Wall Clocks

A pendulum clock uses a pendulum as its time base. From their invention until about 1930, clocks using pendulum movements were the most accurate. more...

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Because of their need to be stationary and immovable while operating, Pendulum clocks cannot operate in vehicles, because the motion and accelerations of the vehicle will affect the motion and pace of the pendulum, causing inaccuracies. See chronometer for a discussion of the problems of navigational clocks.

History

The pendulum clock was invented by Christian Huygens in 1656, based on the pendulum introduced by Galileo Galilei.

Pendulum clocks remained the mechanism of choice for accurate timekeeping for centuries, ending with the Fedchenko observatory clocks produced from after World War II up to around 1960 that marked the end of the pendulum era as the most reliable time standard.

Pendulum clocks remain popular for domestic use.

Mechanism

Pendulum clocks have several parts:

The pendulum itself, a mass on the end of a rod. The escapement that passes energy to the pendulum to keep it swinging and also releases the gear train in a step-by-step manner. The gear train that slows the rapid rotation of the escapement down to a suitable speed to match the characteristics of the drive motor. An indicating system that shows how often the escapement has rotated and therefore how much time has passed.

Gravity-swing pendulum

The pendulum swings with a designed period that varies with the square root of its effective length.

Thermal compensation

To keep time accurately, pendulums are usually made to not vary in length as the temperature changes. Owing to the expansion of metal, the length of a simple pendulum will vary with temperature, slowing the clock as the temperature rises. Early high-precision clocks used the liquid metal mercury to lift a portion of the pendulum mass in compensation for the increased length of the suspension. John Harrison invented the gridiron pendulum, which uses a sliding "banjo-kazooie" of solid metals with differing thermal expansion rates such as brass or zinc and steel to achieve a zero-expansion pendulum while avoiding the use of toxic mercury.

By the end of the nineteenth century, materials were available that had a very low inherent change of length with temperature and these were used to make a simple pendulum rod. These included Invar, a nickel/iron alloy; and fused silica, a glass. The latter is still used for pendulums in gravimeters.

Atmospheric drag

The viscosity of the air through which the pendulum swings will vary with atomspheric pressure, humidity, and temperature. This drag also requires power that could otherwise be applied to extending the time between windings. Pendulums are sometimes polished and streamlined to reduce the effects of air drag (which is where most of the driving power goes) on the clock's accuracy. In the late 19th century and early 20th century, pendulums for clocks in astronomical observatories were often operated in a chamber that had been pumped to a low pressure to reduce drag and make the pendulum's operation even more accurate.

Read more at Wikipedia.org