A CST PUBLICATION

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BYNAVKALA ROYILLUSTRATED BYSURENDRA SINGH RATHOREDESIGNED BYSUBIR ROY

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6. Atomic clock­the ultimatetimekeeper

5 Digital quartzwrist-watch

1. Early man,recording time byobserving shadows 2 Sundial

4. Portabletimepiece

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Miniscule atoms vibrating at 23, 870, 129,300 cycles per second. A radio frequencycharged to match their strength. Trapped in amass of pipes, pumps and tubes. Theresult-an explosioh! Or so one would imagine.

All this is just to tell you the time. Time asrecorded by an atomic clock. And time soaccurate that if you were to go by it, you wouldbe late for school by only one second in1,700,000 years!

The atomic clock has made man's powersalmost magical. Today we can record not onlyevery second but every nanosecond andpicosecond of any occurrence, even theslowing down of the Earth.

Man's need for accuracy and perfection isnot new. More than 4000 years ago he beganto realize the importance" of measuring time.Earlier, he spoke of events that happened somany 'suns' or 'moons' ago or at the time ofthe 'heavy snows' or the 'big flood'.

In ancient India, just as amonth was dividedinto thirty days, a day was divided into thirtymuhurtas. (Day and night were taken together).Distinctive names were given to each of themuhurtas.

Both 'day' and 'night' appeared as naturalunits of time in our earliest literary productions.Expressions like 'many dawns and nights' or'days subdue the nights' occur in the Rig Veda.The 'ahoratra' (that is, day combined withnight) meant a duration of 24 hours.

Aryabhata, one of our greatestmathematicians and astronomers, discussesthe units of time in his masterpiece, theAryabhatiya, written around 500 A.D. In the

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chapter Kalakriya Pada, he talks about ayearbeing divided into twelve months; a month into30 days; a day into 60 nadis and a nadi into60 vinadis. (A nadiwas equal to 24 minutes, avinadi to 24 seconds).

Meanwhile, Babylonian priests, fascinatedby mathematics, were making efforts toachieve precise methods that would divideeach day into 24 hours and each hour into 60minutes.

Those of you who are adventurous, take acue from King Alfred-the Saxon King whoinvented the candle-clock.

Candles of the same size, he noticed, burndown at the same rate. So, Alfred took ahandful of candles, each a foot long, and

The Babylonians noticed that the position ofa shadow changes during the day. So, theyfixed a pole in a sunny place and observed itsshadow as it moved. The shadow, theydiscovered, was long at sunrise and graduallygot shorter and shorter until it reached a certainpoint when it began to lengthen again. Atsunset the shadow was as long as at sunriseand at noon it was the shortest.

marked them with a number of bands. Eachband represented a certain division of time,say, one hour. He could tell how much timehad passed by noting how many divisions hadburned away.

Similarly, the Indians who were studying themovements of the sun, the moon and otherplanets, developed several simpleastronomical instruments like the gnomon,staff, are, wheel and the armillary sphere.

They cast the almanac, that is, a calendarlisting the days, weeks and months of the year,calculated on the basis of the movements ofthe sun, moon and other heavenly bodies.

Even as early societies measured the year,the month and the day, they felt the need fordevices that would monitor and indicate timeprecisely. Gradually, one development led toanother and, as with most things scientific,literary or religious, it was an exchange of ideasand achievements between India on the onehand and West Asia and the Mediterraneanworld on the other that produced the so called'first' clocks.

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The study of shadows led to the making ofshadow clocks and sundials.

A shadow clock consists of a length of woodwith a crosspiece and a traverse bar whoseshadow marks the passage of the sun, whilea sundial is basically a dial face with a pointerin the centre. The pointer, also known asgnomon, is a flat piece of metal which pointsnorth in the northern hemisphere and south inthe southern hemisphere. Its upper edge slantsupwards at an angle correspondiAg to thelatitude or the distance that it is away from theEquator. The shadow of the gnomon movesacross a scale around the dial as the sunmoves across the sky.

ight Time

Shadow clocks and sundials recorded timeduring the day, provided, of course, it was aclear, sunny day, but when night came theywere of absolutely no use. That was whenpeople thought of making water-clocks.

A water-clock consists of a container with asmall hole at the bottom through which thewater can escape. The gradual fall in the levelof the water marks the passage of time. As thelevel drops,. it exposes more and more of ascale marked with the hours. To ensure thatthe water drops steadily, the container musthave sloping sides.

