A watch is something that displays time and sometimes day, date, month and year also. In current usage the word â€œwatchâ€ is used a short form of wristwatch, which is a name for timekeeping devices worn on the wrist.Â Due to the lack of a striking mechanism such as a bell or a gong they are known as timepieces and not clocks.
In the 20th century most watches used were pocket watches. They had hard covers and had to be carried separately in a pocket with a watch chain or fob attached to them. Nowadays, wristwatch is the most common type of watch. It is worn on the wrist and a watchband made of leather, nylon, plastic, metal links or ceramic is used to fasten it.Â Â Not until somewhat recently (that is, in terms of human history) did people find a need for knowing the time of day. As best we know, 5000 to 6000 years ago great civilizations in the Middle East and North Africa initiated clock making as opposed to calendar making. With their attendant bureaucracies and formal religions, these cultures found a need to organize their time more efficiently.
The Egyptians divided the day into parts (analogous to our hours). They used â€œObelisksâ€, slender, tapering, four-sided monument, to divide the day into two parts by indicating the noon. These devices, built around 3500 B.C., used to cast moving shadows which formed a sort of sundial. This sundial was used to partition the day into two parts as well as to find out the longest and shortest day of the year (by measuring the shortest or the longest shadow of the year at noon). Markers were later added around the base of the monument to further subdivide time.Â
Around 1500 B.C. an Egyptian shadow clock or sundial was used to measure the passage of hours. It very well could have been the first portable timepiece. The day was divided into 10 parts plus two twilight hours in the morning and the evening. The long stem which had 5 variably spaced marks was oriented east and west in the morning. A moving shadow over the marks was cast by an elevated crossbar on the east. The device was turned in the opposite direction in noon to measure the afternoon hours.
The â€œmerkhetâ€ which is the oldest astronomical tool known to us was used by Egyptians in 600 B.C. Two merkhets were used together for establishing a north-south line by lining them up with the Pole Star. Time could be determined when certain starts crossed the north-south line in night.
Elements of a Clock
All clocks must have two basic components:
â€¢Â A regular, constant or repetitive process or action to mark off equal increments or time:Â Movement of sun in the sky, sandglasses (hourglasses) were used to measure time.
â€¢Â A means of keeping track of the increments of time and displaying the result: we use position of clock hands and digital time display to keep track of time.
Water clocks were one of the earliest clocks which did not depend on celestial objects for keeping track of time. One of the oldest water clock dating back to 1500 B.C. was found in the tomb of Amenhotep I. Later the Greeks began using them around 325 B.C. Clepsydras, the water clocks, were stone vessels which had sloping sides which allowed water to drip at an almost constant rate through a small hole close to the bottom. Cylindrical or bowl-shaped containers were designed to slowly fill with water which comes in at a constant rate. The passage of time or â€œhoursâ€ could be measured by markings on the inside surfaces as the water level rose. Although these clocks were used to determine hours at night, they may have been used in daylight as well. Another variation of the water clock consisted of a metal bowl with a hole in the bottom. When this bowl was placed in a container of water, the bowl would sink after getting filled with water in certain duration of time.
Between 100 B.C. and 500 A.D. more elaborate and accurate mechanized water clocks were developed by Roman and Greek horologists and astronomers. They were made more complex to regulate the pressure and to provide fancier displays of the passage of time. Different type of attractive indications were used to indicate passage of time such as some clocks rang bells and gongs, some opened doors and windows to show little figures of people, or moved pointers, dials, and astrological models of the universe.
In the first century B.C., construction of the Tower of the Winds, under supervision of a Greek astronomer Andronikos, was done. The Tower of the Winds was an octagonal structure which showed sundials for scholars and mechanical hour indicators for common people. It had a 24-hour mechanized clepsydra and indicated the winds from eight directions. It also displayed the seasons of the year and astrological dates and periods. Though Romans also developed mechanized clepsydras, they didnâ€™t improve much over simple methods for the determination of time or passage of time.
