What is Time? The Race for Accuracy

The last 150 years or so have seen the rapid acceleration of technological advance mirrored by the production of ever more accurate clocks.

(This is the eighth post of Time and Clocks, the thread that follows Time and Calendars.)

Master and Slave Clocks

Master clock No.17 by William Hamilton Shortt, London c.1929

The discovery of electricity was inevitably harnessed by clockmakers. The electric master and slave clocks in the 1850s, such as the one adopted by the Royal Observatory at Greenwich in 1852, used electric contact points to provide steady, regular beats which could drive not only the movements of their own dials but, through cabling, a number of slave dials at other locations. In 1889 Siegmund Riefler invented an almost completely free-swinging pendulum which, because it had less friction from the escapement to counter, is said to have achieved accuracy to one hundredth of a second per day and became the astronomical standard. A decade later, in 1898, the first true pendulum-free clock mechanism was demonstrated by R. J. Rudd, eventually to be replaced by the Shortt clock (invented by W. H. Shortt) which became the choice of all the main observatories in the western world from 1921 onwards. The master pendulum, isolated in a partial vacuum, was entirely independent of its clock mechanism, enabling it to concentrate on maintaining strict accuracy undisturbed by mechanical tasks like moving the hands around a dial. Accuracy was increased to within one second per year.

The Quartz Clock

A major advance was the use of quartz for timekeeping. Quartz clocks and watches had the advantage of ease of manufacture, with no gears or escapements to be set up or to disturb the oscillations. The principle of the clock was based on the piezoelectric property of quartz crystals: placed in a circuit, in a clock, the combination of the mechanical stress and electric field causes the quartz to vibrate and generate a constant signal which can turn hands on a dial or create an electronic display. The first quartz clock was developed in the USA, with the Greenwich Observatory installing one in 1939 which has attained accuracy to within two thousandths of a second in a day.

Four precision 100 kHz quartz crystal oscillators maintained by the US Bureau of Standards (now the National Institute of Standards and Technology that served as the frequency standard for the US in 1929. The quartz crystal oscillator was invented in 1923.

The Caesium Clock

The search for accuracy has reached its ultimate, current goal with the institutionalisation of atomic motion. All chemical elements possess a resonant frequency, an ability to emit and absorb electromagnetic radiation which is stable under almost any range of temperature, pressure or humidity and so can be used as a reference point for time on all parts of the globe and, indeed, on satellites above the earth and spaceships travelling from it. Developments in high-frequency and radio-wave measurements made it possible to create a device that could generate electromagnetic waves which could then be made to interact with atoms.

Caesium, the “heart” of the atomic clock, is contained in a capsule before being put in the “oven” that will vaporise it into a beam of atoms.

In 1957 the USA’s National Institute of Standards and Technology (NIST) created a caesium atomic beam device based on these discoveries, and found that they could achieve the most accurate results in the history of time measurement. Some subsequent measurements have been found to achieve accuracy of one millionth of a second in a year. Such accuracy meant that the second became the preferred international scientific standard unit, measured as 9,192,631,770 oscillations of the caesium atom’s frequency.

By 1967 the frequency of caesium atoms was formally recognised internationally as the universal measure of time.

In the next article on we take a look at the worldwide standardisation of time…