Ah, but a torsion pendulum is inescapably a kind of vertical pendulum as
well?
Don
----- Original Message -----
From: "J. Forster" <[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
Sent: Monday, January 11, 2010 9:22 PM
Subject: Re: [time-nuts] Non electrical time-nuttery
[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.
Big Ben is tuned by adding/removing a penny from the pendulum.
http://en.wikipedia.org/wiki/Big_Ben (search for penny)
That technique gets more interesting in a vacuum, but you might be able
to rig up something equivalent.
Effectively changing the pendulum length by moving the CG relative to the
pivot point.
-John
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