Escapements

Mechanical time pieces usually work on some form of stored kinetic energy, such as that provided by lifting a weight or tightening a spring. When you "wind" a clock, you are imparting that energy to the system.

The problem for clock makers was how to control the release of that energy. On its own, a weight would simply fall to the ground immediately. A spring would immediately unwind. That's where escapements come in.

Escapements allow the potential energy in the clock to be released slowly over a period of time. Usually the release will be in a series of discrete "jumps" - these producing the ticking of the clock.

The earliest mechanical clocks were weight driven. "Winding" the clock consisted of raising the weight and winding its support string around a shaft. If then released on its own, the weight would fall immediately.

There have been numerous escapements designed, here are a couple of the most important:

Verge and Foliot

The earliest real form of escapement was the "verge and foliot", a device believed to have been based on the old alarums. The falling weight turns a crown wheel a little way, at which point it hits an arm extending from the top of a vertical shaft (the "verge"). The wheel then applies an impulse to the verge which spins, releasing the wheel until it contacts a second arm at the bottom of the verge. This process continues, each contact giving a "tick" or "tock" - in reality, more like "bang, bang".

The time it takes for the verge to rotate is controlled by the "foliot", a crossbar placed atop the verge. This has a weight at each end and changing these weights changes the timing of the clock.

Verge and foliot escapements were not highly reliable and were difficult to fine tune. A particular problem was that in high temperatures the foliot mechanism would expand slightly and change the clock's timing.

An improvement to the verge and foliot escapement was to replace the foliot with a metal circle called a balance wheel. This was less prone to temperature changes but still suffered from the general mechanical inaccuracies of the system.

Anchor Escapement

The anchor escapement was a major step forward in horology and was invented by Robert Hooke in the middle of the 17th century. It was designed for use with the newly created pendulum clocks invented by Christiaan Huygens. The first pendulum clocks used a verge escapement with the pendulum taking the place of the foliot. This improved accuracy - the anchor escapement improved it still further.

The anchor escapement is simply a "C" shaped piece of metal that sits atop an escape wheel, its teeth pointing downwards. The movement of the pendulum rocks the anchor back and forth, and first one then the other point locks with the teeth of the escape wheel. This regulates the fall of the weight depending on the frequency of the pendulum swing. Many clocks originally built with a verge escapement were converted to the new anchor mechanism.

Early anchor escapements were "recoil" - the escape wheel actually bounced backwards a little at the end of the pendulum arc. A more advanced version is the "deadbeat" anchor escapement where the anchor stops precisely. These deadbeat escapements are more accurate timekeepers but need to be manufactured with greater precision and were hence more expensive to buy.