How it began
The Dutch mathematician, astronomer and physicist Christiaan Huygens completed a prototype of his first pendulum clock by the end of 1656 and patented it on June 16, 1657. He contracted the construction of his designs to Salomon Coster, a clockmaker in The Hague, who actually built the clocks. Galileo had earlier discovered the property that makes a pendulum useful for timekeeping: its swings take approximately the same time whether they are wide or narrow.
Before it existed
Mechanical clocks before Huygens were of the verge and foliot type, and the spring-driven table clocks of the era were no better: they drifted by about fifteen minutes a day. Minute hands were rare on clock faces. The pendulum was the first harmonic oscillator used in timekeeping, and it reduced that daily error to about fifteen seconds.
How it works
A pendulum is a harmonic oscillator: it swings at a rate set mainly by its length. Its period is approximately the same for swings of different sizes, the property that Galileo had identified, and the clock's movement keeps it swinging. Huygens's early clocks used the old verge escapement and had wide swings of up to 100 degrees. In his 1673 book Horologium Oscillatorium he showed that wide swings made the period vary with the driving force.
How it spread
The gain in accuracy was so large that pendulum clocks spread rapidly, and existing verge and foliot clocks were refitted with pendulums. Around 1670 the anchor escapement reduced the swing to 4 to 6 degrees, which allowed longer, slower pendulums. The seconds pendulum, about one meter long, became common, and the tall narrow cases built around it from about 1680 came to be known as grandfather clocks. Minute hands, previously rare, were added to dials from about 1690.
Who pushed back
Huygens hoped that his clock would solve the problem of finding longitude at sea, which required a timekeeper that stayed accurate on a moving ship. He built several pendulum clocks for the purpose, and they were tested at sea in 1662 and 1686 with mixed results. The American Physical Society notes that his own attempts at a marine design were not sufficiently accurate, and the pendulum clock did not settle the question.
What lasted
Pendulum clocks remained the most accurate timekeepers for hundreds of years, until the quartz clock was invented in 1927. While ill in bed in 1665, Huygens noticed that two of his clocks hung from the same beam fell into step, the first report of coupled oscillation. The first pendulum clock made by Salomon Coster, dated 1657, is preserved in the Museum Boerhaave in Leiden.