The Greeks and Romans used water-clocksfor limiting the time of speeches in the lawcourts. The amount of water put into thecontainer at the start of a speech dependedon the importance of the case to be argued!

Alas! If the winter happened to be too severe,the water froze and these clocks could notoperate.

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As the world progressed, a better method ofmeasuring time had to be thought of. Areasonably accurate and reliable timekeeperdeveloped was the hourglass.

An hourglass is a particular kind ofsand-glass or 'ghantika yantra' as it wasknown in India. It consists of two glass bulbsjoined by a narrow neck and set in a stand.One of the bulbs contains very fine, dry sand.To measure the hour, the glass is inverted sothat the sand-filled bulb is on top. Graduallythe sand drains into the bottom bulb and afterexactly one hour the top bulb is empty. Thesand drains at a steady rate, no matter howmuch is left in the top bulb. This method wasquite successful and very soon sand-glasseswere made in groups of four. There was a 1/4hour, a 1/2 hour, a3/4 hour and an hourglass.

That, incidentally, was the origin of theexpression, 'the sands of time are running out'

Shadow clock Water clock Hourglass

•.-.If you look at the flowers carefully, you will

notice that some open at 6 a.m., some at 7 a.m.and others at 8 a.m. Similarly some flowersclose early and some late. So it was that in

19th century England, flowers were laid out inbeds in the form of a clock face. The bloomingor closing of each bed depicted the hour of theday.

Ding-Dong

While the sun, the sand, shadows and waterhelped man record the hours, there was stillno sign of a clock that ticked.

The exact date of the invention of mechanicalclocks is unknown. The earliest examples,apparently, go back to about 1250 A.D. Thesewere based on the massive clock-work modelsthat had been devised by astronomers andmathematicians to study the heavenly bodies.They were made of wrought iron and wereused atop cathedrals and other buildings. Theregulation of these clocks was clumsy and theywere as much as one hour wrong each day.

By 1389, however, clocks had improvedgreatly. They had bells to ring not only on thehour but at every quarter as well.

Strictly speaking, a clock is an instrumentthat has a bell and strikes the hours andsometimes the quarters too though we tend tocall any instrument that measures the passageof time a clock. Other kinds of time-measurersshould be called timepieces.

It was not until the 16th century that clockssmall enough for the. house were developed.The first movable clocks became possiblewhen weights were replaced by the springdrive. Clocks became the newest and mostexpensive toy in the royal courts. Princes andnoblemen took great delight in organising theirday by it. A banquet would be arranged tobegin at 8 o'clock instead of at sunset andpeople spoke in terms of a place being fivehours away instead of half a day. That was alsowhen 'appointments' were first made.

The clock incidentally was the first complexmechanical device to enter the home.

Around 1660 the pendulum came into use.It was the Italian astronomer, Galileo whocontributed largely to the discovery of thependulum. In 1581, during an earth tremor,Galileo is said to have timed the swings itcaused in a hanging lantern in Pisa against thebeats of his pulse. He found that beginningfrom a central point, the lantern swung thesame distance to the left as to the right, in thesame length of time. From this he formed thetheory of isochronism or things performed inequal times.

In 1641 Galileo started applying this theoryto timepieces. He died in the following year.His son Vincenzio continued his father's workand made drawings for pendulum timepiecesin 1649. But he too died without completing aclock.

Finally in 1656, a Dutchman, ChristianHuygens designed the first practical pendulumclock.

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Despite the discovery of the spiral spring andthe pendulum, some clocks continued to beweight-driven. They were cumbersome andugly. So someone thoughht of enclosing theweights and cords in a cupboard-like case. Loand behold! the grandfather clock was born.

And what a stately clock it turned out to be!Even today grandfather clocks are speciallydesigned for those who like to use it as ac1ock-cum-furniture piece and bring a bit ofold-world charm into their homes.

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By this time the value of a clock wasbeginning to be realized by more and morepeople. They wanted timepieces small enoughto be carried about.

It was a German, Peter Hele, who made thispossible. He invented the coiled spring whichformed the foundation of all future spring­driven clocks and watches. But the power ofa spring becomes less as it uncoils. So Hele'stimepieces, though unique, were no good astimekeepers. A few years later this problemwas overcome by the incorporation of a deviceknown as the fusee and spring which enabledmore reliable small clocks to be made.

By and by further improvements were madeas people the world over worked on variousmethods of keeping time.

