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The History of
Time-Keeping A clock is defined as a device that consists of two qualities:
Relaying the history of time measurement has a degree of
inaccuracy, much like clocks themselves. What follows is, if not completely
accurate, as close as many researchers can ascertain.
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The Egyptians improved upon the sundial with a ?merkhet,? the
oldest known astronomical tool.
It was developed around 600 B.C. and uses a string with a
weight on the end to accurately measure a straight vertical line
(much like a carpenter uses a plumb bob today). A pair of merkhets
were used to establish a North-South line by lining them up with the
Pole Star. This allowed for
the measurement of nighttime hours as it measured when certain stars
crossed a marked meridian on the sundial.
By 30 B.C., there were as many as 13 different types of
sundials used across Greece, Asia Minor and Italy.
Merkhet
?Clepsydras? or Water Clocks were among the first time-keeping devices that didn?t use the sun or the passage of celestial bodies to calculate time. One of the oldest was found in the tomb of ancient Egyptian King Amenhotep I, buried around 1500 B.C.
Around 325 B.C. the
Greeks began using clepsydras (Greek for "water thief") by the
regular dripping of water through a narrow opening and accumulating
the water in a reservoir where a float carrying a pointer rose and
marked the hours. A slightly
different water clock released water at a regulated rate into a bowl
until it sank. These clocks
were common across the Middle East, and were still being used in
parts of Africa during the early 20th century. They could not be relied on to
tell time more closely than a fairly large fraction of an hour.
More elaborate and impressive mechanized water
clocks were developed between 100 B.C. and 500 A.D. by
Greek and Roman horologists and astronomers. 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 Greek astronomer, Andronikos, supervised the construction of the Tower of the Winds in Athens in the 1st century B.C. This octagonal structure showed scholars and marketplace 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 A.D. Third-century Chinese clepsydras drove various
mechanisms that illustrated astronomical phenomena. One of the most
elaborate clock towers was built by Su Sung and his associates
in 1088 A.D. Su Sung's mechanism incorporated a
water-driven escapement invented about 725 A.D.
The Su Sung clock tower, over 30 feet
tall, possessed a bronze power-driven armillary sphere for
observations, an automatically rotating celestial globe, and five
front panels with doors that permitted the viewing of mannequins
which rang bells or gongs, and held tablets indicating the hour or
other special times of the day.
Su Sung's
clock tower, ca. 1088
Water Clock
Tower of the Winds, Athens,
Greece
es?cape?ment (
-sk
p
m
nt)
n.A
mechanism consisting in general of an escape wheel and an anchor,
used especially in timepieces to control movement of the wheel and
to provide periodic energy impulses to a pendulum or
balance.
The mechanical clock was
probably invented in medieval Europe. Clever arrangements of gears and
wheels were devised that turned by weights attached to them. As the
weights were pulled downward by the force of gravity, the wheels
were forced to turn in a slow, regular manner. A pointer, properly
attached to the wheels, marked the hours.
These clocks became
common in churches and monasteries and could be relied upon to tell
when to toll the bells for regular prayers or church attendance.
(The very word "clock" is from the French cloche, meaning
"bell.")
Eventually, mechanical
clocks were designed to strike the hour and even to chime the
quarter-hour. However, they had only an hour hand and were not
enclosed. Even the best such clocks would gain or lose up to half an
hour a day.
A technological advance
came with the invention of the ?spring-powered clock? around
1500-1510, credited to Peter Henlein of Nuremberg, Germany. Because these clocks could fit on
a mantle or shelf they became very popular among the rich. They did have some time-keeping
problems, though, as the clock slowed down as the mainspring
unwound. The development of the spring-powered clock was the
precursor to accurate time keeping.
In 1582, Italian
scientist Galileo, then a teenager, had noticed the swaying
chandeliers in a cathedral. It seemed to him that the movement back
and forth was always the same whether the swing was a large one or a
small one. He timed the swaying with his pulse and then began
experimented with swinging weights. He found that the "pendulum" was
a way of marking off small intervals of time accurately.
Once Galileo had made the
discovery, the regular beat of the pendulum became the most accurate
source used to regulate the movement of the wheels and gears of a
clock.
It wasn't a perfect
system, though, as a pendulum swings through the arc of a circle,
and when that is so, the time of the swing varies slightly with its
size. To make the pendulum keep truly accurate time, it must be made
to swing through a curve known as a "cycloid."
In 1656 Dutch astronomer
Christian Huygens first devised a successful pendulum clock. He used short pendulums that beat
several times a second, encased the works in wood, and hung the
clock on the wall. It had an
error of less than one minute a day.
This was a huge improvement on earlier mechanical clocks, and
subsequent refinements reduced the margin of error to less than 10
seconds per day.
In 1670 English
clockmaker William Clement made use of a pendulum about a yard long
that took a full second to move back and forth, allowing greater
accuracy than ever before. He encased the pendulum and weights in
wood in order to diminish the effect of air currents, thus was born
the "grandfather's clock." For the first time, it made sense to add
a minute hand to the dial, since it was now possible to measure time
to the nearest second.
In 1721George Graham
improved the pendulum clock?s accuracy to within a second a day by
compensating for changes in the pendulum's length caused by
temperature variations. The mechanical clock continued to develop
until it achieved an accuracy of a hundredth-of-a-second a day and
it became the accepted standard in most astronomical observatories.
