[time-nuts] Pendulums & Atomic Clocks & Gravity

Tom Van Baak tvb at LeapSecond.com
Sat May 26 21:43:49 EDT 2007

> At 01:17 PM 5/26/2007, Brooke Clarke wrote...
>>Just starting to adjust the bob on a pendulum clock... Tom has pointed 
>>out that the stability limit on pendulum clocks is in the area of 1E-7 
>>because of the complex effect of the Sun and Moon on the value of "g".
> I'd think it would be significantly less than that for real clocks due 
> to fluctuations in air density caused by barometric pressure (affects 
> friction unless in a vacuum) and in pendulum length due to temperature 
> variation (the linear coefficient of thermal expansion is in the 
> 1.5e-5/degree C range for most metals, and 5e-6 for wood).

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:
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


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