What a pendulum offers
A pendulum is a weight hanging from a rod that swings back and forth. For small swings, the time of each swing depends almost entirely on the pendulum’s length and on gravity, not on how far it swings. A pendulum just under one meter long takes one second for each swing in a single direction, which is why a seconds pendulum is a convenient size and why tall case clocks look the way they do.
This property is exactly what clockmakers wanted. The older foliot bar swung in a rough, unpredictable way. A pendulum is a natural regulator that does most of the timekeeping on its own.
Galileo and the early story
Galileo Galilei studied pendulums, and a popular story says he noticed a swinging lamp in the cathedral at Pisa and timed it against his pulse. That story first appeared in biographies written after his death and should be treated as a legend. What is well documented is that he studied pendulum motion and that he showed interest in using it to regulate a clock late in his life. Descriptions of a design from his last years exist, with accounts that his son built a model, though the details are uncertain.
Huygens builds the clock
The Dutch scientist Christiaan Huygens designed and built a pendulum clock in 1656 and received a patent from the States of Holland in 1657. A clockmaker in The Hague, Salomon Coster, made clocks to his design. Huygens published a full account of the theory in 1673 in a book titled Horologium Oscillatorium.
The first Huygens clocks used a verge escapement, and the pendulum swung through a wide arc. Huygens noticed that a pendulum keeps better time if the arc is small, but a wide swing was needed by that escapement. To cure the problem, he fitted curved metal pieces, called cheeks, on either side of the suspension so that the pendulum swung along a special curve. The idea was mathematically elegant, but practical clockmakers eventually found other solutions.
The improvement in timekeeping was dramatic. Reports suggest that good verge clocks might lose or gain roughly a quarter of an hour in a day, while early pendulum clocks could keep within a minute or so, and later ones did better. The precise figures vary by clock and conditions.
The anchor escapement
Around 1670, English clockmakers introduced the anchor escapement, which is often credited to Robert Hooke or to the clockmaker William Clement, though the credit is disputed. Its pallets are shaped like a ship’s anchor and engage the escape wheel, allowing the pendulum to swing in a much smaller arc of just a few degrees. That arrangement is closer to the ideal conditions for a pendulum, and the long pendulum could now hang behind the dial inside a tall case. The result was the longcase clock, which became a fixture in homes.
In the early 1700s, George Graham improved the design with the deadbeat escapement, which removes the slight backward recoil of the anchor and thus gives the pendulum a more even push. Deadbeat escapements became the standard for precision clocks.
Dealing with temperature
Metal expands when heated, so a pendulum rod grows longer in summer, the clock slows, and in winter it speeds up. For a precise clock the error was significant. In the 1720s, solutions arrived. George Graham developed a pendulum with a mercury-filled jar as the bob, which raises the center of mass when the rod expands. John Harrison, later known for his sea clocks, designed the gridiron pendulum, which combines rods of two metals, such as brass and steel, in an arrangement where their expansions cancel out. Later, alloys with very low expansion improved the situation further.
Improved metalworking techniques, such as better rolled and drawn brass and steel, helped, since clockmakers needed consistent materials.
Clocks for science
Accurate clocks changed observational astronomy. Timing when a star crosses a fixed line in a telescope became more precise, so catalogs of stellar positions improved. Observatories used pendulum clocks as master timekeepers, and the best ones were kept in cellars with steady temperatures and, in the late nineteenth century, even inside sealed cases at constant pressure, as in the Riefler clocks.
Clocks also contributed to measuring gravity. Because a pendulum’s period depends on gravity, expeditions noticed that a clock taken near the equator ran slower than one at higher latitude. This observation contributed to knowledge of the shape of the Earth.
Where a pendulum clock fails
A pendulum clock needs to stay still and upright. On the deck of a ship it is useless, and Huygens himself tried and failed to use pendulum sea clocks. This failure is a main reason the problem of finding a position at sea led to the spring-driven marine chronometer. Pendulums also need to be adjusted for height above sea level, since gravity varies slightly, and they are sensitive to vibration.
Daily life and the public clock
Better clocks changed daily life. Minute hands became common because the clocks were precise enough to make them meaningful. Towns could rely on a central clock, and later railway networks used observatory clocks to distribute standard time. Domestic clocks spread from the wealthy to the middle class during the 1700s and 1800s.
Pendulum clocks were the best timekeepers for roughly 270 years, until quartz and atomic clocks replaced them in the twentieth century. That is a long run for any invention, and it shows how much a simple, well-chosen physical principle can do.