[time-nuts] A different timenuts interest

Don Latham djl at montana.com
Thu Jul 22 02:10:34 UTC 2010


Griffith Park in LA operates a Foucault pendulum that's been going for at
least 70 years (don't ask how I know). They might have a writeup
somewhere. I think te pivot was a simple clamp holding the piano wire.
You'd think it would fail from stress, but the pendulum is very long, so
the angle of the swing is very small. I think the drive actually was done
close to the pivot via a magnet on the wire rather than at the bottom. To
start the pendulum off, a string was tied to hold the pendulum "cocked"
and the string simply burned with a match. A perfect no-torque start...
A simple optical interruptor driving something like a basic stamp and a
ring electromagnet with a PM on the support wire will allow proper timing.
You will be surprised at how little energy is required to keep it going;
it can be roughly calculated from the ball diameter. The beauty of this
system is that the exact frequency of the pendulum is not important. First
order temperature correction can be done in the microprocessor.
Send some pix when you get it going...
Don Latham.

Hal Murray
>
>> ... Foucault pendulum ...
>
>> Has anyone here had any experience with such a system of have any
>> suggestions regarding the sustaining system? This is an interesting and
>> challenging project.
>
> Several years ago, I found a web site for a commercial place that made
> them
> for museums.  (I forget why I was looking for that sort of stuff.)  You
> might
> find interesting stuff/ideas via google but I didn't find a similar site
> with
> a bit of searching.
>
> ----------
>
> Here is what I would try:
>     Put a magnet on the bottom of the pendulum.
>     Put a coil below it.  (obviously centered)
>
>     Use the coil as a sensor to measure the timing.
>     Use the coil as a motor to pull the pendulum every N-th swing.
>
> The question is how accurately centered do the magnet and coil have to be?
>  I
> don't know.  It sounds like a fun mixture of theory and engineering.
>
> One of the variables is how far away is the pendulum when you are pulling.
> The farther away it is, the smaller angle you have from the ideal.  You
> can
> change that by varying the start/stop times on the pull pulse.
>
> I'd probably put the coil on a crude X-Y table, set it up as good as I
> could,
> then see if it worked.  Then I would deliberately move it off a bit and
> see
> what happened.  Or try to servo it to the best place, probably by manual
> changes every day or week or ???
>
> I'm assuming this is for a school or museum.  The required positional
> accuracy is actually a real science experiment.  The idea of "experiment"
> to
> test an idea is more important than the basic Foucault pendulum itself so
> you
> get two exhibits in one.
>
> Of course, another question is how fast does it decay?  Or rather, how
> long
> will it run with no energy input?
>
> This says 2 hours:
>   http://www.cmnh.org/site/AtTheMuseum/OnExhibit/PermanentExhibits/Foucault.as
> px
> for a 270 lb bob, but I don't know how tall that is.  (But it says 6.2
> seconds, so I should be able to calculate it.)
>
>
>
>
>
> --
> These are my opinions, not necessarily my employer's.  I hate spam.
>
>
>
>
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-- 
"Neither the voice of authority nor the weight of reason and argument are
as significant as experiment, for thence comes quiet to the mind."
R. Bacon


Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com





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