On 8/16/2011 11:40 PM, Bob Smither wrote:
> I tried some experiments along the lines you describe.  The 
> experimental pendulum weight was a strong permanent magnet oriented up 
> - down.  By placing the magnet in an (approximate) Helmholtz coil the 
> effective pull of gravity can be reduced.  I managed to increase the 
> period of a small (86") pendulum from 2.96 seconds to 5.02 seconds.  
> Reversing the current in the coil,as expected, gives shorter periods.  
> While encouraging, the result was unsatisfactory as the pendulum bob 
> (the magnet) had a tendency to be drawn off center at higher magnet 
> currents.  I have not had the opportunity to improve on my first crude 
> experiment.

I've of course followed this thread while thinking of magnetic 
levitation, which has been discussed before.

This "extraterrestrial gravitometer" article from January 2000 
Scientific American has been a topic of this list, apparently several 
times. It is effectively a vertical seismometer:

http://www.scientificamerican.com/article.cfm?id=detecting-extraterrestria

The designer of the device has his own page here, longer than the SA 
article:

http://njsas.org/projects/tidal_forces/01/baker/index.html

As he says, one of the biggest problems is temperature variation. His 
device has temperature regulation, but still, my research shows that of 
all magnet types, ceramic has the highest temperature coefficient 
(variation of magnetic force with change in temperature). It appears 
that samarium-cobalt  magnets can be made with near-zero temperature 
coefficient:

http://en.wikipedia.org/wiki/Samarium%E2%80%93cobalt_magnet#Reversible_temperature_coefficient

A while back I tossed together something vaguely similar to this 
gravimeter as a proof-of-concept device, using ceramic ring magnets from 
microwave oven magnetrons. I only have the fixed magnets and the arm, 
which is much longer than the gravitometer design - my arm is a glass 
plate about six inches long with the the razor blade edge at one end, 
and a large ceramic ring magnet glued to the free end. The blade is held 
against a brass plate by small magnets on the other side of the plate, 
and thus has relatively little force on it. Above the ring magnet are 
two more large magnets glued together whose height can be adjusted, and 
likewise two more below. I can adjust the fixed magnets so it has a 
period of about one to two seconds, and it takes a minute or so for the 
oscillation to noticeably decrease. Actually I'm not sure if it ever 
stops oscillating, as it appears to be very sensitive to air currents.

I probably need to make a different case/support structure if I'm to 
take this any further. It's about a 7x9x10" box made of 3/4" plywood and 
thus is very sturdy, but it surely responds to changes in humidity.

I'm thinking this could be made into a magnetically-supported horizontal 
instrument just by making the razor blade hinge vertical, so that the 
arm magnet would be constrained to move side-to-side rather than up and 
down. This may be unstable with only one magnet above and below, but 
this may not be a problem if it's held in place by a force-feedback system.
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