Early humans used natural divisions of time, such as the year, the seasons and the day. Their lengths depended on the movements of the sun, moon and earth. When the day came to be divided into smaller units, with hours and minutes, the use of more accurate time-keeping clocks and watches developed.
The ancient Egyptians probably devised the unit of the hour around 4000 B.C. The 12 daylight hours started at dawn and finished at dusk, when the 12 night hours began. As the length of daylight varies throughout the year, the length of the hours used to vary too. And the night hours were different from the daylight hours. However, astronomers found this system confusing, so they used hours that were all the same length.
In order to tell the time, the ancient Egyptians invented the shadow clock, or sundial. The early sundial consisted of a rod placed upright in the ground. As the sun moved across the sky, it caused the shadow of the rod to change length and position throughout the day. The moving shadow passed over a scale marked on the ground, which enabled the time of day to be seen immediately.
By 1500 B.C., these sticks had been replaced by elaborate posts called obelisks set up in the centers of towns and villages. Many other kinds of sundial were invented later, but none was any use at night, or on days when there was no sun. However, this problem was overcome when the Egyptians invented the clepsydra, or water clock. In its simplest form, the water clock was a vessel containing water, which leaked slowly from a hole in the bottom. The water level indicated the time on a scale marked down the inside of the container.
After the clepsydra came the sand timer, or hour-glass. This used the passing of sand through a narrow-waisted glass container to indicate the passing of a fixed period of time. This device is still used in some kitchens as a three-minute timer for boiling eggs. Originally, it was used for timing longer periods – often an hour, hence the name. Another early timekeeping device was a burning candle. Notches in the wax indicated the hours as the candle burned down. The candle was once widely used, but it tended to be rather inaccurate. Variations in the wax, the length of wick, or the draft in the room could affect the rate of burning.
The first mechanical clock appeared in Europe around A.D. 1275. Instead of showing the time on a dial, it struck a single note on a bell every hour. (The term “clock” evolved from the Latin for bell, clocca.) This was worked by a falling weight pulling a wheel around. Soon after the invention of mechanical clocks, the mechanism became divided into three separate parts. The escapement controlled the speed at which the clock ran, and turned a hand or dial to show the time.
The striking train sounded the hours on a bell. And an alarm mechanism rang a bell at a chosen time. The escapement had a bar that rocked back and forth to interrupt the turning of a toothed wheel. A pair of weights controlled the rocking of the bar and, hence, the running of the clock.
The first major improvement to striking clocks was to make them strike the correct number of times for each hour. This feature was probably introduced in Italy around 1330. A better system was invented in 1676 by a clockmaker, the Reverend Edward Barlow, whose system made repeating clocks possible. These repeat the last hour struck when a cord at the side of the clock is pulled. This was useful for finding the approximate time in the dark. More advanced clocks repeated the last quarter-hour and, sometimes, even chimed the last minute to give a more accurate indication of the time.
For several centuries, domestic clocks had one hand indicating the hours on a dial. Divisions were marked for the quarters, and this was accurate enough for most people. Astronomers, however, needed more accurate clocks and so, from the 1500s, they used minute hands and even second hands on their clocks.
Until about 1475, all clocks worked by the use of weights and, therefore, could not be carried around. If moved, they had to be carefully set up again. Then someone thought of using a coiled spring to drive the mechanism around. This idea resulted in the development of portable clocks and the first watches.
A great advance in accuracy came in the 1650s when the Dutch scientist Christiaan Huygens invented the practical pendulum clock. The regular swinging of the pendulum controlled the escapement and thus regulated the clock. Around 1675, the English clockmaker William Clement designed an escapement (called the anchor, or recoil escapement) especially for use with the pendulum. It allowed a much longer and more accurate pendulum to be used. As a result, Clement was able to reduce the error of a clock from about 15 minutes to 20 seconds per day. With such accuracy available, the use of minute and second hands soon became widespread.
Over the years, other inventors steadily improved the accuracy of clocks and watches. In 1921, a remarkably accurate clock was invented by an English engineer, W. H. Short. The clock was partly electric, and had two pendulums. It was accurate to within one-tenth of a second per year. However, some modern clocks are hundreds of times more accurate.
In recent years, electric and electronic clocks have become extremely popular. Most electric clocks contain a small motor, which drives the hands around. The most popular type of electronic clock contains a quartz crystal. It is made to vibrate at an almost constant rate to regulate the clock. Some types use the hour and minute hands, but in others the time is displayed in digital form (as numbers).
The atomic clock is the most accurate of all. It uses the vibrations of certain atoms to achieve the incredible accuracy of one second in several thousand years.
The mechanical watch is driven by a coiled mainspring. A balance wheel spins back and forth to regulate the speed at which an escape wheel moves around in steps. This, in turn, controls the speed of the watch. The first truly portable watches were made in the early 1500s. Pieces of hog’s bristle were used to give the balance wheel the spring it required.
These early watches were poor timekeepers and had only one hand, which showed the hours. Accurate watches appeared in the late 1600s, and it was then that a minute hand was added. One great improvement made during this period was the introduction of the hairspring. This fine, coiled spring helped to make the balance wheel move back and forth with greater regularity.
Then, in 1759, Thomas Mudge invented a more accurate type of escapement called the lever. The lever escapements found in modern high-grade watches are based on Mudge’s design.
An even more important improvement was made in the 1760s by a craftsman named John Harrison. He made an extremely accurate “marine timekeeper,” designed to be accurate enough for use by navigators. The watch was only five seconds off after a six-month sea voyage.
The problem had been that the balance wheel tended to slow down in the heat, as the hairspring became weaker. To counteract this effect, Harrison arranged for any change in temperature to alter the effective length of the hairspring automatically. By careful design, one change could be made to counteract the other and timekeeping errors were greatly reduced.
Since the early 1700s, jeweled bearings made from ruby have been used in watches. This hard material can support the main moving parts without being worn by the constant rubbing, so the watch tends to last longer. But even better reliability can be obtained from an electronic watch, which may have no moving parts at all, except for a few switches.
The first watches to use electrical power instead of a coiled spring appeared in the 1950s. Pulses of magnetism pushed a balance wheel to keep it moving. The source of power was a tiny electric cell. In some watches of the 1960s, the balance wheel system was replaced by a tiny tuning fork. This, too, was kept vibrating by means of magnetism. In both types of watch, mechanical hands moved over a dial, as in a mechanical watch.
Modern electronic watches are regulated by a vibrating quartz crystal. The time may be displayed on a dial or as numbers displayed in digital readout, or in both forms. The day and date are also usually displayed, and the instrument may act as a stopwatch.
Source: Britannica.com: Clockworks: From Sundials to the Atomic Second