Now an automatic calling system, knownas Subscriber Trunk Dialling (STD), operatesin nearly all the major cities of the world. Itis slightly more complicated than the localdialling system as it needs electronicequipment which operates in much the sameway as an electronic brain. Not only is STDable to pick the particular route for a longdistance call, but it can choose an alternativepath if the first is busy. Additionally it worksout the cost of a call and transfers thatinformation to a meter which ticks up theamount to be charged to the subscriber's bill.

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    The system of charging for calls isinteresting. A meter connected to thesubscriber's line is arranged to mark up oneunit of charge each time it receives six pulsesfrom the exchange equipment. The rate atwhich it ticks up the money depends on thespeed of the pulses reaching it. The chargefor a call depends on two factors, time anddistance. So, for long-distance calls the speedat which the charging-up pulses are fed tothe meter increases and for short-distancecalls it decreases.

    The latest in electronic components is the'Command Dialer' a - programmable phonethat can recognize words.If you want to make a call with the Command Dialer, all you do is utter two words.The first tells the machine whose number to

    Relief, fear, joy, anxiety, shock! A telephonering can convey all these and more. It is nowonder 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 friendswhom I knew not. You have given me an "entry into homes not my own. You havebrought the distant near and turned strangersinto brothers of mine.)

    look up while the second tells it to go aheadand dial. And this could be anywhere in theworld, provided the same person speakseach time, because the Command Dialer isa one-man-machine and it will not respondto different voices.

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    Others in this series include:

    by CST 1986Reprinted 1989, 1990, 19"93, 1995, 1997, 1998, 2001, 2003.

    I;

    .

    ISBN 81-7011-316-4184S.15.00

    The Television The Motor Car The Aeroplane The Clock The Ship The Railway Train

    Published by Children's Book Trust, Nehru House,4 Bahadur Shah Zalar Marg, New Delhi-110002 and printedat its Indraprastha Press. Ph: 23316970-74 Fax: 23721090e-mail: [email protected] Website: www.childrensbooktrust.com

    This book, one of a series of informationbooks, introduces the child to the marvels ofthe telephone-how it works and how itdeveloped.


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