Just for your information and enjoyment 

AN EVOLUTION OF CLOCKS

Early examples of such processes included the movement of the sun across the sky, candles marked in increments, oil lamps with marked reservoirs, sand glasses (hourglasses), and in the Orient, knotted cords and small stone or metal mazes filled with incense that would burn at a certain pace. Modern clocks use a balance wheel, pendulum, vibrating crystal, or electromagnetic waves associated with the internal workings of atoms as their regulators.

Our ways of keeping track of the passage of time include the position of clock hands and digital time displays. The history of timekeeping is the story of the search for increasingly more consistent actions or processes to regulate the rate of a clock.

As best we know, 5000 to 6000 years ago great civilizations in the Middle East and North Africa began to make clocks to augment their calendars.

SUN CLOCKS

The Egyptians formally divided their day into parts something like our hours. Obelisks were built as early as 3500 BC. Their moving shadows formed a kind of sundial, enabling people to partition the day into morning and afternoon. Obelisks also showed the year's longest and shortest days when the shadow at noon was the shortest or longest of the year. Later, additional markers around the base of the monument would indicate further subdivisions of time.

Another Egyptian shadow clock or sundial, possibly the first portable timepiece, came into use around 1500 BC. This device divided a sunlit day into 10 parts plus two "twilight hours" in the morning and evening. When the long stem with 5 variably spaced marks was oriented east and west in the morning, an elevated crossbar on the east end cast a moving shadow over the marks. At noon, the device was turned in the opposite direction to measure the afternoon "hours."

The merkhet, the oldest known astronomical tool, was an Egyptian development of around 600 BC. A pair of merkhets was used to establish a north-south line (or meridian) by aligning them with the Pole Star. They could then be used to mark off nighttime hours by determining when certain other stars crossed the meridian.

In the quest for better year-round accuracy, sundials evolved from flat horizontal or vertical plates to more elaborate forms. One version was the hemispherical dial, a bowl-shaped depression cut into a block of stone, carrying a central vertical gnomon (pointer) and scribed with sets of hour lines for different seasons. The hemicycle removed the useless half of the hemisphere to give an appearance of a half-bowl cut into the edge of a squared block. By 30 BC, there were 13 different sundial styles in use in Greece, Asia Minor, and Italy.

WATER CLOCKS

Water clocks were among the earliest timekeepers that didn't depend on the observation of celestial bodies. One of the oldest was found in the tomb of an Egyptian pharaoh buried around 1500 BC. Later named clepsydras ("water thieves") by the Greeks, who began using them about 325 BC, these were stone vessels with sloping sides that allowed water to drip at a nearly constant rate from a small hole near the bottom. Other clepsydras were cylindrical or bowl-shaped containers designed to slowly fill with water coming in at a constant rate. Markings on the inside surfaces measured the passage of "hours" as the water level reached them. These clocks could be used to determine hours at night as well as used in daylight. Another version consisted of a metal bowl with a hole in the bottom. When placed in a container of water the bowl would fill and sink in a certain time. These were still in use in North Africa in the 20th century.

More elaborate and impressive mechanized water clocks were developed between 100 BC and 500 AD. The added complexity was aimed at making the flow more constant by regulating the pressure, and at providing fancier displays of the passage of time. Some water clocks rang bells and gongs; others opened doors and windows to show little figures of people, or moved pointers, dials, and astrological models of the universe.

A Macedonian astronomer supervised the construction of the Horologion, known today as the Tower of the Winds, in the Athens marketplace in the first half of the first century BC. This octagonal structure showed scholars and shoppers both sundials and mechanical hour indicators. It featured a 24 hour mechanized clepsydra and indicators for the eight winds from which the tower got its name, and it displayed the seasons of the year and astrological dates and periods.

In the Far East, mechanized astronomical/astrological clock making developed from 200 to 1300 AD. Third-century Chinese clepsydras drove various mechanisms that illustrated astronomical phenomena. One of the most elaborate clock towers was built in 1088 AD. The mechanism incorporated a water-driven escapement invented about 725 AD. The clock tower, over 30 feet tall, possessed a bronze power-driven sphere for observations, an automatically rotating celestial globe, and five front panels with doors that permitted the viewing of changing manikins which rang bells or gongs, and held tablets indicating the hour or other special times of the day.

