The 240 Year Old Pendulum Clock That’s More Accurate Than your Watch

If you have a quartz watch on your wrist, chances are it’ll be pretty accurate. Probably +/- 15 seconds a month. Not shabby, given the low price of a mainstream quartz. If you have a modern mechanical watch, +/- 15 seconds a day would be normal. Still impressive, particularly with a balance wheel inside that has to revolve nearly 700,000 times each day.

But how about a pendulum clock built from a set of 240 year old plans? How long would it take to gain or lose a second? Just to make the question more interesting, imagine it was designed by a man who’d started life as a rural carpenter who made his first clock out of wood. Then imagine he’d decided not to lubricate the mechanism either.

How accurate would a clock like that be over 100 days?

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This is Clock B, made by clockmaker Martin Burgess from John Harrison’s pendulum clock theory; the eighteenth century English clockmaker who should have won the Admiralty’s Longitude Prize and pretty much invented the accurate marine timekeeper. In January this year it finally vindicated its designer’s 1774 claim that he could design a pendulum timepiece that was accurate to within a second over a 100-day period.


Harrison was living proof that you need more than talent to get on in life. He endured knockback after knockback throughout his 63 year career. The British Admiralty picked fault with each of his chronometers. Rivals criticised his work and publicly undermined him. And his final assertions that his pendulum clock with his own grasshopper escapement could be accurate to a second in a hundred days were met with derision. Little wonder that his snappily titled final work, “A description concerning such mechanism as will afford a nice, or True mensuration of time”, was so bitter. Later clockmakers referred to it as “…the ramblings of superannuated dotage.”

Two things in Harrison’s background seem to have combined to make him a remarkable and innovative clockmaker. First, he was – in effect – a natural materials scientist. He’s often portrayed as ‘just’ a rural carpenter, but that understates his affinity for, and experience with, the materials he used.

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Rory McEvoy, Curator of Horology at Greenwich and a Harrison expert, explains, “Take the way he used metals in H3, his third marine chronometer. The brass gear wheels in the movement are wide and very lightly made, yet they’re perfectly true. If you or I tried to produce wheels like that, with the inherent tensions within an untreated sheet of brass, we’d end up with something shaped like a crisp (potato chip).”

Not only this, but Harrison instinctively understood the need to reduce – and even remove – lubricants from clock mechanisms. As McEvoy explains, “Oil was the Achilles heel of any clock or watch. Harrison did away with lubrication altogether in his pendulum clocks and large timekeepers.” Modern watchmakers are still trying to find ways to do this.

Second, he was self-taught. That meant that he was able to think outside contemporary clockmaking practice. As McEvoy explains it, “Harrison came at things from a different angle, almost from first principles. He wasn’t indoctrinated with current watchmaking ideas.”


It was this fresh thinking that led to the plans for Clock A, an ultra-accurate pendulum clock, being realised by clockmaker Martin Burgess.

Clock A was commissioned from Burgess in 1975 by the Gurney family, a Norwich banking family. Completed in 1987 (proving you can’t rush good clockmaking) they gave the clock as a gift to the city of Norwich where it now ticks happily in a local shopping centre, the Castle Mall. Burgess had also started another Harrison pendulum clock, Clock B, but not finished it. The parts for the clock gathered dust on a shelf in Burgess’ workshop until 1993.

In 1993, he delivered a paper at a Harvard horology symposium where he talked about the ‘scandalous neglect’ of Harrison’s work in pendulum clock innovation – and, crucially, mentioned Clock B. Art historian and clock collector Donald Saff read the paper, tracked him down and persuaded him to sell him the unassembled and unfinished Clock B. Saff then commissioned English clockmaker Charles Frodsham to complete the project.

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Once the clock was completed in 2014, it attracted the sort of attention from horologists that premier league footballers would be familiar with. The began studying the clock in March 2014 and how it worked…

It quickly became clear that Clock B was something very special indeed. McEvoy continues, “We looked at the behaviour of Clock B very deeply, and we found that any fluctuations in its timekeeping were cyclical. In other words, they weren’t a problem because they were wholly predictable.” So although the clock’s timekeeping varied by a few fractions of a second, in effect, it evened itself out.

Finally, to determine whether Harrison’s words were indeed “…the symptoms of insanity” as The London Review of English and Foreign Literature suggested, Clock B was sealed in a perspex case in January this year and trialled for 100 days.

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To ensure there was no horological tinkering, the National Physical Laboratory and Worshipful Company of Clockmakers oversaw the trials. At the start, Harrison’s B clock was running a quarter second behind GMT. After 100 days of running, it was a mere 5/8ths of a second behind.

Guinness World Records have confirmed Clock B as the “most accurate mechanical clock with a pendulum swinging in free air.” But that bald description understates the achievements of both Harrison and Burgess. This is a clock that is so accurate, its curator was able to measure the impact of barometric pressure on its going. “As the barometer moved up, so the clock slowed down with air density,” explains Rory McEvoy. ‘When we adjusted the clock to take barometric pressure into account, it was 96% accurate. On most clocks, you wouldn’t even notice the error, let alone be able to correct it. We don’t see this sort of accuracy until at least 150 years after Harrison’s death.”

You’d think that with pin-sharp accuracy like this, Clock B would be a horological prima donna, throwing timekeeping tantrums if it was stopped or started. But not so, says McEvoy, “Once it’s adjusted, Clock B is remarkably stable. You can stop and start it without any problems.”


So why did Harrison do it? Why did he persist until almost the day he died in developing, defending and promoting pendulum clocks? He had a vision that, one day, every port would have a public pendulum clock, accurate to within fractions of a second, for mariners to set their marine chronometers by. This would mean they were able to calculate their position at sea to within a few nautical miles, thus missing shoals, sandbanks and rocks. Harrison realised that accurate timekeeping wasn’t just a theory, it was a lifesaving practice.

So, next year, on the 24th of March, raise a glass to Mr Harrison’s 240th anniversary. A remarkable man very much ahead of his time. This year, take a trip to Greenwich and take a look at Harrison’s other remarkable marine chronometers – it’s well worth the visit.

Notes and extras

The theory behind the clock fascinated a group of horologists, researchers and clockmakers who became The Harrison Research Group (HRG), meeting once a year. As Rory McEvoy, Curator of Horology at Greenwich puts it. “The work of the HRG culminated in this practical attempt to prove Harrison’s theories were valid.”

for more on Clock B and John Harrison, check out:

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Mark developed a passion for watches at a young age. At 9, he was gifted an Omega Time Computer manual from a local watch maker and he finagled Rolex brochures from a local dealer. Today, residing in the Oxfordshire village of Bampton, Mark brings his technical expertise and robust watch knowledge to worn&wound.
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