The pendulum clock in Trinity College, Cambridge was installed in 1910. It is remarkably accurate, known to be better than one second per month. In 2009 a system was installed to monitor the "going". The work was done as part of a 4th year MEng project at the Engineering Department. A sensor on the pendulum detects the going of the clock, which is compared to the accurate time signal from a GPS receiver.

Here is a graph for the last 30 days of drift (updated every 3 hours):

For an entirely mechanical clock the accuracy is remarkable.

A barometric compensator was installed at clock change - midnight 28 March 2010.

The clock is regulated as infrequently as possible so as to stay within +/- 5 seconds of the correct time. This enables the steady-state physics of the clock to be examined without unnecessary interference.

The black dots ● indicate that there is a comment relating to the time indicated. Blue arrows pointing down indicate that an adjustment was made to the Going of the clock by removing weights from the pendulum. Red arrows up indicate addition of weights to speed the clock up.

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20 Jul 2014 [12:00]
top-up wind with newly-doctored Stephanie Hill and Simon Jones. Off to Bath.
20 Jul 2014 [11:56]
the wear patterns on the quarters strike plate are looking normal [link] . Perhaps just filing it down and greasing it is all that was needed.
17 Jul 2014 [11:00]
visit to the clock by TC computer office guys Andrew Cullen and George Townsend, with Rick Lupton showing them how the web-based monitoring system works. They also did a top-up wind.
16 Jul 2014 [11:00]
Great to have Graham Newman helping with the clock - fixing the Quarters anomaly: from time to time the quarters will skip and the quarter-past does an hour, the half-past does quarter-past etc. Why? A real mystery. [link]
12 Jul 2014 [11:00]
top-up wind with Malcolm Good. Investigating the Quarters mystery: why does the clock occasionally miss striking quarters?
11 Jul 2014 [00:33]
winding after the Geoengineering debate for Cambridge Summer School: Matt Watson (no winding) @matthew__watson, Malcolm Good, Terje Doeviken, @giovannibisutti, isobel smith, Alanna Punch, Katharina Tomaschek who used the last key!
06 Jul 2014 [12:06]
winding after a boozy Annual Gathering (well done!) by Miss Li Yan, Dr Richard Manns
29 Jun 2014 [00:05]
winding: Beatrice Hunt, Matthew Dougherty, Harriet Kilduff @littlehduff, Hannah Kilduff @hannahkilduff, Louise Dorignon @therightscale
27 Jun 2014 [10:23]
winding with Dan Safka @Rubicycling