Pendulum clock
From Freepedia
A pendulum clock uses a pendulum as its time base. From their invention until about 1930, the most accurate clocks were pendulum clocks. Pendulum clocks cannot operate on vehicles, because the accelerations of the vehicle drive the pendulum, causing inaccuracies. See chronometer for a discussion of the problems of navigational clocks.
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History
The pendulum clock was invented by Christian Huygens in 1656, based on the pendulum introduced by Galileo Galilei.
Ths Shortt clock was the first device accurate enough to detect seasonal variations in the rotation of the Earth.
Mechanism
Pendulum clocks have several parts:
The pendulum swings with a designed period. To keep time accurately, pendulums are usually made to not vary in length when the temperature changes. John Harrison invented the grid pendulum, which used the differential expansion of brass and steel to achieve a zero-expansion pendulum. Modern clocks use a low-expansion alloy such as invar. Astronomical pendulums were often constructed of fused quartz, which changes length even less because of temperature.
Pendulums are frequently polished and streamlined to reduce the randomizing effects of turbulent air flow 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 vacuum to make the pendulum's operation even more accurate.
The escapement drives the pendulum, usually from a gear train. It is the part that ticks. Escapements have a locking state, and a drive state. In the locking state, nothing moves. The motion of the pendulum switches the escapement to drive, and the escapement then pushes on the pendulum for a brief part of the pendulum's cycle.
In the late 19th century, electromechanical escapements were developed. In these, a switch or phototube turned an electromagnet on for a brief section of the pendulum's swing. These are the most precise escapements known. They were usually employed with vacuum pendulums on astronomical clocks. The pulse of electricity that drove the pendulum would also drive a plunger to move the gear train.
In the 20th century W.H. Shortt invented a twin pendulum clock with an accuracy of one hundredth of a second per day. In this system the time keeping pendulum does no work and its movement is monitored by electrical devices which drive a slave pendulum which impulses the master pendulum. This form of clock became a standard for use in observatories.
To convert the motion of the escapement into an accurate analogue representation using 'hands' a gear train divides the motion of the escapement. Usually there are at least two gears: an hour gear, and a minute gear. These two gears are directly connected to the indicators (hands).
It is customary to make smaller gears more precisely, from more expensive materials in order to reduce wear.
Modern gear trains use involute gears, with tooth shapes that are an engineered compromise between efficiency and wear. Older clocks use cycloidal gears. The oldest clocks had hand-cut gears, some use gears made from interpenetrating cages of rods known as lantern pinons.
The indicators and clock face show the current time. Premium pendulum clocks often drive bells, whistles (cuckoo clocks) and dolls in order to help announce the time.
The slowest part of the gear train is attached to an energy storage device. This is either a spring, or a set of weights that pull on a cogwheel.