Mechanized and astronomical clocks were developed in the Far East from 200 to 1300 A.D. Su Sung and his associates made one of the most elaborate clock towers in 1088 A.D. the mechanism developed by Su Sung incorporated a water-driven escapement which was invented around 725 A.D. The clock tower developed by Su Sung was more than 30 feet tall and had a bronze power-driven armillary sphere for observations along with a celestial globe which used to rotate automatically. It had five front panels with doors which allowed viewing the changing of manikins which rang bells or gongs, and held tablets which indicated the hour or other special times of the day. As it was very difficult to control the rat of flow of water accurately, a clock based on that flow could never be expected to be accurate beyond a point. To achieve greater accuracy people were led to other approaches.
The technological advancement came to a standstill during the Middle Ages (around 500 â€“ 1500 A.D.). Even though some new styles of sundials were evolved, they were not technologically much better than those of Egyptians. In this era, simple sundials placed above doorways were used to identify midday and four â€œtidesâ€ of the sunlit day. Various kinds of sundials were being used by the 10th Century. An English model identified tides and even compensated for seasonal changes of the sunâ€™s altitude.
The desire for accuracy in timekeeping increased in the 15th century with increase in navigation and mapping. On a ship the latitude could be measured relatively easily by looking at the stars but to measure the shipâ€™s longitude it had to compare the midday (high noon) time of its local longitude with that of a European meridian which usually was Paris or Greenwich. This process was very unreliable until John Harrisonâ€™s marine chronometer was introduced. It is due to this reason only that most maps belonging to period between 15th century and 19th century have precise latitudes but distorted longitudes.Â Â Â Â Â
Simple weighted pendulums were used to make first fairly accurate mechanical clocks. These clocks were not fit to be used at sea as they gave inaccurate time when there was an irregular movement of the fulcrum. Spring mechanism proved a very crucial invention for the development of portable clocks.Â Â Â Â Â Â Â Â Â Â Â Â
In early 15th century Peter Henlein, a German locksmith, created the first pocket watch. In these pocket watches the heavy drive weight were replaced which allowed making smaller and portable clocks and watches. These clocks were nicknamed â€œNuremberg Eggsâ€ after the name of the place from which Peter hailed. These timepieces had a tendency to slow down gradually with the unwinding of the mainspring but they were still popular with wealthy individuals due to their small size. Due to the small size they could be put on a shelf and didnâ€™t need to be hung from the wall. Another of their disadvantage was that they didnâ€™t have a minute hand. They had only an hour hand to indicate the time. The minute hand wasnâ€™t added as it would have been pretty useless in view of the inaccuracy of the clock. Initially there was no glass protection also. The minute hand appeared in 1670 and the glass cover over the face of the watch didnâ€™t come till the 17th century.Â
Following are the mechanisms which are found in any mechanical watch of classical design. The first two mechanisms are the key mechanisms while the third one is optional.
1.Â The escapement- It is a mechanism which controls as well as limits the unwinding of the watch. It converts the simple unwinding into a regular, periodic back and forth motion. It is done when the escapement interlocks with a gear in a simple manner which switches between a â€œdrivenâ€ and â€œfreeâ€ state and abrupt locking at the end of the cycle. For the same reason he escapement produces the ticking noise which is characteristic of mechanical watches.
2.Â The balance wheel: The balance wheel together with the balance spring forms a simple harmonic oscillator, which controls the motion of the gear system of the watch in a similar way the pendulum controls the motion of a pendulum clock.
3.Â The tourbillon- It is a rotating frame for the escapement. Its purpose is to remove or reduce the effects of bias of gravitational origin in timekeeping, which might be a result from the watch being kept in a particular position for long durations. Technically, it is quite a challenge to create a tourbillon of high quality. Tourbillon of high quality made by specialists are highly valued and found in prestige watches.
Galileo is sometimes credited with the invention of pendulum but it was Christian Huygens, a Dutch scientist, who made the first pendulum clock which worked on the principle of a natural period of oscillation in 1656. Huygensâ€™ pendulum clock was highly accurate with an error of less than 1 minute a day. It was for the first time that such a high accuracy was achieved. Further improvements by Huygens reduced the clockâ€™s error to less than 10 seconds per day. Huygens developed the balance wheel and spring assembly around 1675. It can still be found on some present dayâ€™s watches. William Clement started building clocks with the new â€œanchorâ€ or â€œrecoilâ€ escapement in 1671. It was an improvement over the verge technique as it interfered less with the motion of the pendulum.
