Rolling ball clock in the British Museum

A Congreve clock (also known as Congreve's Rolling Ball Clock or Oscillating Path Rolling Ball Clock) is a type of

clock A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and the ...

that uses a ball rolling along a

zigzag A zigzag is a pattern made up of small corners at variable angles, though constant within the zigzag, tracing a path between two parallel lines; it can be described as both jagged and fairly regular. In geometry, this pattern is described as a ...

track rather than a

pendulum A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the ...

to regulate the time. It was invented by

Sir William Congreve Lieutenant General Sir William Congreve, 1st Baronet (4 July 1742 – 30 April 1814) was a British military officer who improved artillery strength through gunpowder experiments. Personal life William Congreve was born in Stafford on 4 July 1 ...

in 1808. The ball takes between 15 seconds and one minute to run down the zigzag track, where it trips the

escapement An escapement is a mechanical linkage in mechanical watches and clocks that gives impulses to the timekeeping element and periodically releases the gear train to move forward, advancing the clock's hands. The impulse action transfers energy to ...

which in turn reverses the tilt of the tray and at the same time causes the hands of the clock to move forward. Thus the angle of the plate reverses and the clock hands move forward between one and four times every minute. On versions of the clock with a dial to indicate seconds, the second hand jumps forward either 15 or 30 seconds on each oscillation depending on the length of the track. Congreve appears to have been unaware that rolling ball clocks had been invented earlier by both

Nicolas Grollier de Servière Nicolas Grollier de Servière (1596–1689) was a French inventor and ornamental turner who became well known for creating a series of fantastic machines. Grollier de Servière, a cousin of Jean Grolier de Servières (1489/90–1565), Treasu ...

and

Johann Sayller Johann, typically a male given name, is the German form of ''Iohannes'', which is the Latin form of the Greek name ''Iōánnēs'' (), itself derived from Hebrew name ''Yochanan'' () in turn from its extended form (), meaning "Yahweh is Gracious" ...

. Congreve's version differed from Grollier's rolling ball clock in that the ball travelled down a zigzag rather than straight path. While Sayller's version had also used a zigzag path, it had utilized a number of balls and a fixed table rather than the single ball and tilting table employed by Congreve. The "Extreme Detached Escapement" (as Congreve referred to his escapement design) was patented in 1808. Congreve, who was not a clock maker, hired Gravell & Tolkien to produce the first working version, which he then presented to the

Prince of Wales Prince of Wales ( cy, Tywysog Cymru, ; la, Princeps Cambriae/Walliae) is a title traditionally given to the heir apparent to the English and later British throne. Prior to the conquest by Edward I in the 13th century, it was used by the rulers ...

in 1808. This version was weight-driven, but the second design, which appears to have been constructed by John Moxon, was spring-driven. The second model is in the collection of

Buckingham Palace Buckingham Palace () is a London royal residence and the administrative headquarters of the monarch of the United Kingdom. Located in the City of Westminster, the palace is often at the centre of state occasions and royal hospitality. It ...

. Congreve clocks are unreliable timekeepers — the time taken for the ball to travel along the track varies greatly depending on the cleanliness of the track and ball, and since the plate is aligned horizontally, it is easy for dust to accumulate. CongreveClock

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