A second in one-hundred days – the results
The last time I wrote about Clock B, we had just commenced an official 100-day trial to see if it would keep up with John Harrison’s expectations. In the last blog post the conditions and aims of the trial were outlined.
In this post I will add a little detail as to how the clock was monitored and chart the clock’s behaviour throughout the trial. Firstly, I ought to reiterate that throughout the trial, the clock was kept within a Perspex case and that the purpose of the case was only to protect the clock from dust, spiders and humans; it did not provide a hermetic seal, so the pendulum was swinging in free air and therefore affected by any change in temperature, barometric pressure or humidity. The clock case was wired shut in April, 2014, and rendered tamper-proof by wax impressions that were kindly applied by the National Physical Laboratory (NPL) and the Worshipful Company of Clockmakers.
To ensure that there can never be any doubting that Clock B was fully exposed to atmospheric conditions throughout the trial, an environmental sensor was sealed inside the case. This provided a continuous log of air pressure, temperature and relative humidity alongside the clock’s rate and amplitude (arc of pendulum swing).
The electronic data gives a very good overview of the clock’s characteristics and will serve as a useful reference to run through the environmental effects on the clock’s running.
Barometric pressure fluctuated with some particularly large swings during the middle of the trial period, the largest of which was from 977 to 1038 millibars. From the graph above, there is no discernible effect on the clock’s rate, but when Tom van Baak put the raw data under the microscope, he was able to demonstrate that there was a small effect and that around 96% of barometric influence was compensated for.
From the graph it is evident that the amplitude of the pendulum is mainly affected by barometric pressure, which is to be expected as the barometric compensation relies on changes in amplitude to regulate the speed of the pendulum using the suspension spring and cheeks. If one looks at the coldest spell during the trial, it is clear to see that the cooler conditions also affected the pendulum’s amplitude, further reducing the arc of swing because cold air is denser than warm air.
The most pronounced effect on the clock’s rate was caused by temperature change. Looking at the above graph, one can see a sharp dip in temperature between days 18 and 28. During this part of the trial the clock reacted positively to drop of around 4 degrees Celsius and the pendulum sped up by almost 0.000001 seconds per swing. As there are 86,400 seconds in a day, this change in rate equates to a gain of 0.086 seconds per day. This small temperature effect only became apparent after the barometric compensation had been adjusted as can be better seen in Tom’s Tom’s scatter plots. The 2012/13 plot shows that the clock was slightly over-compensated for temperature change (the clock clock ran faster in heat) before the adjustment for barometric compensation. Afterwards, without any change to the pendulum’s temperature compensation, the clock ran slower in heat.
Here, it is reasonable to infer that around 94% of the physical changes in the components of the clock were compensated for and that this new characteristic was caused by changing density of air. As cold air is denser, it adds more drag to the pendulum and the pendulum uses up more energy, reducing the arc of swing and so causes the clock to run slightly faster.
The effect of relative humidity on the clock’s rate is minimal, if not zero. Any quantification of this effect is beyond the scope of the current data set.
Because the electronic data capture is not infallible, it was decided that a manual record should be taken of the clock’s going throughout the trial and should serve as the primary data. As agreed with the NPL, who acted as peer reviewer of the trial, an MSF radio-controlled clock was our main time standard, which was regularly checked against an NTP clock. At the beginning of the trial an eye and ear method was used to rate the clock. The telephone time signal was checked against the MSF clock and then used to gauge Clock B‘s rate. As the trial progressed, the observations were improved by recording a slow motion movie of the pendulum and the MSF clock (see below) to better gauge the fractional part of the second.
The graph below shows the both the electronic and manual plots. The two lines are separated by around one quarter of a second because electronic data assumes a zero starting point and the manual record takes into account the fact that the Clock B was showing UTC -1/4 second at the beginning of the trial.
As can be seen, the electronic equipment disconnected for a few days towards the end of the trial and we lost a little of the electronic data, but the manual record continued and was verified on day 100 that the clock was well within the one-second margin. The final figure after 100 days was an error of -5/8 of a second, gaining Clock B, John Harrison, Martin Burgess, Don Saff, the many others involved in the research and finishing a Guinness World Record for the ‘most accurate mechanical clock with a pendulum swinging in free air.’
Burgess Clock B has shown us that the principles of John Harrison’s pendulum clock system were indeed correct and hopefully, this result will inform further study and encourage other clock-building projects to further test out the theory.
What next for Clock B?
The clock is still running in the horological workshop at the Royal Observatory, Greenwich and plans are afoot to improve the clock by fitting an adjustable temperature compensation. Once this is completed the clock is expected to outperform Harrison’s proposed “…nicety of 2 or 3 seconds in a year.”
Shortly after the latest conference, Tom and I spent a day studying the clock with an advanced monitoring set-up that enabled precise examination of the clock’s behaviour for each beat of the pendulum. To give a flavour of the type of high-resolution data that we hope to gather and study in future, the above graph shows the cyclical changes that are inherent with the remontoire mechanism.
Furthermore, we are working to publish the proceedings of the two conferences. More information on this will be released as the project begins to crystallize, but in the meantime, Clock B can be viewed at the Royal Observatory in the horological workshop from our Time for the Navy gallery.