̽��ֱ�� of Cambridge - clock

Opinion: Inside Big Ben: why the world’s most famous clock will soon lose its bong

Anonymous — Wed, 04 May 2016 11:03:30 +0000

London is soon going to lose one of its most familiar sounds when the world-famous Big Ben falls silent for repairs. ̽��ֱ��“bonging” chimes that have marked the passing of time for Londoners since 1859 will fall silent for months beginning in 2017 as part of a three-year £29m conservation project.

Of course, “Big Ben” is the nickname of the Great Bell and the bell itself is not in bad shape – even though it does have a huge crack in it. ̽��ֱ��bell weighs nearly 14 tonnes and it cracked in 1859 when it was first bonged with a hammer that was way too heavy. ̽��ֱ��crack was never repaired. Instead the bell was rotated one eighth of a turn and a lighter (200kg) hammer was installed. ̽��ֱ��cracked bell has a characteristic sound which we have all grown to love, so maybe best leave it alone.

Big Ben strikes. UK Parliament918 KB (download)

Instead, it is the Elizabeth Tower (1859) and the clock mechanism (1854), designed by Denison and Airy, that need attention.

Big Ben in 1858. ̽��ֱ��Illustrated News of the World December 4 1858

Any building or machine needs regular maintenance – we paint our doors and windows when they need it and we repair or replace our cars quite routinely. It is convenient to choose a day when we’re out of the house to paint the doors, or when we don’t need the car to repair the brakes. But a clock just doesn’t stop – especially not a clock as iconic as the Great Clock at the Palace of Westminster.

Repairs to the tower are long overdue. There is corrosion damage to the cast iron roof and to the belfry structure which keeps the bells in place. There is water damage to the masonry and condensation problems will be addressed, too. There are plumbing and electrical works to be done for a lift to be installed in one of the ventilation shafts, toilet facilities and the fitting of low-energy lighting.

Marvel of engineering

̽��ֱ��clock mechanism itself is remarkable. In its 162-year history it has only had one major breakdown. In 1976 the speed regulator for the chimes broke and the mechanism sped up to destruction. ̽��ֱ��resulting damage took months to repair.

Big Ben’s clock has had only one major breakdown, in 1976. UK Parliament, CC BY

̽��ֱ��weights that drive the clock are, like the bells and hammers, unimaginably huge. ̽��ֱ��“drive train” that keeps the pendulum swinging and that turns the hands is driven by a weight of about 100kg. Two other weights that ring the bells are each over a tonne. If any of these weights falls out of control (as in the 1976 incident), they could do a lot of damage.

̽��ֱ��pendulum suspension spring is especially critical because it holds up the huge pendulum bob which weighs 321kg. ̽��ֱ��swinging pendulum releases the “escapement” every two seconds which then turns the hands on the clock’s four faces. If you look very closely, you will see that the minute hand doesn’t move smoothly but it sits still most of the time, only moving on each tick by 1.5cm.

Pendulum suspension from a Smith of Derby clock. Hugh Hunt, Author provided

̽��ֱ��pendulum swings back and forth 21,600 times a day. That’s nearly 8m times a year, bending the pendulum spring. Like any metal, it has the potential to suffer from fatigue. ̽��ֱ��pendulum needs to be lifted out of the clock so that the spring can be closely inspected.

̽��ֱ��clock derives its remarkable accuracy in part from the temperature compensation which is built into the construction of the pendulum. This was yet another of John Harrison’s genius ideas (you probably know him from longitude fame). He came up with the solution of using metals of differing temperature expansion coefficient so that the pendulum doesn’t change in length as the temperature changes with the seasons.

In the Westminster clock, the pendulum shaft is made of concentric tubes of steel and zinc. A similar construction is described for the clock in Trinity College Cambridge and near perfect temperature compensation can be achieved. But zinc is a ductile metal and the tube deforms with time under the heavy load of the 321kg pendulum bob. This “creeping” will cause the temperature compensation to jam up and become less effective.

So stopping the clock will also be a good opportunity to dismantle the pendulum completely and to check that the zinc tube is sliding freely. This in itself is a few days' work.

What makes it tick

But the truly clever bit of this clock is the escapement. All clocks have one - it’s what makes the clock tick, quite literally. Denison developed his new gravity escapement especially for the Westminster clock. It decouples the driving force of the falling weight from the periodic force that maintains the motion of the pendulum. To this day, the best tower clocks in England use the gravity escapement leading to remarkable accuracy – better even than that of your quartz crystal wrist watch.

In Denison’s gravity escapement, the “tick” is the impact of the “legs” of the escapement colliding with hardened steel seats. Each collision causes microscopic damage which, accumulated over millions of collisions per year, causes wear and tear affecting the accuracy of the clock. It is impossible to inspect the escapement without stopping the clock. Part of the maintenance proposed during this stoppage is a thorough overhaul of the escapement and the other workings of the clock.

