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A Nobel Prize Clock-Watching in Great Court Compromising With Issue 10 • Spring 2010 A Nobel Prize Clock-watching in Great Court Compromising with Apartheid Restoration in the Wren A Trinity Memoir New Fellows Ethics and the Genome French Cricket SPIDERS AND PIGEONS: THE PRESSURES ON THE GREAT COURT CLOCK By Hugh Hunt This is a double anniversary year for Smith’s mechanism to attain still the Great Court Clock. It was 400 greater accuracy. years ago in 1610 that the clock was At the heart of the monitoring first installed. Its dial was small and system is a tiny infrared beam that modest, but in 1726 our masterful is interrupted every 1½ seconds by master Bentley had it replaced by the swing of the pendulum—once something grander. The 1610 clock is per “tick”. The signal is recorded by still running in the tower of Orwell’s a PC which also records the one- village church. pulse-per-second output of a GPS As Graham Chinner reported in The receiver—accurate to 10 nanoseconds, Fountain of Autumn 2009, the Bentley so they say. A computer compares the clock mechanism was coincidentally ticks with the GPS to compute the replaced a century ago, in 1910, thanks drift of the clock and the amplitude to a bequest from Lord Grimthorpe. of the pendulum’s swing, producing a His “double three-legged gravity new data point every three seconds. escapement” was made famous in 1859 A weather station also records when installed in the Great Clock in temperature, barometric pressure is 104 milliseconds per day per gram. Westminster (“Big Ben”). In the 50 years and humidity. Data is uploaded to the Our smallest regulation weight, at that followed, the design was adopted website every three hours. 5ms/day, is 50mg —not much heavier and perfected by all the best turret- This system, which has been than a grain of rice. clock makers. running since April 2009, has given Data for the months of April – July J. Smith of Derby built our own invaluable help in regulating the clock 2009 are shown in the graph below. precision timepiece. For its 100th and recording much interesting data. The red diamond indicates that a anniversary we have set up a web page For example, you can watch how regulating mass was added to the at www.trin.cam.ac.uk/clock so that the clock is regulated day by day. pendulum in early September. For anyone can monitor its accuracy. We This is done by placing small brass the next three months the clock kept have also devised an engineering- weights on a platform halfway up the time with remarkable accuracy, to student project to see if we can help pendulum. The regulation constant within one second (most wrist watches will not keep time to better than one Clock Drift second per week). 6 Unfortunately, the regulation 4 didn’t stay perfect for long. Something 2 summer time ends clearly happened late in November and 0 December, causing the clock to gain drift (seconds) several seconds in less than two months. Aug 2009 September October November December Jan 2010 One might suppose that low winter Barometric Pressure 1040 temperatures were the likely cause, but ) our clock is fitted with a temperature 1020 compensated pendulum—as has been common practice for three centuries. 1000 Barometric pressure is a less obvious pressure (mbar 980 influence. Close inspection, however, time (months) shows that the clock slows down during 4 THE FOUNTAIN SPRING 2010 SPIDERS AND PIGEONS: THE PRESSURES ON THE GREAT COURT CLOCK By Hugh Hunt periods of high pressure, and that it was an extended period of low pressure that had caused the acceleration. Three effects are at work here. The biggest is buoyancy, because the pendulum bob displaces air whose density depends on air pressure. The weight of the displaced air reduces the apparent value of the gravity constant g and therefore the period of the pendulum’s swing. For more than a century we have known that pendulum clocks lose time at a rate of around 10ms/day for every millibar rise in barometric pressure. Two other effects to consider are the aerodynamic drag on the occurred only in daylight hours. to see the graphs. I am convinced, for pendulum as it swings, and the mass Is it therefore possible for pigeons instance, that a spider spinning its of the boundary layer of air that the to stop the clock by sitting on the web around the pendulum rod caused pendulum carries along with it. minute hand? A quick calculation of the clock to gain 300 milliseconds The little black dots in the graph power balance can help us decide. overnight on 3–4 May 2009. During indicate other interesting events. For Suppose a pigeon weighing 1 kg this “spider” incident, a scatter plot instance, in October the clock had to be is sitting on the minute hand at a of “going” vs “amplitude” shows a put back an hour at the end of Summer distance of 50 cm from the pivot. beautiful straight line with a gradient Time. The pendulum continued to The hand would be rising at about 1 of –580ms/day for every 1mm change swing during this adjustment but it mm/sec and would therefore require in the amplitude of pendulum swing. was not driven. The change in the about 5mW (milliWatts) of power to This agrees perfectly with small- breadth of the pendulum’s swing lift it. The clock is driven by an 80kg amplitude pendulum theory. caused the recorded drift to shift by weight which falls 9 metres in 7 days, What of the future? The original about half a second. A similar “glitch” equivalent to 12mW of power. It seems motive for setting up all this came in March, as will happen in March that there is only just enough power to instrumentation was to try to detect and October of every year. lift a single pigeon. A couple of pigeons the tidal variations in gravity due to the But what of the flurry of black dots could easily stop the clock. moon and the sun. To do this it looks in late September and early December? Three consecutive pigeon-events as if we need to devise a barometric These glitches seemed inexplicable. It occurred around 10.45am, stopping compensator for the pendulum, and appeared that they always occurred the clock for a total of 4 minutes on this will be part of a new student around quarter-to-the hour. The minute 8 December. This was in the middle project in 2010–11. Look out for our hand is horizontal then and rising. of the Admissions Interview season. results in the Autumn issue of The We also noticed that pigeons gather in Already nervous candidates might have Fountain! large numbers around the clock dial, thought that their punctuality was being But perhaps we should sort out the especially when the sun shines. The tested as a fiendish new admissions test! pigeons and spiders first… black-painted south-facing dial must There are other events that can only Dr Hugh Hunt (e1990) Fellow, Engineering be a warm place to sit. The glitches be understood if you go to the website THE FOUNTAIN SPRING 2010 5.
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