Maharaja Jai Singh II of Jaipur wasmotivated by a strong desire to set up highlyefficient, modern observatories for producingaccurate astronomical data. With the result,

during the early 18th century a large numberof giant instruments, including the SamratYantra (Sundial) were constructed in stone.This was the first time that sundials were setup in India. Jai Singh constructed severalobservatories. The one at Jaipur is the mostfamous. It has a huge masonry gnomon orsundial pin, whose shadows show the hour ofthe day perfectly. New Delhi's 'Jantar Mantar'(observatory) is a big draw.

Jaipur observatory

Chronometer designed by John Harrison

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Ocean navigations were still controlled bycalculations based on the position of the sun,moon and stars. This inaccurate methodinvariably produced navigation errors becauseevery minute lost by mistiming could put a shipas much as 15 kilometres off course.

John Harrison of England spent many yearsdesigning a clock for sailors. His fourth attemptproved successful and in 1761 a chronometeraccurate to half a minute a year was produced.A chronometer keeps time in all variations oftemperature and tells people at sea exactlyhow long they have been plying east or west.

The world gets its time from the sun-a daybeing recorded from noon to noon. But, owingto the rotation of the earth on its axis,noontime would differ from place to place. Toavoid this confusion the world has beendivided into 24 time zones by imaginary lines,known as meridians. Each time zone is15 degrees apart and represents a differenceof one hour.

At Greenwich Observatory, England, is theprime meridian, where Greenwich Mean Time(G.M.T.) is recorded. This is the absolutestandard of time, on which all other times arebased.

When the sun is directly over the primemeridian, it is 12 noon at Greenwich. At

In scientific technological work, G.M.T. hasnow been replaced by Coordinated UniversalTime (UTC) which is based on data collectedby Bureau International de I'Heure (BIH),

Stockholm, in the next zone east, it is past1 o'clock. On the other hand the time zones tothe west of Greenwich are hours behind. NewYork, for instance, is five hours behind G.M.T.

If we were to strictly follow the one hourdifference between each time zone,especially in a country like India, whichoccupies more than one zone, we would haveto adjust our watches while travelling fromBombay to Calcutta!

Indian Standard Time (I.S.T.) is based onthe meridian which passes through Mirzapurin U.P. and is 82.5° E of Greenwich. That ishow we in India are 5-1/2 hours ahead ofG.M.T. I.S.T. is maintained and disseminatedby the National Physical Laboratory in Delhi.

Paris. BIH collects data from atomic clocksaround the world and provides an internationalatomic time scale which is more accurate thanG.M.T.

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Meridians showing 24 time zones

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It was in 1900 that the possibility oftransmitting electrical impulses from a centralsource to drive clocks was developed. Electricclocks were made in 1918 by Henry EllisWarren, an electrical engineer fromMassachusetts.

The French brothers, Pierre and PaulJacques Curie, had identified the 'piezoelectric effect' in 1880, which meant thatcertain crystals vibrate at a constant frequencywhen a controlled alternating voltage is appliedacross them and also that bending or strikingthe crystals produces an electric charge.

In 1929, Warren Alvin Marrison of NewJersey was the first to apply quartz crystals toelectric clocks.

With this it became possible to measure timespans of a few thousandths of a second.

Today with the atomic clock being used inhigh-speed calculations, all one seems to bedoing is catching up with time!

Day after day time moves on-not a secondslower or faster than the previous day!

How does this happen? What is it that makesthe hands ofthe clock go round so accurately?

Let us take an ordinary mechanical clock andturn it inside out.

The first thing you do is to wind the clock.When you do this, you are actually winding acoiled spring or the mainspring within theclock. One - two - 'three - four - round andround goes the coil until it is tight. Butremember, easy does it. For, an extra turnand-snap-the spring goes!

So, you have wound the mainspring justenough and left it. Now what? The coil at oncebegins to unwind. In this way it releases acertain force which drives all the wheels thatactually make up a clock.

But if these wheels just went round andround merrily, it would serve no purpose. Theirmovement has, therefore, to be regulated.

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Inside a clock

1. Winding key 2. Mainspring 3. Centre wheel4. Escape wheel 5. Escapement 6. Hairspring7 Balance-wheel

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The heart of the c10c is the part known asthe regulator. This is a device that controls therate at which a clock keeps time. There aredifferent kinds of regulators. The balance­wheel is one that is commonly used in mostmechanical watches. This is a delicate wheelwith a pivot at the centre which is connectedto a very fine spiral spring called a hairspring.

