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The History of Time-Keeping
In respect to human history, time keeping is a relatively recent
desire – probably 5000 to 6000 years old. It was most likely
initiated in the Middle East and North Africa.
A clock is defined as a device that consists of two qualities:
A regular, constant or repetitive process or action to mark off
equal increments of time. Early examples of such processes
included movement of the sun across the sky, candles marked in
increments, oil lamps with marked reservoirs, sand glasses
("hourglasses"), and in the Orient, small stone or metal mazes
filled with incense that would burn at a certain pace.
A means of keeping track of the increments of time and displaying
the result.
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.
Contents
Keeping time with the Sun
Keeping time with the Stars
Keeping time with Water
Mechanical Time
Quartz Clocks
Atomic Clocks
Links to Horology Sites
Using the Sun
The Egyptians are the first group of people that we can reasonably
prove that took time-keeping seriously as a culture. Many believe
that the Sumerians were thousands of years ahead of the game, but
proof of this is only speculative.
Around 3500 B.C. the Egyptians built “Obelisks” -- tall four-sided
tapered monuments -- and placed them in strategic locations to cast
shadows from the sun. Their moving shadows formed a kind of sundial,
enabling citizens to partition the day into two parts by indicating
noon. They also showed the year's longest and shortest days when the
shadow at noon was the shortest or longest of the year. Later,
markers added around the base of the monument would indicate further
time subdivisions.
Around 1500 B.C., the Egyptians took the next step forward with a
more accurate “shadow clock” or “sundial.” The sundial was divided
into 10 parts, with two “twilight” hours indicated. This sundial
only kept accurate time (in relative terms) for a half day. So at
midday, the device had to be turned 180 degrees to measure the
afternoon hours.
A sundial tracks the apparent movement of the sun around the earth's
celestial pole by casting a shadow (or point of light) onto a
surface that is marked by hour and minute lines. That is why the
shadow-casting object (the gnomon or style) must point towards the
north celestial pole, which is very near Polaris, the North Star.
The gnomon serves as an axis about which the sun appears to rotate.
The sharper the shadow line is, the greater the accuracy. So,
generally speaking, the larger the sundial the greater the accuracy,
because the hour line can be divided into smaller portions of time.
But if a sundial gets too large, a point of diminishing returns is
reached because, due to the diffraction of light waves and the width
of the sun's face, the shadow spreads out and becomes fuzzy, making
the dial difficult to read.
In the quest for more 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, thought to have been invented about 300 B.C., removed the
useless half of the hemisphere to give an appearance of a half-bowl
cut into the edge of a squared block.
Obelisk
Copper Sundial
Using Stars
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
Using Water
“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 (-skpmnt)
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.
Mechanical Clocks
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 (sz-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.
Links to Clock, Watch, Time and Horology Websites
“Ultrasonic Ferroelectrics Frequency Control”
http://www.ieee-uffc.org/fc
“Walsh Brothers Craftsmen Watchmakers & Jewelers”
http://www.walshbrothers.co.uk/
“Clockmakers Newsletter”
http://www.clockmakersnewsletter.com/
“Clockstop.com”
http://www.clockstop.com/
“Horology – The Hands of Time”
HTTP://horology.magnet.fsu.edu
"All Clock-Wise"
http://www.allclockwise.com
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