The pendulum clockâ€™s accuracy was reduced to 1 second per day after George Graham compensated for changes in the pendulumâ€™s length caused by temperature variations. John Harrison, a carpenter and self-taught clock-maker, improved upon Grahamâ€™s techniques of temperature compensation and added new methods to reduce friction. In 1761, he build a marine chronometer with a spring and balance wheel escapement which won the award offered by British government for a means to determine longitude with accuracy of half a degree. The chronometer was able to keep time about one-fifth of a second a day aboard a ship, which is almost as good as a pendulum clock could do on land.
The developments in the next century paved the way for Siegmund Rieflerâ€™s clock (1989) with an almost free pendulum. It had accuracy of a hundredth of a second a day and later became the standard in many astronomical observatories. When R. J. Rudd introduced the principle of true free pendulum, it led to development of many free-pendulum clocks. The W. H. Shortt clock, based on Ruddâ€™s principle, quickly replaced Rieflerâ€™s clock as the timekeeper in many observatories.Â Â This clock consisted of two pendulums working as master and slave. The slave pendulumâ€™s function is to give gentle pushes to master pendulum needed to maintain its motion. It drives the clockâ€™s hands as well thus making the master pendulum free from mechanical tasks which may affect its regularity.
In the 1930s and 1940s quartz crystal clocks replaced the Shortt clock as the standard. The clocks using quartz crystal had improved timekeeping performance surpassing that of pendulum and balance-wheel escapements.
The operation of a quartz clock is based on the piezoelectric property of quartz crystals. The operation of quartz clock is based on the piezoelectric property of quartz crystals. When an electric field is applied to the crystal, a change in its shape is produced and when the crystal is squeezed or physically deformed, it generates an electric field. When the crystal is put in a suitable electronic circuit, the interaction between mechanical stress and electric field induces the crystal to vibrate and generate a constant frequency electric signal which can be used to operate an electronic clock display.
Quartz crystal clocks are better as by doing away with mechanical parts such as gears and escapements the disturbance in the frequency is removed. The crystal clocks relied on a mechanical vibration whose frequency was highly depended on the crystalâ€™s size and shape. Hence, no two crystals can be precisely alike and have exactly the same frequency. These quartz clocks are still dominant in market as they have excellent performance and they are not expensive. But the accuracy of the quartz clocks has been surpassed with a huge margin by atomic clocks.
The atomic clocks were based on the principle that atoms and molecules have resonances i.e. they absorb and emit electromagnetic energy at a frequency which is characteristic to that element or compound. These resonances are very stable over time and space as they vibrate with the same frequency all the time and irrespective of the location. The scientists saw an opportunity to make a clock using these atoms or molecules as a â€œpendulumâ€.
The microwaves (a kind of electromagnetic waves) required to interact with the atoms were found in the process of development of radar and very high frequency radio communications in the 1930s and 1940s. The focus of the research aimed at developing an atomic clock was focused initially on microwave resonance in the ammonia molecule. NIST (National Institute of Standards and Technology) build the first atomic clock based on ammonia. It did not perform much better than the standards which existed at that time. Hence, the attention was shifted on Cesium, which held more promise in terms of accuracy.
The first cesium clock was build by NIST in 1957 and a second unit was build soon for comparison testing. By 1960, the cesium standards were refined enough to be incorporated into the official timekeeping system of NIST.
The natural frequency of the cesium atoms was formally recognized as the new international unit of time in 1967. A second was defined as exactly 9,192,631,770 oscillations or cycles of the cesium atomâ€™s resonant frequency and replaced the old second which was defined on the basis of earthâ€™s motions. The best primary cesium standard of today is accurate to about one-millionth of a second per year.
In the present world several industries and sectors such as aviation, rail transport, communication, and manufacturing are dependent on highly accurate clocks. So the demands of the technology as well as the scientific research provide an impetus for the search for even more accurate clocks. NIST is working n next generation of cesium time standards at its Boulder laboratory.