Time Measurements over the Centuries

TIME MEASUREMENT OVER THE CENTURIES

T.V. PRAFULLA KUMAR

The oldest time keepers that have been unearthed by excavations when one civilization after another disappeared over the centuries, are “Sun-dials and water Clocks”. A part of a simple sundial used in Egypt dating to above 1500 B.C. is kept in Neues Museum. Berlin, A water­-clock also of Egypt, dating to about 1380 B.C. is in the Cairo Mu­seum. The sundial was used to meas­ure time during the day and water clock during night. Both sundial and water clock measured 12 hours so that day and night together made 24 hours. A simple sundial had a rock surface from the centre of which a short rod projected at a certain angle. During day-time, the shadow of the rod fell on the sundial. The period from sunrise to sunset was divided into 12 equal parts and each part was called an hour. The Egyptian water clock was called ‘clep­sydra’. It consisted of an albaster bowl with sloping sides and a small hole at the bottom. The small hole was pro­vided with a metal outlet pipe. On the inner sloping surface of the bowl, there were hour-scales, twelve in number, corresponding to each month of the year. When water was filled to the top most mark of the hour-scale, water flowed out at a fairly uniform rate and the water level showed the hour during the night time.

During the Greek and Roman civilization also the sundials and water clocks were used with slight modifica­tions. A sundial called the hemicycle of Berosus was invented in about 300 B.C. It consisted of a hollow bowl instead of a flat surface and a horizon­tal rod projected from its centre. It was placed in the sun facing south and the tip of the rod fell on the hours scale in the bowl. Between the 8th and 11th century A.D. large water clocks with moving figures were constructed in China. They had bells also to sound the hours.

During  the 14th century sandglasses were made in Europe. The sandglasses consisted of two conical glass bulbs, connected by a very nar­row neck; one of the bulbs was filled with sand almost completely. When sandglass is placed such that the empty bulb is below, sand slowly falls into the bottom bulb and the time taken by sand to fill the bottom bulb is taken as an hour. After the bottom bulb is filled, the sand is kept upside down. Sandglass was also used to measure small durations of time. Candle clocks, and oil-lamp clocks were also used upto a certain extent. The hours scales were marked on the side of the candle clock and the glass reservoir of the oil-clock. During this period sundials were also used and some portable sundials were also made.

The first all mechanical weight-­driven clocks were probably made around 1300 AD. Their function was not to show time on a dial. They were made to sound hourly bells and give astronomical indications. They used them in Christian monasteries and public bell towers to attend to timely prayers. The oldest such clock made in 1386 is in Salisbury Cathedral, Eng­land. By then, clocks were made in Europe indicating the time on dials.

The spring-driven clocks made their appearance in the second half of 15th century. But they were not accu­rate, because the pull of the spring is greater when the spring is fully wound and the pull decreases as the spring unwinds. In 1582, Galileo observed that when the principle of oscillations of pendulum was applied to the spring-driven clocks great accuracy could be achieved. Unfortunately he died in 1642, before the clock designed by him, was made. In Holland, Huygens designed a spring-driven pendulum clock independently in 1656. During this time, balance wheel was developed for smaller clocks. In 1675, Huygens successfully used a small circular spring to control the oscillations of a balance wheel in small clocks. The system is being used till today with minor modifications, in table mechanical clocks and mechani­cal watches.

Electric clocks made their ap­pearance in 1840. Alexander Bain of Scotland and Sir Charles Wheatstone of England worked separately and used electric power to replace the weights and spring mechanism: The electrical clocks developed by William Hamilton Shortt have synchronous electric motors and were used as the standard clocks of the Greenwich observatory from 1924 to 1942. Dur­ing this time it was realised that greater accuracy could be achieved by using the vibrations of quartz crystal.

Quartz is a special form of sand formed deep inside the earth just as coal. Quartz has what is known as “piezo-electric” property. That is when pressure is applied on quartz, electric charges and voltages appear on its surface. These voltages are am­plified and fed-to the crystal, so that the crystal vibrates. The vibra­tions of the quartz crystal are of the order of 1,00,000 vibrations per sec­ond. These vibrations are reduced to 60 vibrations per second electronically and made to run a synchronous motor of an electrical clock. W. A. Morrison was the first to construct a quartz clock on this principle. Greenwich Ob­servatory replaced the Shortt’s dock; with quartz clock in 1942.

The Atomic clocks are the most accurate clocks, which utilize the oscillations of atoms and molecules. These Atomic clocks are so accurate that the error is only one second in 3,000 years.

Thus man has progressed from the approximate measurement of time by a sundial to the most accurate by an Atomic clock through the last 3,500 years.