̽��ֱ�� of Cambridge - timekeeping

Opinion: ̽��ֱ��remarkable accuracy of the Trinity College clock – and what makes it tick

Anonymous — Wed, 06 Apr 2016 13:22:15 +0000

This is a brief history of time – or at least how one clock tells it. In 1910, a new clock was installed in Trinity College, Cambridge. ̽��ֱ��maker, Smith of Derby, had long been recognised for its top quality “tower clocks”, clocks that sit high up a tower, usually a church, and often have only one small dial. Its main function is to ring bells to announce the hour, and perhaps the half hour and quarters.

̽��ֱ��most famous tower clock is the one in London’s Elizabeth Tower, commonly known as “Big Ben” – although this actually is the name of its Great Bell. It was designed by Edmund Beckett Denison, 1st Baron Grimthorpe, who was educated at Trinity College, and constructed by the famous clockmaker Edward John Dent. It was completed in 1859 and boasted Denison’s brand new “double three-legged gravity escapement”.

In a clock, the escapement converts the force of a falling weight into the periodic alternating impulses needed to keep the pendulum going. ̽��ֱ��weight also turns the hands of the clock. Denison’s “gravity” escapement has the virtue of remarkable accuracy because it decouples the driving force of the falling weights from the periodic force that maintains the motion of the pendulum. ̽��ֱ��well-established “remontoire” mechanism that was commonplace in Europe does a similar decoupling, but this new English gravity escapement is so much simpler and arguably more accurate.

To this day, the best tower clocks in England use the gravity escapement – and the Trinity College clock is one of them. It was not the college’s first clock. ̽��ֱ��clock tower was rebuilt to house the first clock in 1610 and in 1726 the then Master Bentley insisted on having one of his own. It didn’t last very long, however, and the college was soon without a clock.

In 1910, Lord Grimpthorpe’s great nephew paid for a new clock that boasted the gravity escapement – about 50 years after his great uncle had invented it. By then, Smith of Derby had perfected it. Indeed, the gravity escapement in the Trinity clock manages to keep the amplitude of the pendulum’s swing virtually constant.

Pigeons, wind and snow

This is no mean feat. For instance, if snow settles on the hands or if there is a driving wind then a conventional escapement will be disturbed and the clock will speed up or slow down. This is because the force driving the pendulum and the force turning the hands both come from the single falling weight. But the two “legs” of a gravity escapement are separate from the hands and so the pendulum is given a gentle tap on each swing that is not disturbed by wind and snow.

Pigeons can also play havoc with time. This is no fault of the escapement, just a bad design flaw in the dial that means there is plenty of room for pigeons to sit on the hands. And if two birds perch on the minute hand at quarter-to-the-hour, the clock will stop. Again, no fault of the escapement. ̽��ֱ��falling weight doesn’t have the power to lift two birds.

To counteract this, in February 2011, I fitted an anti-pigeon wire to the minute hand. Since then, the clock has been free from stoppages due to overweight pairs of pigeons.

How accurate is it?

One of the hardest things to do is to measure how fast or slow a clock is running. Have you ever tried to time the second hand on your watch? ̽��ֱ��first thing you need is a more accurate time reference than the clock you are measuring. Then you need to figure out a way of automatically comparing the two clocks.

In 2009, a monitoring system was set up with the help of Rick Lupton, a fourth year engineering student at Cambridge ̽��ֱ��. He fitted an infrared switch to the pendulum so that it would cut the infrared beam each time it swung past. He then compared the pendulum signal with the one-pulse-per-second output of a GPS receiver. This was achieved by feeding the two signals into the stereo sound jack of a PC and processing the data with some C++ and Python code.

Not only can precise pendulum timing be deduced, but temperature, pressure and humidity data are also collected every 30 seconds. ̽��ֱ��data are stored on the PC (running Linux – other operating systems are not up to the job) and they are all uploaded to an internet server.

Temperature compensation

Temperature can interfere with time, too – by causing the pendulum to expand or contract and to become longer or shorter. But in 1726, John Harrison came up with a neat way of controlling the change of length of a pendulum due to thermal expansion.

̽��ֱ��gridiron temperature compensated pendulum. Leonard G/Wikimedia

He invented the gridiron pendulum which uses two metals with different thermal expansion coefficients (such as steel and brass) so that the overall length is insensitive to temperature change. From then on, clocks became accurate to within a few seconds a week. ̽��ֱ��technique was adapted and perfected over the centuries and the Smith of Derby compensated pendulum comprises concentric steel and zinc tubes.

Atmospheric pressure disturbance

̽��ֱ��remarkable gravity escapement and the astonishing bi-metal temperature compensation technology enable accuracy to within one or two seconds a week. But better accuracy is hampered by the effect of barometric pressure on air density and so the buoyancy of the pendulum bob.

Strange as it may seem, an increase in air pressure of only 20mbar will cause any pendulum clock with a steel bob to run slow by about a second a week. This means that the clock might lose a few seconds during a prolonged spell of settled high-pressure dry weather and then regain this lost time when stormy unsettled low-pressure weather arrives.

It has been known for some time that a mass suspended by a pressure-sensitive actuator (known as an aneroid stack) could be attached to the pendulum and used to compensate for barometric pressure variations. Such a system was installed in the Trinity clock in 2010 and the effect has been remarkable. Since November 24 2015, until the present day (this article was written on April 4 2016) the clock has lost less than one second over a period of over four months.

This remarkable accuracy is normally expected of modern electronic timepieces, but the quartz watch on your wrist is probably only good enough to achieve a few seconds' accuracy a week. ̽��ֱ��Trinity clock with its various clever compensation systems is a remarkable testament to the importance of sound science and the longevity of good engineering.

