Trinity’s Clock is cool with the heat

In Great Court the grass is brown, the air is still and visitors walk slowly. But Trinity’s historic clock is speeding up in the heat – currently it’s 2.7 seconds ahead of the correct time.

So the Keeper of the Clock and Trinity Fellow, Dr Hugh Hunt, is experimenting with a hood over the upper part of the pendulum to keep the temperature down.

Of course, makers of mechanical clocks such as Trinity’s, knew about thermal expansion. In 1726, the famous clock maker, John Harrison, invented the grid-iron pendulum, made of zinc and steel. This perfect combination of materials does not expand or contract with temperature changes unlike steel. Dr Hunt, who is Reader in Engineering Dynamics and Vibration at Cambridge, said:

Without compensation the effect is huge. A regular steel pendulum would run 12 seconds per day slower in the summer than in the winter, assuming a 20C difference in temperature just because it gets longer by 4mm.

Dr Hunt beside the Clock and its new hood

The first  tower clock was installed in 1610. Dr Hunt says we’re lucky to have a newer one – dating from 1910 – because its pendulum uses Harrison’s steel and zinc combination in the form of concentric tubes.

So why is it running fast? Dr Hunt explains:

The pendulum is about 2.2 metres long and when the weather gets really hot we’re finding that the top end of the pendulum is warmer than the bottom end. This is because hot air rises and recently it’s been 30oC up in the Clock Tower. Harrison’s compensation works well if the pendulum warms up uniformly so I’ve tried to keep the pendulum at a uniform temperature by enclosing it in a hood.

I hope I’m on the right track with this and that a proper wooden case will make a difference, not only in the hot summers but in the cold winters. Harrison would be very pleased with all of this!

Dr Hunt checks the latest data

Dr Hunt is tracking the effects of this experiment as part of the Trinity Clock Monitoring Project. A team of students and alumni led by Dr Hunt have created bespoke instrumentation and computer programmes that monitor data…round the clock!

The programme also monitors the Clock Tower itself, one of the oldest buildings at the College. In Medieval times it was the entrance to King’s Hall (one of the two institutions preceding Trinity) and was taken down and rebuilt 30 metres north, in the wake of Henry VIII’s founding of the College of the Holy and Undivided Trinity.

The heat of the sun is beating down on the south-facing wall of the Clock Tower and thermal expansion is causing it to tilt, ‘not by much,’ says Dr Hunt, ‘but a tilt of 0.005o is enough to notice.’

Let’s not forget that this is one of the most accurate tower clocks in the world, made by John Smith of Derby. We’re so lucky to have such a gem of mechanical engineering and genius to work with.

It’s hard to know if what we’re doing will make much difference – it already keeps time to better than a second a month which is quite remarkable.

Photographs: Graham CopeKoga

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