[time-nuts] Pendulums & Atomic Clocks & Gravity

Brooke Clarke brooke at pacific.net
Sat May 26 19:49:53 EDT 2007


Hi Mike:

Back in the 1800s clock makers found ways to temperature compensate the 
pendulum such as putting a Mercury thermometer at the bottom, using metals with 
dissimilar expansion coefficients (Harrison used steel and bronze (no zinc 
then)) or materials with almost zero COE like Invar.

The Dent clock at Greenwich in 1885 had an arenoid type compensator to remove 
barometric pressure effects, later clocks were run in vacuum.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com



Mike S wrote:
> 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).
> 
> 
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