Tom Van Baak wrote:
Depends on what you mean by "real clocks". The best pendulum clocks, made in the early 1900's, solved almost all the normal sources of error and instability. That left gravitational tides as the one of the few remaining sources of "error", down well below the 1 ppm level. I say error in quotes because if you call them gravimeters instead of clocks, they we're just doing their job and did it well. These were real clocks; most of the pendulum clocks, vintage or modern, that you see are toys.Specifically, these used aged invar rods, at very low amplitude, running in partial vacuum inside brass chambers. That helped reduce buoyancy, humidity, and barometric pressure issues. They were also either well temperature compensated and were operated deep in constant-temperature basements. Some modern attempts at world-class pendulum clocks have tried fused quartz instead of invar to avoid the reputation that fresh invar has for long-term instability. A great example is: http://www.precisionclocks.com/ You should know Bill had hp cesium clocks in his home clock collection long before I did. Those of you really interested in the history, art, and science of pendulum clocks should see the following six books: Accurate Clock Pendulums by Robert J. Matthys Precision Pendulum Clocks, A Trilogy of Books by Derek Roberts My Own Right Time, by Philip Woodward The Science of Clocks & Watches, by Arthur L. Rawlings /tvb http://www.LeapSecond.com
Tom Even fused silica is unstable (see attachment). Single crystal materials should be significantly better. Ageing Invar doesn't do much for its dimensional instability. Bruce
<<inline: DimensionalInstability2.gif>>
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