The hairspring turns the balance-wheel. Butagain, not round and round. Its movement isarrested by a lever called a rocker which hastwo pallets. Whichever way the balance-wheelturns, the rocker stops it and the hairspringimmediately pulls it back the other way. Thusthe balance-wheel is kept twisting a half turnone way, a half turn the other way. And this isrepeated over and over again. Once thebalance-wheel is set in motion it will move toand fro in the same length of time.

The balance-wheel is connected to a deviceknown as the escapement which, in turn, is

The rocker engages one tooth ofthe escape wheel and the clockgoes 'tick'

The Escapement

1. Hairspring2. Balance-wheel3. Rocker4. Escape wheel

The rocker engages the next toothof the escape wheel and the clockgoes'tock'.

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connected to the mainspring through a mainwheel, known as the driving wheel. (I hope youare counting the wheels!)

The escapement consists of a toothed wheelknown as an escape wheel and the rocker (thesame as the one mentioned earlier). It has twojobs. One is to ensure that power 'escapes'from the mainspring. The other, that it

'escapes' at the rate laid down by the regulator,so that eventually the hands of the clock canturn accurately. How does it do this?

We said earlier that when the balance-wheelturns, its movement is arrested by a rocker andimmediately the hairspring pulls it back. Thismovement is exactly in step with themovement of the escapement. For, when the

balance-wheel takes a half turn one way, oneof the pallets of the rocker releases a tooth ofthe escape wheel and allows it to rotate till itis stopped again by the other pallet. The timetaken by a tooth of the escape wheel to pushaside the first one and then the other pallet ofthe rocker is the time taken by the clock to gofirst 'tick' and then 'tock'. Thus the clockmoves in a series of jerks controlled by theescapement and the regulator.

Now we have a perfect, isochronic motiongoing. This means, a movement that goes toand fro in the same length of time constantly.This movement has to be transmitted to thehands of the clock.

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The hour, the minute and the second handmust necessarily go round at different speeds.For this reason their movements are controlledby gear wheels.

Gear wheels, as you may know, are discswith teeth cut out of them. These teeth linktogether with the teeth from other wheels sothat, when one wheel turns, the others alsoturn-but in the opposite direction.

If two gear wheels of the same size areinterconnected, they would rotate at the samespeed. If one of the wheels is half the size ofthe one that it is connected to, the smallerwheel will go round twice in the same time asthe big wheel would take to go round once.

The arms of the clock work on this principle.They are thus connected to gears of differentsizes so that they can go round at differentspeeds.

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Falling weights were used to power theearliest clocks, especially the grandfather andgrandmother clocks. They worked through aseries of wheels driven by a falling weight ona cord. The pendulum, which acted as therocker, regulated the rate at which the wheelsturned. The pendulum and escapementtogether produced the 'tick' or forward jerk inthe clock. This was transmitted to a finger thatshowed the time.

A pendulum is a very good regulator. This isbecause the swings or oscillation:;; of apendulum are regular and steady. You can seethis for yourself if you tie a weight on a pieceof string and allow it to swing back and forth.

The Pendulum

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1. Rocker 2. Escape wheel3. Pendulum rod 4. Bob5. Weignt

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While the method of working remains thesame for all modern clocks, the material usedvaries. For instance, in research laboratoriesand for astronomical observations twoextremely precise kinds of electronic clocksare used today. They are the quartz crystalclock and the atomic clock.

Quartz is one of the commonest minerals.Sand is made up almost entirely of small grainsof quartz. Usually it is milky white or has shadesof various colours, but sometimes it is as clearas glass. This form is called rock crystal. Inbulk form quartz is crystalline.

.Pure quartz has interesting electricalproperties which make it suitable for regulatingthe mechanism of a clock. When an electriccurrent is passed through a quartz crystal, thecrystal vibrates at an almost perfectly constantrate. The rate of the vibration depends on thethickness of the crystal. The thinner it is, thefaster it vibrates. The faster it can be made to

The front of a quartzcrystal clock

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The atomic clock is the most accuratetime-keeping machine to date. It can measuretime intervals of a millionth of a second. It usesthe vibrations of either ammonia or caesiumatoms as a regulating device. These atomsvibrate very regularly and rapidly-thousandsof millions of times every second..

The principle of the atomic clock wasworked out in 1946 by the American physicist,Dr. Willard Frank Libby. The first such clockwas made two years later at the NationalBureau of Standards in Washington D.C. In1969 the U.S. Naval Research Laboratory builtan atomic clock, counting the oscillations ofthe ammonia atom. The National PhysicalLaboratory at New Delhi, has five caesiumatomic clocks.

