Rolling ball clock (Congreve clock)
A ball zigzags across a tilting tray, trips a lever, and the tray flips to send it back: a hypnotic early 19th-century invention that was better theater than timekeeper.
What is a rolling ball clock?
A rolling ball clock uses a small ball running down a zigzag track, rather than a pendulum or balance wheel, to pace the clock. The most familiar form is named after Sir William Congreve (1772-1828), who patented it in 1808. He was also a Member of Parliament, sitting for Plymouth from 1818 until his death, according to Britannica.
The British Museum's example sits in a satinwood case with columns and a domed glass hood, leaving the moving tray on full display. Three silvered rings show hours, minutes and seconds above it.
Congreve was a prolific inventor. He is chiefly remembered for the military rockets that carry his name, which Britannica says were used in the bombardments of Boulogne and Copenhagen and at Fort McHenry in 1814. National Museums Scotland adds that he also devised a paper for bank notes intended to resist forgery, and Britannica credits him with proposing armor plating for warships as early as 1805.
How it works
The heart of the clock is a flat tray tilted slightly to one side, with a long groove cut back and forth across it. A ball released at the high end rolls down the zigzag path. When it reaches the bottom, it hits a lever that unlocks the tray. The spring-driven movement then tips the tray the other way and locks it, and the ball sets off on its return run.
Each crossing advances the hands by a fixed step. National Museums Scotland says each run of its clock moves the hands on by 15 seconds, which adds up to 5,760 trips a day, while the British Museum describes the ball on its example as timed to take about thirty seconds. The ball does not power the clock: it sets the rate, while the mainspring and gear train do the work.
A small slide above the tray on the British Museum's clock shows seconds through windows in five-second steps, counting 0 to 30 on one run and 30 to 60 on the next. On National Museums Scotland's example the clock is meant to run about two weeks on one winding, though the museum says its weakening spring sometimes stops it sooner.
See also: an online stopwatch to time each run of the ball on OnlineClock.app.
History and invention
Congreve claimed the idea and patented it in 1808, together with what the British Museum's records call his extreme detached escapement. The Science Museum Group dates the original design to 1798-1802. National Museums Scotland, however, owns a rolling ball clock made by the Edinburgh clockmaker Robert Bryson in 1804, four years before Congreve's patent, so the invention's origins are not as tidy as the name suggests.
Congreve hoped the design would keep better time. The Science Museum Group notes that it needed less energy to keep in motion and suffered less wear than some other designs. In practice the British Museum judges it a failure as a precision timekeeper, though a popular one: makers have produced versions from the early 1800s to the present day.
Why it is a poor timekeeper
Everything about the ball's run is sensitive. Dust on the track slows it, and changes in temperature and humidity alter both the length of the groove and the size of the ball. The clock and its table must be perfectly level, and the ball needs regular polishing.
Darren Cox, a conservator at National Museums Scotland, reset the museum's Bryson clock every working day for two months and found that it gained or lost up to 45 minutes a day; he estimated that even in good conditions it might manage about 15 minutes a day. In 1837 staff at Buckingham Palace, tasked with keeping one running, called it a most complicated and troublesome machine. Even the seconds ring misleads: on the British Museum's example, the seconds hand runs only from 60 to 30 as the tray tilts, so it does not show true seconds.
See also: why a mechanical clock gains or loses time on OnlineClock.app.
Famous examples and how to recognize one
Notable examples include the 1804 Bryson clock on display at the National Museum of Scotland in Edinburgh, and a British Museum clock signed French Royal Exchange London 470, the work of Santiago James Moore French of Sweetings Alley, Royal Exchange. Even museum dating can be uncertain: the British Museum's catalogue gives 1805-1815 for that clock, while its curator's notes suggest about 1830.
Interest has never quite faded. In the 1970s a copy was produced with the Master Clock Makers Company, and the Science Museum Group, which now holds it, tentatively attributes it to Thwaites and Reed. The British Museum's clock, by contrast, still sits in its original satinwood case in the Sheraton style, glazed all round.
Visitors to Edinburgh should not use the Bryson clock to catch a train. The museum resets it only once a week, so on any given day it may be hours out.
A rolling ball clock is hard to mistake. Look for an exposed tray with a zigzag groove beneath the dials, often under a glass dome or in a glazed case, and a single ball that runs from one end to the other before the tray rocks over and sends it back.
Questions people ask
Who invented the rolling ball clock?
Sir William Congreve patented it in 1808, which is why it is often called a Congreve clock. A rolling ball clock made by Robert Bryson in Edinburgh in 1804 shows the idea was already in use a few years earlier.
Does the ball power the clock?
No. A mainspring drives the clock and tilts the tray. The rolling ball only regulates the rate, doing the job a pendulum would do in an ordinary clock.
Are Congreve clocks accurate?
Not very. Dust, temperature, humidity and leveling all affect the ball's speed, and a museum test of one 1804 example found it drifting by up to 45 minutes a day.