Types of 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.

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.

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.

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'.

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.