J. Forster wrote:
[email protected] said:
The reason that I remember a hard vacuum is not used is because the
low pressure is used to rate the pendulum (fine tune) by slightly
increasing or decreasing, and in a hard vacuum metals tend to weld
together and oils evaporate so the mechanical bits seize up.
How does the pressure change the frequency?
I don't think it does.
Why are oils a problem? I thought typical pendulums used a spring rather
than a bearing.
I think a torsion pendulum suspended by a fiber woul have no oil to be an
issue.
That does raise an interesting issue. How would you fine tune a pendulum?
A back-and-forth pendulum is tuned by adjusting it's active length. If
it's suspended by a flat spring, you can adjust the spring length,
adjusting the period.
With a torsional pendulum you could also adjust the spring rate or the
wheel moment of inertia with symmetric radial screws.
If you can get close enough, then you can tweak things by varying the
amplitude, or temperature.
I don't think that's a good idea. With a back-and-forth pendulum amplitude
adjusts non-linearity. A good pendulum should be temp independent.
Prior to the advent of alloys like Glucydur used in balance wheels, and
Elinvar, used in balance springs, serious efforts were made to attempt to
automatically correct for temperature dependent shifts in spring constant,
and balance wheel size. These worked pretty well, but it was a very twidgety
job to get them just right. The first watchmaker to clean and lubricate the
watch/clock usually forever ruined that calibration. In the end, it was
found to be better to use springs and balance wheels that didn't change with
temperature.
We would laugh at the watches and clocks that were considered state of the
art from those days.
-Chuck Harris
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