The ticking of the Trinity Clock

How does the old clock work?
25 February 2020

Interview with 

Hugh Hunt, University of Cambridge


A close up of an open pocket watch


How do we measure time? Adam Murphy went to see engineer Hugh Hunt, and a very large clock in Trinity College Cambridge, the setting for a scene in Chariots of Fire, to find out...

Adam - What you're hearing right now is the clock at Trinity College at the University of Cambridge heard from the inside, from within the clock tower. It's not just any clock, it's also the clock from Chariots of Fire. We're in the film. Students have to run around the whole of the Great Court of Trinity before the 12th strike of the bells. I was lucky enough to get up close to the mechanism of the Trinity College clock with Hugh Hunt who looks after it as we stopped to chat on the roof overlooking the Great Court, so I could learn some history.

Hugh - Well, this clock was installed in 1910. It's the third clock that's been in this tower. The first clock, from 1610, was actually not a bad clock for its time, but in 1726 the new master of the college wanted his own fancy clock and that turned out not to be a great clock at all. Fortunately, we got a new clock at a time when clocks were at their zenith, that mechanical clocks were at their very best. There's a pendulum, Isaac Newton would have told you that the period of a pendulum is relative to its length, and you can get very accurate time keeping with a pendulum. Then there are some weights that go down the height of the tower. They go down about seven metres. It takes about a week for them to fall the distance, so the clock needs winding once a week. Those weights keep the pendulum going. They give the pendulum a little nudge through a thing called the escapement. Now the clever part is the mechanism that links the pendulum to the weights to the escapement, to make it just go steadily, reliably, day after day, week after week, month after month. Then there's an extra part which rings the bells and that's kind of an added extra.

Adam - And having stood right beside those bells I can say that it's really something. But how good is the rest of the clock?

Hugh - This clock is accurate, at its best, to maybe a second a month. And I find that absolutely astonishing because most people's wristwatch, if you've got a quartz crystal wristwatch, that's accurate to a second a day if you're lucky. The more accurately you measure things, the more funny things you find. That's what science is all about, you know, you answer one question and you find five more riddles. We discovered that when the sun is shining, we find that the clock tends to slow down. We're trying to figure out, well why is that? Turns out that the sun shining on the south-facing wall of the tower, by thermal expansion, causes the tower to tilt over. It's a couple of millimetres of movement at the top of the tower, but that then means the pendulum is no longer swinging in a vertical plane anymore. It means that the pendulum is slightly leaning over and that means that the pendulum starts to twist because it's moving in a bit of a curve rather than a straight line.

Adam - So it's a well-maintained, really incredible piece of history that still might have some secrets to offer up. But it's amazing just how simply things can get in the way.

Hugh - You might hear in the background up here that there are some birds flying around. Some of those birds are pigeons. They are a bit of a nuisance because they love to sit on the minute hand when the sun is shining on a lovely warm morning, just to bask in the sun. Now at about a quarter to the hour when the minute hand is horizontal, a couple of pigeons standing on the hand is enough to stop the clock. The weight of the pigeons. I was looking at some documents in the library from about sixty, seventy years ago where the people, who were looking after the clock then, were complaining that the clock kept on stopping and they had to get the clockmakers to come and fix the clock. And they said the clock stopped at 7:43 again today. Aaah, it always stopped at about a quarter to! So sixty years ago, I'm certain they were having problems with the pigeons but they didn't figure it out. So now what we've got is a wire on the minute hand, which is stretched across the minute hand, so that the pigeons can't stand on the minute hand anymore. Problem solved. So this kind of sixty, eighty year old problem now, in fact, it's probably been like that ever since the clock was put in! It's just great. Love it.


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