Hugh Hunt, Reader in Engineering Dynamics and Vibration, ̽��ֱ�� of Cambridge

This article was originally published on ̽��ֱ��Conversation. Read the original article.

̽��ֱ��opinions expressed in this article are those of the individual author(s) and do not represent the views of the ̽��ֱ�� of Cambridge.

Hugh Hunt (Department of Engineering) discusses the history of the Trinity College clock and how it keeps time.

Trinity College clock

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Longitude project to chart new waters in untold story

bjb42 — Wed, 12 May 2010 00:00:00 +0000

In a new project announced today (Wednesday, May 12th), funded by the Arts and Humanities Research Council, researchers will embark on the first full investigation of the barely-studied archives of the British Board of Longitude, the eighteenth century organisation which oversaw the search for an accurate method of determining how far east or west a ship was at sea.

̽��ֱ��aim is not just to write the first history of the Board, but to reclaim its place in the story of a long series of breakthroughs, arguments, projects and schemes that until now have largely been associated with the efforts of a lone, self-educated clockmaker, John Harrison.

Harrison, whose life has been the subject of both a best-selling book and a film, was responsible for the development of a marine timekeeper, later called the chronometer. This key piece of technology enabled Longitude's accurate measurement and thus helped crack a conundrum that had baffled the world's most eminent scholars and navigators for generations.

As the project will seek to prove, however, the achievement was not his alone. Researchers will examine how he was one of an array of astronomers, inventors and craftspeople whose talents were harnessed and exploited by the Board's actions, and how it continued to sponsor innovations in science, exploration and industry long after Harrison had claimed his prize.

̽��ֱ��study, which is a collaboration between the ̽��ֱ�� of Cambridge and the National Maritime Museum, Greenwich, will produce the first ever complete history of the British Board of Longitude in time for 2014, the 300th anniversary of the Longitude Act that established it. To mark the launch, a short film about the project will be available from today on the ̽��ֱ��'s YouTube Channel (https://www.youtube.com/cambridgeuniversity).

" ̽��ֱ��Board of Longitude has had a pretty bad career in history, because it has either been forgotten or condemned," Professor Simon Schaffer, from the ̽��ֱ�� of Cambridge, who will lead the research, said. "Its creation was a turning point in British history, but after it was abolished in 1828 it was largely forgotten and its impact was never properly assessed."

"Part of the reason is that we still like to believe that we are a nation of enthusiastic amateurs like Harrison, making huge breakthroughs against the odds and in spite of a state hostile to scientific progress. In fact, we have a long history of state-sponsored ingenuity which made Britain into a military and technological world player. ̽��ֱ��Board is in many ways that history. By writing it we want to change the narrative."

̽��ֱ��Board of Longitude was set up to administer a prize of £20,000 (almost £3 million in modern money) to anyone who could solve the Longitude Problem. In addition, however, it had the discretion to support any sufficiently "promising" experimental work that might help along the way.

After Harrison claimed the prize, its patronage extended further still. ̽��ֱ��Board became involved with a wide range of scientific and maritime initiatives, including Captain Cook's voyages of exploration, the worldwide survey of geomagnetism, the establishment of the first overseas state observatory and the search for a North-West passage.

̽��ֱ��research will attempt to piece together its entire story, from 1714 to 1828, by focusing on two collections that represent the Board's Legacy.

̽��ֱ��first, an internationally-important collection of instruments and materials at the National Maritime Museum, attracts two million visitors a year, not least because it includes Harrison's own timekeepers. In addition, the team will open up the Board of Longitude papers at Cambridge's ̽��ֱ�� Library; a vast archive of manuscripts, letters, log-books (including those of Captain Cook) and other documents that have never been systematically studied. These papers include invaluable material ranging from climate records across the world to reports of encounters between Europeans and other peoples.

Researchers argue that while Harrison has been hugely important in popularising Longitude, it is important to rectify what they claim is a national myth that elevates his heroic role at the expense of the whole truth.

"One of the things we will be doing is taking apart the timekeepers Harrison made, which can give us an alternative version of the story," Richard Dunn, Curator of the History of Navigation at the National Maritime Museum, said. "If you look inside the first clock, it quickly becomes clear that several people were involved in making it. Clearly this wasn't just about a lone genius working by himself."

̽��ֱ��project also promises to illuminate the lives of a multitude of people who worked with the Board, corresponded with it, or wrote about and sometimes satirised it in newspapers and magazines of the time. ̽��ֱ��majority - artisans vying for the prize - would have been lost to history themselves without the archive's existence.

"Essentially the Board represents the germs of our national science policy," Professor Schaffer added. " ̽��ֱ��materials and correspondence it left behind is a window on to the cosmology of an entire class of people, and also on to the beginnings of Government-sponsored science in Britain."

"State-backed science is still an issue which matters a lot now, whether it's on stem cell research or climate change. We don't always know whether to trust it, and we don't know how to respond when scientists and the state fall out. If we can find out what worked as that relationship was beginning - and why - then we will have lessons to teach from the project we are starting now."

̽��ֱ��forgotten story of the British organisation that enabled the development of a system for measuring Longitude, only to disappear from memory after its demise, is to be told in full for the first time.

̽��ֱ��Board of Longitude has had a pretty bad career in history, because it has either been forgotten or condemned.

Professor Simon Schaffer

Norman B. Leventhal Map Center from Flickr

longitude

This work is licensed under a Creative Commons Licence. If you use this content on your site please link back to this page.

Yes