Pendulum and Precision

A well-known story says that a young Galileo, sitting in the cathedral at Pisa, watched a lamp swinging from the ceiling and timed its swings against his own pulse, noticing that they took the same time whether the swing was wide or narrow (check; the tale is probably embroidered). Whatever the truth of the story, the idea behind it changed timekeeping. A pendulum has a natural rhythm of its own, set by its length. Harnessing that rhythm took the best clocks from losing a quarter of an hour a day to losing a few seconds. This chapter follows that leap in accuracy, and the people and the mechanisms that brought it about.

The Idea of the Pendulum

A weight on a string or rod swings back and forth at a rate that depends almost entirely on its length, not on its weight or, for small swings, on how far it swings. This property, called isochronism, is only approximately true, but for small swings it is true enough to be very useful (check the details of the physics).

Huygens and the First Pendulum Clock

Galileo studied the pendulum and, late in life, is said to have designed a clock using one, but the first working pendulum clock is credited to the Dutch scientist Christiaan Huygens, who built one around 1656 and published a description soon after (check the dates). Huygens' early clocks were reported to keep time to within about a minute a day, and later versions did better (check the figure). It was an enormous improvement on the verge and foliot of Chapter 4. Huygens also worked out the mathematics of the pendulum in detail, in a book published in 1673 (check).

All figures are rough guides only (check).

The Anchor Escapement and the Long-Case Clock

The early pendulum clocks still used the verge escapement, which forced the pendulum to swing through a wide arc, undermining the accuracy gain. In England in the later seventeenth century, an escapement shaped like a ship's anchor, now called the anchor or recoil escapement, was introduced, and the credit is often given to the English clockmaker William Clement or to Robert Hooke, though the matter is disputed (check). It let the pendulum swing through a small arc, where the rule of isochronism holds best.

A small arc allows a long pendulum, and a long pendulum needs a tall case. The result was the long-case clock, the grandfather clock of the popular name, which became a status piece of the British home. The name "grandfather clock" itself is said to come from a nineteenth-century American song (check).

Fighting the Weather

A metal pendulum rod lengthens a little in warm weather and shortens in cold, so a clock runs slow in summer and fast in winter. The best makers devised compensation to cancel this. Two famous designs are the mercury pendulum, in which the expansion of liquid mercury balances the rod, and the gridiron, which uses rods of different metals (check the makers and dates). Changes in air pressure and humidity also have an effect, and observatory clocks were sometimes kept in sealed chambers.

Clockmakers in Britain

From the seventeenth century, London was one of the leading centres of clockmaking, and the Worshipful Company of Clockmakers received its charter in 1631 (check the date). Names such as Thomas Tompion and George Graham are often cited among the finest makers of the following century (check). Edinburgh had its own guild of craftsmen, and clockmakers in Scotland made long-case clocks that survive in museums and houses (check the guild details).

Exercises

Next chapter: Longitude and the Marine Chronometer, which covers why finding a ship's position needed an accurate clock, and the story of John Harrison.