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The first clocks were made
in the 1200's. They had no hand dials. They told time by ringing a bell
to indicate the hour. Weight driven clocks were powered by a heavy weight
that hung from a chord on a chain. When the clock was wound, so was the
chain. Weights slowly turned turning the drum. The drum turned the gears
which turned the hands. The atomic clocks are the most accurate clocks ever
made. They are based on vibrations of certain atoms or molecules. These
particles most always vibrate the same number of times per second. They
would gain or lose only a few seconds in one hundred thousand years.
The prehistoric people most
likely told the time of day by watching the shadows. When the sun moved,
the lengths of the shadows changed. When the shadows were short, the prehistoric
people knew the time was near noon.
Sundials developed more than
4000 years ago. They are the oldest known instruments for telling time.
As the sun crosses the sky, it casts a shadow on the dial. The sundial tells
time by measuring the length or the angle of the shadow.
Other earlier time instruments
are hour glasses and water clocks. Sand or water flowed from one container
to another at a steady rate. People could tell how much time had passed
because they knew how long it took for all the sand or water to empty.
The first mechanical clock was invented in
China in the late 1000's. This invention was never developed further. Later
Chinese clocks were based on European models.
Historians believed the first mechanical clocks in
western civilizations were developed during the late 1200's. These clocks
were weight driven, but they had no pendulum or hands. A bell rang to indicate
the hour. By the mid 1300's the dial and hand had been added. The first
spring driven clocks were probably in Italy during the late 1400's.
Early clocks ran unevenly and inaccurately. The pendulum and the balance
spring greatly improved time keeping accuracy. Minute and second hands became
common. By the mid 1700's, inventors developed most of the mechanisms found
in clocks.
Electric clocks, introduced in
the mid-1800's, were in many homes by the 1920's. Quartz-based clocks appeared
during the 1930's. Scientists developed the first atomic clock in the 1940's.
Digital clocks became popular in the 1970's, particularly as wrist watches.
In the 1980's the chip was added into clock mechanisms. Besides displaying
time, watches with electronic chips can store information and serve as calculators
and game boards. -