To look at, the atomic clock is nothing likean ordinary clock. It is a complicated mass oftubes, pipes, pumps and electronic gadgets.A radio frequency is charged to match the

vibrate, the smaller is the time interval that canbe measured.

In a quartz clock the crystal is made so thinthat it vibrates 1,000,000 times a second. Thismakes it possible to measure time intervals ofa few thousandths of a second.

At major sports tracks events are filmed anda quartz-crystal timing mechanism in thecamera runs all through the race. It directs thefigures through a prism on to the film to givesplit-second timing of the finish.

A quartz crystal clock can have a dial ordigital face. The battery makes the crystaloscillate at its natural rate. A divider circuit inthe watch converts the natural vibration rate toone pulse per second. A motor converts thesevibrations into one-second 'ticks'. These aretransmitted to the clock face again by a set ofwheels.

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The atomic beam chamber of a caesium atomic clock

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strength of the atom that is used. The radiofrequency (maintained at the correct rate byconstant electronic checking) makes the clockadvance one second after the number of

vibrations has been counted off electronically.Atomic clocks are used for high-speed

navigation calculations an9 by advancedresearch scientists and astronomers.

For the rest of us there are a variety of clocksavailable these days. Aside from mechanicalclocks there are those that run on electricity.

Some of them merely use electricity as asource of power to drive the clock mechanism.In some battery-operated ones, the electricitypowers a small electric motor which rewindsthe mainspring of the clock every few minutes.In large public clocks electricity is used topower a motor which winds up a weight drivingthe clockwork.

The most common electric clocks, however,use electricity not only to power them but alsoto regulate them. They are called synchronousclocks. In these clocks the motor driving themechanism keeps in step with the rapidalternation of the electric current.

And if you attempt to listen to the 'ticks' inan electric clock, you will not be able to hearthem, because an electric clock has no'tick-tacks'. It has no rocker and no hooks. A

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Electric clocks

small electric motor turns the wheels and thewheels turn the hands. As the electric motoralways runs at the same speed the hands ofthe clock point to the right time without the'tick-tack' sound.

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Many of us have come across deligh fulclocks that chime or say 'cuckoo' andwondered how they work. At times one haseven been tempted to go behind the 'cuckooclock' and see if there is a real bird in there!

There are actually two whistles that makethe 'cuckoo' sound. Little boxes, calledbellows, blow air through the whistles. If youlisten carefully, the first 'coo' is shorter thanthe second. The two whistles tooting, one afterthe other, make the 'cuckoo' sound.

At one o'clock the clock 'cuckoos' one time.At two o'clock two times and so on. Whatmakes the clock 'cuckoo' the right number oftimes?

A catch flips out of a notch in a special wheelwhen it is time for the clock to 'cuckoo'. Thewheel turns a short way, the little door popsopen, the bird comes out and bows and thewhistles go 'cuckoo'! As long as this wheelturns, the clock 'cuckoos' over and over again.

The wheel turns when the catch is out of thenotch. The catch slides on the edge of theturning wheel. The wheel stops when the catchfalls into the next notch. With that the 'cuckoo'sound also stops.

A cuckoo clock'cuckoos' on thehour

From the moment the alarm clock wakes usup we know it is one mad rush-to catch thebus, to school, to work, to get back home, toplay and to sleep. All this must be done within

the 24 hours we have allotted ourselves eachday.

It is not surprising then that people discardtheir watches to counteract the pressure ofwork when they want to go away to a quietplace on a real holiday!

When we say a clock has jewels, whatexactly does it mean?

The moving parts of a watch, that is, theescape wheel, the balance-wheel and so onturn in bearings often made of jewels. Suchbearings, called jewelled bearings, are usedbecause they are hard and can withstand yearafter year of constant use. So next time yousee a watch that claims 17 jewels, don't lookpuzzled!

This book, one of a series of informationbooks, introduces the child to a variety ofclocks-ancient and modern. It explains ina simple manner how the clock works.

Others in this series include:

• The Television• The Telephone• The Mator Car• The Aeroplane• The Ship• The Railway Train• The Computer

@by CBT 1988Reprinted 1990, 1993, 1995, 1997, 1999, 2001, 2003, 2004, 2006.

Published by Children's Book Trust, Nehru House,4 Bahadur Shah Zafar Marg, New Delhi-11 0002 andprinted at its Indraprastha Press. Ph: 23316970-74Fax: 23721090 e-mail: cbtnd@vsnl.comWebsite: www.childrensbooktrust.com

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ISBN 81-7011-398-9

788170113980