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In
Europe during most of the Middle Ages (roughly 500 to 1500 A.D.), technological
advancement was at a virtual standstill. 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" of
the sunlit day. By the 10th Century, several types of pocket sundials were
used. One English model identified tides and even compensated for seasonal
changes of the sun's altitude.
Then, in the early-to-mid-14th century,
large mechanical clocks began to appear in the towers of several large
Italian cities. There is no evidence or record of the working models preceding
these public clocks that were weight-driven and regulated by a verge-and-foliot
escapement. Verge-and-foliot mechanisms reigned for more than 300 years
with variations in the shape of the foliot. All had the same basic problem:
the period of oscillation of this 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, a German
locksmith from Nuremberg. Replacing the heavy drive weights permitted smaller
(and portable) clocks and watches. Henlein nicknamed his clocks "Nuremberg
Eggs". Although they slowed down as the mainspring unwound, they were popular
among wealthy individuals due to their size and the fact that they could
be put on a shelf or table instead of hanging from the wall. They were
the first portable timepieces. However, they only had an hour hand, minute
hands did not appear until 1670, and there was no glass protection. Glass
over the face of the watch did not come about until the 17th century. Still,
Henlein's advances in design were precursors to truly accurate timekeeping.
Accurate Mechanical Clocks
In 1656, Christian
Huygens, a Dutch
scientist, made the first pendulum clock, regulated by a mechanism with
a "natural" period of oscillation. Although Galileo
Galilei, sometimes credited with inventing the pendulum, studied its
motion as early as 1582, Galileo's design for a clock was not built before
his death. Huygens' 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 errors to less than 10 seconds a day.
Around 1675, Huygens developed the
balance wheel and spring assembly, still found in some of today's wrist
watches. This improvement allowed 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 interferes less with the motion of the pendulum.
In 1721, George Graham improved the
pendulum clock's accuracy to 1 second a 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 added new methods of 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 (of over $2,000,000
in today's currency) offered for a means of determining longitude to within
one-half degree after a voyage to the West Indies. 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.
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 consists
of two pendulums, one a slave and the other a master. The slave pendulum
gives the master pendulum the gentle pushes needed to maintain its motion,
and also drives the clock's hands. This allows the master pendulum to remain
free from mechanical tasks that would disturb its regularity.
Quartz
Clocks
The Shortt clock was replaced as
the standard by quartz crystal clocks in the 1930s and 1940s, improving
timekeeping performance far beyond that of 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 a constant frequency electric signal 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 and shape. Thus, no two crystals can be
precisely alike, with exactly the same frequency. Such quartz clocks continue
to dominate the market in numbers because their performance is excellent
and they are inexpensive. But the timekeeping performance of quartz clocks
has been substantially surpassed by atomic clocks.
Information and illustrations provided
by the National Institute of Standards and Technology and the U.S. Department
of Commerce
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Pendulum
used to keep time
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