PENDULUM CLOCK

A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. From its invention in 1656 by Christiaan Huygens until the 1930s, the pendulum clock was the world's most accurate timekeeper, accounting for its widespread use. Pendulum clocks must be stationary to operate; any motion or accelerations will affect the motion of the pendulum, causing inaccuracies, so other mechanisms must be used in portable timepieces. They are now kept mostly for their decorative and antique value.

The pendulum clock was invented in 1656 by Dutch scientist Christiaan Huygens, and patented the following year. Huygens was inspired by investigations of pendulums by Galileo Galilei beginning around 1602. Galileo discovered the key property that makes pendulums useful timekeepers: isochronism, which means that the period of swing of a pendulum is approximately the same for different sized swings. Galileo had the idea for a pendulum clock in 1637, partly constructed by his son in 1649, but neither lived to finish it. The introduction of the pendulum, the first harmonic oscillator used in timekeeping, increased the accuracy of clocks enormously, from about 15 minutes per day to 15 seconds per dayleading to their rapid spread as existing 'verge and foliot' clocks were retrofitted with pendulums.

These early clocks, due to their verge escapements, had wide pendulum swings of up to 100°. Huygens discovered that wide swings made the pendulum inaccurate, causing its period, and thus the rate of the clock, to vary with unavoidable variations in the driving force provided by the movement. Clockmakers' realization that only pendulums with small swings of a few degrees are isochronous motivated the invention of the anchor escapement around 1670, which reduced the pendulum's swing to 4°-6°. In addition to increased accuracy, this allowed the clock's case to accommodate longer, slower pendulums, which needed less power and caused less wear on the movement. The seconds pendulum (also called the Royal pendulum) in which each swing takes one second, which is about one metre (39.1 in) long, became widely used. The long narrow clocks built around these pendulums, first made by William Clement around 1680, became known as grandfather clocks. The increased accuracy resulting from these developments caused the minute hand, previously rare, to be added to clock faces beginning around 1690.

The 18th and 19th century wave of horological innovation that followed the invention of the pendulum brought many improvements to pendulum clocks. The deadbeat escapement invented in 1675 by Richard Towneley and popularized by George Graham around 1715 gradually became standard in precision regulators and is now used in most modern pendulum clocks. Observation that pendulum clocks slowed down in summer brought the realization that thermal expansion and contraction of the pendulum rod was a large source of error. This was solved by the invention of the mercury pendulum by George Graham in 1721 and the gridiron pendulum by John Harrison in 1726allowing the construction of precision regulator.