MECHANICAL CLOCKS

In Europe during most of the Middle Ages (roughly 500 AD to 1500 AD), technological advancement virtually ceased. Sundial styles evolved, but didn't move far from ancient Egyptian principles. During these times, simple sundials placed above doorways were used to identify midday and four "tides" (important times or periods) of the sunlit day. By the 10th century, several types of pocket sundials were used.

Then, in the first half of the 14th century, large mechanical clocks began to appear in the towers of several large Italian cities. We have no evidence or record of the working models preceding these public clocks, which were weight-driven and regulated by a verge-and-foliot escapement. Variations of the verge-and-foliot mechanism reigned for more than 300 years, but all had the same basic problem: the period of oscillation of the escapement depended heavily on the amount of driving force and the amount of friction in the drive. Like water flow, the rate was difficult to regulate.

Another advance was the invention of spring-powered clocks between 1500 and 1510 by Peter Henlein of Nuremberg. Replacing the heavy drive weights permitted smaller (and portable) clocks and watches. Although they ran slower as the mainspring unwound, they were popular among wealthy individuals due to their small size and the fact that they could be put on a shelf or table instead of hanging on the wall or being housed in tall cases. These advances in design were precursors to truly accurate timekeeping.

ACCURATE MECHANICAL CLOCKS

In 1656, Christiaan Huygens, a Dutch scientist, made the first pendulum clock regulated by a mechanism with a "natural" period of oscillation. Huygens' early pendulum clock had an error of less than 1 minute a day, the first time such accuracy had been achieved. His later refinements reduced his clock's error to no more than10 seconds a day.

Around 1675, Huygens developed the balance wheel and spring assembly, still found in some of today's wristwatches. This improvement allowed portable 17th century watches to keep time to 10 minutes a day. And in London in 1671, William Clement began building clocks with the new "anchor" or "recoil" escapement, a substantial improvement over the verge because it interfered less with the motion of the pendulum.

In 1721, George Graham improved the pendulum clock's accuracy to 1 second per day by compensating for changes in the pendulum's length due to temperature variations. John Harrison, a carpenter and self-taught clock-maker, refined Graham's temperature compensation techniques and developed new methods for reducing friction. By 1761, he had built a marine chronometer with a spring and balance wheel escapement that won the British government's 1714 prize. It kept time on board a rolling ship to about one-fifth of a second a day, nearly as well as a pendulum clock could do on land, and 10 times better than required to win the prize.

Over the next century, refinements led in 1889 to Siegmund Riefler's clock with a nearly free pendulum, which attained an accuracy of a hundredth of a second a day and became the standard in many astronomical observatories. A true free-pendulum principle was introduced by R.J. Rudd about 1898, stimulating development of several free-pendulum clocks. One of the most famous, the W.H. Shortt clock, was demonstrated in 1921. The Shortt clock almost immediately replaced Riefler's clock as a supreme timekeeper in many observatories. This clock contained two pendulums, one a slave and the other a master. The slave pendulum gave the master pendulum the gentle pushes needed to maintain its motion, and also drove the clock's hands. This allowed the master pendulum to remain free from mechanical tasks that would disturb its regularity.

QUARTZ CLOCKS

The performance of the Shortt clock was overtaken as quartz crystal oscillators and clocks, developed in the 1920s and onward, eventually improved timekeeping performance far beyond that achieved using pendulum and balance-wheel escapements.

Quartz clock operation is based on the piezoelectric property of quartz crystals. If you apply an electric field to the crystal, it changes its shape, and if you squeeze it or bend it, it generates an electric field. When put in a suitable electronic circuit, this interaction between mechanical stress and electric field causes the crystal to vibrate and generate an electric signal of relatively constant frequency that can be used to operate an electronic clock display.