Wall Clock from the
1870s
early mechanical
clock
Galileo
Christian Huygens
George Graham
Early Graham
clock
17th Century
Pocket Watch
Quartz Clocks
a quartz
crystal
chemical name: SiO2 , Silicon dioxide
The running
of a Quartz clock is based on an electric property of the quartz
crystal. When an electric field is applied to a quartz crystal, it
changes the shape of the crystal itself. If you then squeeze it or
bend it, an electric field is generated. When placed in an
electronic circuit, the interaction between the mechanical stress
and the electrical field causes the crystal to vibrate, generating a
constant electric signal which can then be used to measure time.
Quartz clocks continue to
dominate the market because of the accuracy and reliability of their
performance and by their low cost when produced in mass quantities.
A modern quartz digital watch that not only keeps
accurate time,
but can check your heart rate,
too.
"The Black Watch", released in 1975
by the Sinclair Co., was one of the first digital watches ever
produced, and probably the worst. If you were unlucky enough
to buy one of these lemons you could expect various kinds of
trouble:
- The internal chip could be ruined by static from your nylon shirt, nylon carpets or air-conditioned office. This problem also affected the production facility, leading to a large number of failures before the watches even left the factory. The result was that the display would freeze on one very bright digit, causing the batteries to overload (and occasionally explode).
- The accuracy of the quartz timing crystal was highly temperature-sensitive: the watch ran at different speeds in winter and summer.
- The batteries had a life of just ten days; this meant that customers often received a Black Watch with dead batteries inside. The design of the circuitry and case made them very difficult to replace.
- The control panels frequently malfunctioned, making it impossible to turn the display on or off - which again led to exploding batteries.
- The watch came in a kit which was almost impossible for hobbyists to construct. Practical Wireless magazine advised readers to use two wooden clothes pegs, two drawing pins and a piece of insulated wire to work the batteries into position. You then had to spend another four days adjusting the trimmer to ensure that the watch was running at the right speed.
- The casing was
impossible to keep in one piece. It was made from a plastic which
turned out to be unglueable, so the parts were designed to clip
together--which they didn't.
- A very high percentage of Black Watches were returned, leading to the legend that Sinclair actually had more returned than had been manufactured. The backlog eventually reached such monstrous proportions that it still hadn't been cleared two years later.
Atomic Clocks
Termed NIST F-1, the new
cesium atomic clock at NIST, the National Institute of Science and
Technology, in Boulder, Colorado is the nation's primary frequency
standard that is used to define Coordinated Universal Time (known as
UTC), the official world time. Because NIST F-1 shares the
distinction of being the most accurate clock in the world (with a
similar device in Paris), it is making UTC more accurate than ever
before. NIST F-1 recently passed the evaluation tests that
demonstrated it is approximately three times more accurate than the
atomic clock it replaces, NIST-7, also located at the Boulder
facility. NIST-7 had been the primary atomic time standard for the
United States since 1993 and was among the best time standards in
the world.
NIST F-1 is referred to
as a fountain clock because it uses a fountain-like movement of
atoms to obtain its improved reckoning of time. First, a gas of
cesium atoms is introduced into the clock's vacuum chamber. Six
infrared laser beams then are directed at right angles to each other
at the center of the chamber. The lasers gently push the cesium
atoms together into a ball. In the process of creating this ball,
the lasers slow down the movement of the atoms and cool them to near
absolute zero.
Two vertical lasers are
used to gently toss the ball upward (the "fountain" action), and
then all of the lasers are turned off. This little push is just
enough to loft the ball about a meter high through a
microwave-filled cavity. Under the influence of gravity, the ball
then falls back down through the cavity.
The fountain action of
the cesium clock.
As the atoms interact
with the microwave signal?depending on the frequency of that
signal?their atomic states may or may not be altered. The entire
round trip for the ball of atoms takes about a second. At the finish
point, another laser is directed at the cesium atoms. Only those
whose atomic states are altered by the microwave cavity are induced
to emit light (known as fluorescence). The photons (tiny packets of
light) emitted in fluorescence are measured by a detector.
This procedure is
repeated many times while the microwave energy in the cavity is
tuned to different frequencies. Eventually, a microwave frequency is
achieved that alters the states of most of the cesium atoms and
maximizes their fluorescence. This frequency is the natural
resonance frequency for the cesium atom?the characteristic that
defines the second and, in turn, makes ultra precise timekeeping
possible.
The 'Natural frequency' recognized currently as the measurement of time used by all scientists, defines the period of one second as exactly 9,192,631,770 oscillations or 9,192,631,770 cycles of the Cesium Atom's Resonant Frequency. The cesium-clock at NIST is so accurate that it will neither gain nor lose a second in 20 million years!
The cesium atomic
clock at the NIST.
This new standard is more
accurate by a wide margin than any other clock in the United States
and assures the nation's industry, science and business sectors
continued access to the extremely accurate timekeeping necessary for
modern technology-based operations.
ce?si?um (s
z-m). n.
Symbol Cs
A soft, silvery-white
ductile metal, liquid at room temperature, the most electropositive
and alkaline of the elements, used in photoelectric cells and to
catalyze hydrogenation of some organic compounds. Atomic number 55;
atomic weight 132.905; melting point 28.5°C; boiling point 690°C;
specific gravity 1.87; valence 1.
Discovered by spectroscopy
in 1860 by Robert Bunsen and Gustav Kirchhoff.
One
gram of cesium is an ample supply for a typical atomic clock to run
for one year.
A gram of cesium could be found in about a
cubic foot of ordinary granite. Natural cesium is pure cesium-133
(55 protons and 78 neutrons in the nucleus, 55+78=133): it is
non-radioactive.
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