Until the 13th century, the only way people had to measure time was by observing the sun, or by devices in which the constant flow of a substance like water or sand can be measured and correlated with the passing of time. Thus in a water clock, or clepsydra, the water drips into a recipient and the hours are measured according to the level the water has reached. Both of these inventions achieved high levels of sophistication, but could never become super-accurate.
The real break-through came when it was realised that time could be chopped into small, measurable intervals rather than envisaged as a continuous flow. The first mechanical clocks appeared in Italy around 1300, but accuracy remained a problem for another 300 years. Even the word «hour» acquired its meaning of a precise stretch of time only in the 15th century; the words «minute» and «second» in their common modern meaning followed in the 16th century.
All modern clocks – down to the latest atomic clock accurate to one second every 30 million years – depend on oscillation. An oscillator is a device which moves backwards and forwards at a regular speed. This regular movement chops time into segments, which can then be counted.
The best known example is the pendulum, which is designed by the clockmaker to swing a precise number of times per second. (For example, in most pendulum wall clocks, it swings once every second.) The time of the swing depends on the length of the pendulum.
A pendulum cannot be transported, and a different device fulfilling the same function is needed for a watch.
In a mechanical watch, the oscillation is provided by a wound spring through a set of gear wheels (the wheel train) which ends in a mechanism known as an escapement. There are a number of different types of escapement, but the purpose is to interrupt the movement of the wheels at regular intervals and to convert the rotation of the wheel train into controlled and regular steps.
The purpose of the gear wheels is twofold. On the one hand they slow down the ticking rate of the oscillator, and on the other they drive the hands of the clock or watch.
In order to keep working, the spring of the mechanical watch has to wound periodically.
In a quartz watch the oscillator is a quartz crystal, which has the property that it vibrates in the presence of an electric field. The high frequency of the vibrations means that a quartz timekeeper is very accurate – to within about one minute a year.
The quartz is used in an electrical circuit, where its rate of oscillation is carefully regulated. Although the properties of quartz had been discovered towards the end of the 19th century (and were used, for example, in early radio sets), it was not until the 1960s that it became possible to manufacture integrated circuits small enough to be used in wristwatches.
Where the source of energy in a mechanical watch is the spring, in a quartz watch it is a miniature battery which lasts for several years.
The atomic clock uses the oscillations of atoms of caesium-133. The advantage of using atoms is that they always oscillate extremely fast and at exactly the same rate. Unlike quartz crystals, for example, they are unaffected by outside influences, such as temperature changes.