Quartz crystal clocks were better because they had no gears or escapements to disturb their regular frequency. Even so, they still relied on a mechanical vibration whose frequency depended critically on the crystal's size, shape and temperature. Thus, no two crystals can be exactly alike, with just the same frequency. Such quartz clocks and watches continue to dominate the market in numbers because their performance is excellent for their price. But the timekeeping performance of quartz clocks has been substantially surpassed by atomic clocks.

The development of radar and extremely high frequency radio communications in the 1930s and 1940s made possible the generation of the kind of electromagnetic waves (microwaves) needed to interact with atoms. Research aimed at developing an atomic clock focused first on a microwave resonance in the ammonia molecule. In 1949, NIST built the first atomic clock, which was based on ammonia. However, its performance wasn't much better than the existing standards, and attention shifted almost immediately to more promising atomic-beam devices based on cesium.

The first practical cesium atomic frequency standard was built at the National Physical Laboratory in England in 1955, and in collaboration with the U.S. Naval Observatory (USNO), the frequency of the cesium reference was established or measured relative to astronomical time. While NIST was the first to start working on a cesium standard, it wasn't until several years later that NIST completed its first cesium atomic beam device, and soon after a second NIST unit was built for comparison testing. By 1960, cesium standards had been refined enough to be incorporated into the official timekeeping system of NIST.

The cesium atom's natural frequency was formally recognized as the new international unit of time in 1967: the second was defined as exactly 9,192,631,770 oscillations or cycles of the cesium atom's resonant frequency, replacing the old second that was defined in terms of the Earth's motions. The second quickly became the physical quantity most accurately measured by scientists. As of January, 2002, NIST's latest primary cesium standard was capable of keeping time to about 30 billionths of a second per year. Called NIST-F1, it is the 8th of a series of cesium clocks built by NIST and NIST's first to operate on the "fountain" principle.

Other kinds of atomic clocks have also been developed for various applications; those based on hydrogen offer exceptional stability, for example, and those based on microwave absorption in rubidium vapor are more compact, lower in cost, and require less power.

Much of modern life has come to depend on precise time. The day is long past when we could get by with a timepiece accurate to the nearest quarter-hour. Transportation, communication, financial transactions, manufacturing, electric power and many other technologies has become dependent on accurate clocks. Scientific research and the demands of modern technology continue to drive our search for increasingly more accurate clocks. The next generation of time standards is presently under development at NIST, USNO, in France, in Germany, and other laboratories around the world.

Although implied above, it must be noted that successful proliferation of so called atomic clocks is dependent on a service provided by the NIST that provides a standard that is used by these clocks. These clocks actually receive a radio signal that controls their accuracy based on the actual atomic clocks existing.  In essence the atomic clocks we buy are in actuality radio controlled clocks.

Since 1923, NIST radio station WWV has provided round-the-clock short-wave broadcasts of time and frequency signals. A sister station, WWVH, was established in 1948 in Hawaii.

Broadcast frequencies are 2.5 MHz (megahertz), 5 MHz, 10 MHz, and 15 MHz for both stations, plus 20 MHz on WWV. The signal includes UTC time in both voice and coded form; standard carrier frequencies, time intervals and audio tones; information about Atlantic or Pacific storms; geophysical alert data related to radio propagation conditions; and other public service announcements. An accuracy of one millisecond (one thousandth of a second) can be obtained from these broadcasts if corrections for the distance from the stations (near Ft. Collins, Colorado, and Kauai, Hawaii) to the receiver are accomplished.

In 1956, low-frequency station WWVB, which offers greater accuracy than WWV or WWVH, began broadcasting at 60 kilohertz. The broadcast power for WWVB was increased in 1999 from about 10 kilowatts to 50 kilowatts, providing much improved signal strength and coverage to most of the North American continent. This has stimulated commercial development of a wide range of inexpensive radio-controlled clocks and watches for general consumer use.

As atomic timekeeping has grown in importance, the world's standards laboratories have become more involved with the process, and in the United States today, NIST and USNO cooperate to provide official U.S. time for the nation. You can see a clock synchronized to the official U.S. government time provided by NIST and USNO at http://www.time.gov.