[time-nuts] Non electrical time-nuttery

[Don Latham]

Ah, but a torsion pendulum is inescapably a kind of vertical pendulum as 
well?
Don
----- Original Message ----- 
From: "J. Forster" <jfor at quik.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Monday, January 11, 2010 9:22 PM
Subject: Re: [time-nuts] Non electrical time-nuttery


>
>> namichie at gmail.com 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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