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.
In 1656, Christiaan Huygens, a Dutch scientist, made the first pendulum clock, regulated by a mechanism with a "natural" period of oscillation. (Galileo Galilei is credited with inventing the pendulum-clock concept, and he studied the motion of the pendulum as early as 1582. He even sketched out a design for a pendulum clock, but he never actually constructed one before his death in 1642.) Huygens' early pendulum clock had an error of less than 1 minute a day, the first time such accuracy had been achieved. His later refinements reduced his clock's error to less than 10 seconds a day.
Around 1675, Huygens developed the balance wheel and spring assembly, still found in some of today's wristwatches. This improvement allowed portable 17th century watches to keep time to 10 minutes a day. And in London in 1671, William Clement began building clocks with the new "anchor" or "recoil" escapement, a substantial improvement over the verge because it interferes less with the motion of the pendulum.
In 1721, George Graham improved the pendulum clock's accuracy to 1 second per day by compensating for changes in the pendulum's length due to temperature variations. John Harrison, a carpenter and self-taught clock-maker, refined Graham's temperature compensation techniques and developed new methods for reducing friction. By 1761, he had built a marine chronometer with a spring and balance wheel escapement that won the British government's 1714 prize (worth more than $10,000,000 in today's currency) for a means of determining longitude to within one-half degree after a voyage to the West Indies. It kept time on board a rolling ship to about one-fifth of a second a day, nearly as well as a pendulum clock could do on land, and 10 times better than required to win the prize.
Over the next century, refinements led in 1889 to Siegmund Riefler's clock with a nearly free pendulum, which attained an accuracy of a hundredth of a second a day and became the standard in many astronomical observatories. A true free-pendulum principle was introduced by R.J. Rudd about 1898, stimulating development of several free-pendulum clocks. One of the most famous, the W.H. Shortt clock, was demonstrated in 1921. The Shortt clock almost immediately replaced Riefler's clock as a supreme timekeeper in many observatories. This clock contained two pendulums, one a slave and the other a master. The slave pendulum gave the master pendulum the gentle pushes needed to maintain its motion, and also drove the clock's hands. This allowed the master pendulum to remain free from mechanical tasks that would disturb its regularity. Ths Shortt clock was the first device accurate enough to detect seasonal variations in the rotation of the Earth.
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 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.