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How Pendulum Clocks Work

Gear driven clocks replaced water clocks in the Medieval world in the late 1200s. Water clocks were based on the slow regular movement of water from a higher level to a lower level (the same idea as the hour glass). Often the flow of water drove gears and dials so that some such devices had a fairly modern appearance. However, they were only accurate to within about 15 minutes per day. Thus the advancement to mechanical clocks did not seem a great leap. Yet the invention of the escapement was an essential step in the increasing accuracy of time pieces. No one knows exactly who invented the escapement, but the first recorded description came in about 1250 AD by Villard de Honnecourt, a French architect.

Anchor Escapement Image.
An escapement is a device used to release just a bit of stored energy at regular intervals in even amounts to turn the gears of a clock in as precise a manner as the engineers of the age could devise. The type of mechanical escapement that is most familiar is the one associated with the pendulum (which has very regular motion). It is actually a simple device by modern standards. As a pendulum swings, a bar releases a tooth from a gear holding a coiled spring or a suspended weight. The gear then turns incrementally being pulled by the weight or pushed by the spring. But before the gear can spin past the first tooth, the pendulum moves back, and catches the next tooth. Then it swings back again releasing the gear again. The pendulum escapement continues this motion until all of the energy stored behind the gear is used up. Typically, this means that the clock spring has fully unwound or hanging weights reach the full extent of their tether.

The gear regulated by the escapement bar is attached to other gears which reduce or speed up the spin of the various dials and hands on a clock. Christian Huygens is credited with developing the first pendulum clock in 1656, although Galileo is credited with coming up with the idea in the first place. Our image shows an anchor escapement. It was called this because of its resemblance to a ship's anchor. Note that because of the shape of the gear, the teeth give the pendulum a slight shove as it swings back from its release. This helps to prevent friction from eventually stopping the pendulum's motion.

When gear-driven, mechanical clocks were first developed, most were designed so that the hands would move "clockwise"; this was the direction that the shadow on a sundial moves in the northern hemisphere. There are a couple of drawbacks to mechanical clocks. Although they are accurate enough to be very useful for use in daily life. They lack the accuracy necessary for very precise work, such as modern astronomical observation or the functioning of modern computers.

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