̽��ֱ��Westminster clock is a remarkable icon for London and for England. For more than 150 years it has reminded us of each hour, tirelessly. That’s what I love about clocks – they seem to carry on without a fuss. But every now and then even the most famous of clocks need a bit of tender loving care. After this period of pampering, “Big Ben” ought to be set for another 100 or so years of trouble-free running.

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 mechanism that makes Big Ben chime, and why it needs repairing.

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

Yes

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

Core values set new date for birth of the Earth

bjb42 — Fri, 09 Jul 2010 00:00:00 +0000

An international team of researchers used geochemical information taken from the Earth's mantle, and compared it with similar data from meteorites to create a new set of models showing how the planet might have been born.

̽��ֱ��results suggest that the length of time between the date at which the solar system was formed, about 4.567 billion years ago, and the point at which the Earth reached its present size, may have been far longer than traditionally presumed.

Scientists have typically suggested that the Earth's development - a process known as "accretion" - happened over the course of 30 million years.

Writing in the journal Nature Geoscience, however, the researchers argue that while the Earth probably grew to 60% of its size relatively quickly, the process may well have then slowed, taking about 100 million years in all.

" ̽��ֱ��whole issue hinges on working out how long it took for the core of the Earth to form, which is one of the big unknowns in this area of science," co-author Dr. John Rudge, from the ̽��ֱ�� of Cambridge, said.

"One of the problems has been that scientists usually presume Earth's accretion happened at an exponentially decreasing rate. We believe that the process may not have been that simple and that it could well have been a much more staggered, stop-start affair."

̽��ֱ��accretion of the Earth involved a series of collisions between dozens of smaller planetary bodies, referred to as "planetary embryos". Parts of these proto-planets blended together as they collided, and the intense heat of impact caused their interiors to melt. Over time, this led to the formation of a molten metal core at the heart of the Earth, with a silicate mantle overlying it.

Many scientists believe that the final part of the process happened when a body roughly the size of Mars collided with the Earth and caused part of the planet to break off, forming the Moon.

While they are broadly in agreement about the style of accretion, working out how long accretion took and thus how old the Earth really is has proven much more difficult.

̽��ֱ��research team behind the new study used information taken from isotopes of elements which would have undergone a process of radioactive decay while accretion was happening, to create a set of mathematical models revealing the different ways in which accretion might have occurred.

This hinges on the principle that some elements are naturally attracted, or have an "affinity" for, the silicate mantle of the Earth, while others are drawn to the metal at the centre.

Some elements have isotopes which decay to form isotopes of other elements with a different affinity. One good example is 182-hafnium, an isotope of a silicate-loving element, which decays to 182-tungsten, an isotope of a metal-loving element, most of which would have sunk to the Earth's core.

Traces of 182-tungsten can now be found within the mantle of the Earth, and some of this appeared after the formation of the core ceased as a result of the decay of 182-hafnium.

These geochemical signatures can be compared with what are called "chondritic" meteorites: primitive meteorites that have fallen to Earth in modern times, but have never undergone any sort of metal segregation.

Differences in the isotopic values of Earth tungsten and that taken from chondritic meteorites can provide some sense of how long accretion took, because we already know how long it takes for tungsten to go through each stage of decay.

These periods, known as "half lives", are millions of years long, which makes the calculation far from precise. Typically, however, scientists have used the hafnium-tungsten radioactive clock to estimate that accretion took 30 million years, presuming that the process of accretion was relatively steady.

In the new study, the researchers combined this data with a similar set of readings for uranium-lead decay, which also happened during accretion. ̽��ֱ��half-life readings for lead are much longer, but when they were put together with the tungsten measurements, overlaps could be determined.

Dr. Rudge modelled every single way in which the Earth could have undergone a process of accretion while matching the hafnium-tungsten and uranium-lead observations. Critically, the team never presumed that accretion happened at any particular rate.

While a wide variety of options emerged, the modelling process showed that the Earth almost certainly could not have formed within 30 million years. Instead, the results suggested that the planet initially grew very quickly, reaching two-thirds of its size within about 10 to 40 million years. Accretion then slowed down, however, and took perhaps another 70 million years to complete.

"If correct, that would mean the Earth was about 100 million years in the making altogether," Dr. Rudge said. "We estimate that makes it about 4.467 billion years old - a mere youngster compared with the 4.537 billion-year-old planet we had previously imagined."

̽��ֱ��Earth could be up to 70 million years younger than scientists previously thought, a study has found.

̽��ֱ��whole issue hinges on working out how long it took for the core of the Earth to form, which is one of the big unknowns in this area of science.

Dr. John Rudge

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