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# What is a Pendulum Clock?

Barbara R. Cochran
Barbara R. Cochran

A pendulum is a relatively long, thin rod made of wood or metal that terminates in a disk. In the earliest versions of the pendulum clock, the disk weighed down the pendulum rod as it moved from one side to the other. An escapement gear, which converted the movement of a gear into the swing of a pendulum, allowed energy to go to the pendulum to overcome friction so that it could continue swinging. Later versions of the pendulum clock were powered by springs, and finally batteries.

Galileo is credited with coming up with the concept of a pendulum as an tool for measuring time, having studied its motion as early as 1582. Although he drew up design plans for a pendulum clock, Galileo died in 1642 before he could actually build one. Dutch mathematician and astronomer Christiaan Huygens is the man who is actually credited with the invention of the pendulum clock, in 1656.

Huygens earliest pendulum clock measured time with an error of less than one minute per day. With later models, he was able to reduce that error down to less than 10 seconds. In 1721, George Graham increased pendulum clock accuracy by reducing error to one second per day. He achieved this by making adjustments for changes in the pendulum rod's length, which were caused by temperature variations.

Siegmund Riefler and W.H. Shortt increased pendulum clock accuracy even more. Riefler’s clocks, which used an escapement he perfected in 1889, became widely used in astronomical observatories, and were accurate down to a hundredth of a second per day. The Shortt clock, introduced in 1921, operated on the two-pendulum slave and master principle and replaced Riefler’s as clock of choice for observatories.

Quartz clock technology was introduced during the 1920s and became the standard for timekeeping in subsequent decades. The pendulum clock came to take on the status of a collector’s item, both in its antique and more contemporary forms. Three common types of pendulum clocks that might appear in a collection are the cuckoo clock, the grandfather clock, and the mantel clock. For cuckoo clocks and grandfather clocks, the pendulum typically swings twice every second, and once every two seconds, respectively.