Pendulum Clocks
The introduction of the pendulum represented a major leap forward for horology. All mechanical clocks face the same problem: how to control and regulate the release of energy to turn it into a timekeeper.The energy stored in (say) a raised weight or coiled spring must be released in a regulated fashion rather than all at once. This is achieved by use of an escapement. Early escapements were very limited in accuracy - the introduction of pendulum regulated escapements allowed for far greater accuracy.
The most well known examples of pendulum driven clocks today are probably longcase (grandfather) clocks.
History
The principle of the pendulum was studied in the 1580s by Galileo Galilei. He noted that a given pendulum always took the same length of time to transcibe a full arc, regardless of whether this was a wide or narrow arc. The pendulum was thus a natural mechanical timekeeper. Galileo recognised the potential value of this discovery to horology, however he didn't actually build a pendulum clock.The first known pendulum clock is credited to Christiaan Huygens who built a pendulum based clock in 1656. This first clock achieved an accuracy of within one minute a day.
The early pendulum clocks used traditional escapements. The full accuracy of the pendulum was only realised when Robert Hooke developed the anchor escapement. This was a C-shaped piece of metal that sat on top of the escape wheel. It interacted with the pendulum to produce the most accurate timekeeping achieved to date.
The periodicity of a pendulum is related to its length. One limiting factor on the accuracy of early pendulum clocks was that the metal in the pendulum would expand and contract depending on the temperature. To counter this, later pendulums were made from a combination of brass and steel which have different expansion characteristics. This allowed clocks to be built with an acuracy within one second a day.
In 1921 W.H. Shortt created the Shortt Clock. This used two pendulums, one master and one slave. The slave pendulum actually drives the clock hands, avoiding mechanical disturbances to the master pendulum. The Shortt clock was the most accurate mechaniccal clock devised and was the standard for timekeeping until the coming of the quartz clock in the 1930s.