Geoff,

You have stated the problem quite clearly.

A specially designed spring has been the only thing found to work 
sufficiently well in practice.  What you need is a spring arrangement 
strong enough to support the mass, but designed so that when the mass 
moves up and down, the spring changes its lifting force only *very* 
slightly.  One way of looking at its effect is that such a spring 
setup makes the mass think that gravity has become very small, 
behaving something like a rig they used to train the astronauts 
headed for the moon.

An early design geometry, the LaCoste configuration, involving a 
"zero-length" coil spring did exactly that.  But now almost all 
instruments use a thin "U" shaped leaf spring, like in Sean-Thomas' 
STM-8.  Magnets have the problem that their force varies quite a lot 
with temperature and if you make your instrument very sensitive, it 
will drift out of range with even a small temperature change.  That's 
a big problem with springs, too, but the effect is maybe 5x 
smaller.  With the leaf spring, the geometry of its design is 
critical, with only a few hundredths of an inch change in its length 
or attachment point making a significant difference in how it behaves.

The only effective way I know of to deal with the temperature drift 
is with electronics.  You sense the position of the mass and make a 
circuit which will apply a gentle force using a coil and magnet to 
slowly nudge the mass back toward where it should be.  Way more 
complicated than that, but most modern seismometers use something of 
the sort.  That approach is usually extended to provide other 
benefits, too, making the signal distortion smaller and broadening 
the instrument frequency response while making it more predictable and stable.

Regards,
Brett

At 12:44 AM 2/29/2012, you wrote:
>Hey Guys,
>
>Ive been looking at the pull of gravity
>and what you need to do to lengthen period.
>
>
>Like here's what seems to be:
>
>
>Given any proper spring extended by whatever mass 10 inches
>the free period on earth is like 1 second.
>
>
>if the gravity is made less and you use the same spring
>here is what happens.
>
>to extend the spring to 10 inches on the moon
>you need to increase the mass until the wight
>will deflect the spring 10 inches. then
>you get like 1 * sqr( gEarth/gmoon ) or 2.4 seconds.
>
>This means to me you need to reduce gravity considerably
>like 1/25 to 1/100 to get the free periods you are looking for.
>
>
>Does anyone know how to build a circuit based upon this idea.
>
>I mean,
>
>Like,
>
>Magnetic levitation to effectively reduce
>gravity by 1/25 or 32.17/25 = 1.28 f/(sec^2)
>
>I mean in layman terms,
>
>How can one reduce the pull of gravity
>so that gravity effect is like 1.28
>instead of 32.17 using any practical means
>known today ?
>
>You are playing with forces here
>which means you will need to
>take most of the wight off the spring
>and use a smaller spring coefficient
>with the very same mass.
>
>I have been thinking like
>buoyancy in whatever OF magnetic field
>or electrostatic or mechanical
>advantages ???
>
>Any ideas ?
>
>I am relatively sure everything has
>already been tried even tho no one
>talks about it.
>
>Any relevant answer is appreciated.
>
>Regards,
>geoff


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