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Types of Clocks
Sun clocks
| Water clocks | Mechanical
clocks | Quartz Clocks
| Atomic Clocks
Sun
Clocks
With the disappearance
of any ancient civilisation, such as the Sumerian culture, knowledge
is also lost. Whilst we can but hypothesise on the reasons of
why the equivalent to the modern
wristwatch was never completed, we know that the ancient Egyptians
were next to layout a system of dividing the day into parts,
similar to hours.
'Obelisks' (tall four-sided
tapered monuments) were carefully constructed and even purposefully
geographically located we believe around 3500 BC. A shadow was
cast as the Sun moved across the sky by the obelisk, which it
appears was then marked out in sections, allowing people to
clearly see the two halves of the day. Some of the sections
have also been found to indicate the 'year's longest and shortest
days', which it is thought were developments added later to
allow identification of other important time subdivisions.
Another ancient Egyptian 'shadow
clock' or 'sundial' has been discovered to have been in use
around 1500 BC, which allowed the measuring of the passage of
'hours'. The sections were divided into ten parts, with two
'twilight hours' indicated, occurring in the morning and the
evening. For it to work successfully then at midday or noon,
the device had to be turned 180 degrees to measure the afternoon
hours.
The Egyptians also used the
'Merkhet', the oldest known astronomical tool, which is believed
to have been developed around 600 BC. Two merkhets were used
to establish a north-south line which was achieved by lining
them up with the 'Pole Star'. This enabled the measurement of
night-time hours, when certain stars crossed the marked meridian.
By 30 BC, 'Vitruvius' describes thirteen different sundial styles
being used across Greece, Asia Minor, and Italy, inherently
demonstrating how the development must have grown to be more
complex.
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Water
Clocks
'Water clocks' were among
the earliest time keeping devices that didn't use the observation
of the celestial bodies to calculate the passage of time. The
ancient Greeks, it is believed, began using water clocks around
325 BC. Most of these clocks were used to determine the hours
of the night, but may have also been used during daylight. An
inherent problem with the water clock was that they were not
totally accurate, as the system of measurement was based on
the flow of water either into, or out of, a container which
had markers around the sides. Another very similar form was
that of a bowl that sank during a period as it was filled of
water from a regulated flow. It is known that water clocks were
common across the Middle East, and that these were still being
used in North Africa during the early part of the twentieth-century.
In the Far East, mechanised
'astronomical' and 'astrological' clock-making is known to have
developed between 200-1300 AD. In 1088 AD, 'Su Sung' and his
colleagues designed and constructed a highly complex mechanism
that incorporated a water-driven escapement, invented about
725 AD. It was over seven metres in height and had all manor
of mechanisms running simultaneously. During each hour an observer
could view the movement of a power-driven armillary sphere,
constructed of bronze rings, an automatically rotating celestial
globe, together with five doors that allowed an enticing glimpse
of seeing individual statues, all of which rang bells, banged
gongs or held inscribed tablets showing the hour or a special
time of the day. The appearance and actions would have appeared
similar to the automaton we know so well today.
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Mechanical
Clocks
In 1656, 'Christian Huygens'
(Dutch scientist), made the first 'Pendulum clock', with a mechanism
using a 'natural' period of oscillation. 'Galileo Galilei' is
credited, in most historical books, for inventing the pendulum
as early as 1582, but his design was not built before his death.
Huygens' clock ,when built, had an error of 'less than only
one minute a day'. This was a massive leap in the development
of maintaining accuracy, as this had previously never been achieved.
Later refinements to the pendulum clock reduced this margin
of error to 'less than 10 seconds a day'.
Huygens, in 1657, developed what is known today as the 'balance
wheel and spring assembly', which is still found in some of
today's wrist watches. This allowed watches of the seventeenth-century
to keep accuracy of time to approximately ten minutes a day.
Meanwhile, in London, England (UK) in 1671, 'William Clement'
began building clocks with an 'anchor' or 'recoil' escapement,
which interfered even less with the perpetual motion of the
pendulum system of clock.
'George Graham', in 1721, invented
a design with the degree of accuracy to 'one second a day' by
compensating for changes in the pendulum's length caused by
temperature variations. The mechanical clock continued to develop
until they achieved an accuracy of 'a hundredth-of-a-second
a day', when the pendulum clock became the accepted standard
in most astronomical observatories.
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Quartz
Clocks
The running of a 'Quartz clock'
is based on the piezoelectric property of the quartz crystal.
When an electric field is applied to a quartz crystal, it actually
changes the shape of the crystal itself. If you then squeeze
it or bend it, an electric field is generated. When placed in
an appropriate electronic circuit, this interaction. between
the mechanical stress and the electrical field. causes the crystal
to vibrate, generating a constant electric signal which can
then be used for example on an electronic clock display. The
first wrist-watches that appeared in mass production used 'LED',
'Light Emitting Diode' displays. By the 1970's these were to
be replaced by a 'LCD', 'Liquid Crystal Display'.
Quartz clocks continue to dominate the market because of the
accuracy and reliability of the performance, also being inexpensive
to produce on mass scale. The time keeping performance of the
quartz clock has now been surpassed by the 'Atomic clock'.
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Atomic
Clocks
Scientists discovered some
time ago that atoms and molecules have 'resonances' and that
each chemical element and compound absorbs and emits
'electromagnetic radiation' within its own characteristic 'frequencies'.
This we are told is highly accurate even over 'Time and Space'.
The development of radar and
the subsequent experimentation with high frequency radio communications
during the 1930s and 1940s created a vast amount of knowledge
regarding 'electromagnetic waves', also known as 'microwaves',
which interact with the atoms. The development of atomic clocks
focused firstly on microwave resonances in the chemical Ammonia
and its molecules. In 1957, 'NIST', the 'National Institute
of Standards and Technology', completed a series of tests using
a 'Cesium Atomic Beam' device, followed by a second programme
of experiments by NIST in order to have something for comparision
when working at the atomic level. By 1960, as the outcome of
the programmes, 'Cesium Time Standards' were incorporated as
the official time keeping system at NIST.
The 'Natural frequency' recognized
currently is 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'. From the 'Macrocosm', or 'Planetary Alignment',
to the 'Microcosm', or 'Atomic Frequency', the cesium now maintains
an accuracy with a degree of error to about 'one-millionth of
a second per year'.
Much of modern life has come
to depend on such precise measurements of time. The day is long
past when we could get by with a timepiece accurate to the nearest
quarter hour. Transportation, financial markets, communication,
manufacturing, electric power and many other technologies have
become dependent on super-accurate clocks. Scientific research
and the demands of modern technology continue to drive our search
for ever more accuracy. The next generation of Cesium Time Standards
is presently under development at NIST's 'Boulder Laboratory'
and other laboratories around the world.