World's Largest Wheatstone Bridge



Use a laser and a mirror.
They already measure the distance from earth to the moon that way to a few cm.
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What you are trying to make is a strain gauge. This scheme wont work because the gauge has to be attached to the substrate along it's whole length not just strung up like a power line. But attaching it to the earth in any meaningful way over distance would be next to impossible. Yes, you can compensate for the temperature coefficient of resistance but how do you compensate for the change in length due to temperature, coefficient of expansion, a very different animal. I can't see a strain gauge being a solution for seismic motions.
The best way is a laser interferometer for small displacements and GPS or gross measurements. Only two points need to be attached to the earth with these schemes.
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Tie it down every few feet.

If you know the coefficient of expansion then you can correct for it, either mechanically or later in the calculations.

Is there any other way to measure very low frequency / very small displacements?

What's the accuracy?
Bret Cahill
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Tie it down every few feet. ==========================================Do manual work in a dust storm or rainstorm? That was one of your objections to laser ranging, right? 'Faced with changing one's mind, or proving that there is no need to do so, most people get busy on the proof.'- John Kenneth Galbraith 'There is nothing so easy but that it becomes difficult when you do it with reluctance.'- Marcus Tullius Cicero
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because
Tie it down every few feet.

If you know the coefficient of expansion then you can correct for it, either mechanically or later in the calculations.

Is there any other way to measure very low frequency / very small displacements?

What is the accuracy of a laser interferometer? Fractions of a wave length of light: nanometers. How accurate do you want it?
If you think you can make a strain gauge by hanging a some wire on poles space a few feet apart, then test your theory and set up such a scheme in your back yard. See if you can detect the vibration of nearby traffic or the earth tides every time the sun goes across the sky. I think you'll find it WONT WORK! But, prove me wrong. If you can show it works then great, more power to ya.
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Mounting a conventional strain gage just at the ends would give you the same output assuming you had the dexterity to glue just the ends. dl/l should be const. over the entire area of the strain gage site.

Correcting for thermal expansion would require temperature data over the entire length.

Adding a +/- nanometer error to a +/- meter error just gives you a +/- meter error.
GPS wouldn't be useful for the actual data.

They claim laser interferometry won't work over long distances, i.e., 50 km, but they are probably thinking of measuring an entire unknown distance when all that is really necessary is a _change_ in distance.
Maybe use several different meters at different wavelengths so that you don't skip a wave.
Bret Cahill
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Only in your (extremely small) mind.
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On Mon, 3 May 2010 09:46:39 -0700 (PDT), Bret Cahill

Differential GPS and laser rangefinders make a lot more sense.
John
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On Mon, 03 May 2010 17:29:32 -0700, John Larkin

Sense? You're talking to Cahill, here.
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On Mon, 03 May 2010 19:37:55 -0500, " snipped-for-privacy@att.bizzzzzzzzzzzz"

oh. sorrreee.
John
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On May 3, 8:44pm, John Larkin

And still, sensing the earth shake, doesn't tell you ... . .??? there is all sorts of low level shaking going on. How many 5.0 earth quakes per year? And how big is the shake from a 18 wheel semi going by your site.. or logging going on near by....(tale from Ligo)
George H.
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" snipped-for-privacy@att.bizzzzzzzzzzzz" wrote:

AKA: The world's biggest loaded dummy.
--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.
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Laser rangefinders can work over dozens to hundreds of miles in a dust storm or rainstorm?
To the nearest few microns?
Bret Cahill
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Hundreds of miles, nope, but when combined with GPS you don't need lasers to go hundreds of miles.

Over hundreds of miles microns are meaningless and in the noise of numerous factors.
--
Jim Pennino

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How does string work over hundreds of miles ins a dust storm?
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Apparently one nails it down every yard to get dust up one's nose.
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On Mon, 3 May 2010 09:46:39 -0700 (PDT), Bret Cahill

--
Oh, My...

To call you an idiot would be kind, since there's nothing in any of your
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The problem is just to the East of Laguna Salada:
http://maps.google.com/maps?f=q&hl=en&q=32.2006,-115.4625 (M3.0+-+Baja+California,+Mexico+-+2010+May+04+02:35:27+UTC)&t=h&z=9&iwloc=A
String a thin steel wire from Aqua Matias to Cucapa returning with a larger diameter copper wire with a resistance << than the steel.
All those << M 1.0 movements can now be documented and studied.
Bret Cahill
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http://maps.google.com/maps?f=q&hl=en&q2.2006,-115.4625 (M3.0+-+Baja+California,+Mexico+-+2010+May+04+02:35:27+UTC)&t=h&z=9&iwloc=A
Obviously you have no clue what "Wheatstone Bridge" means and are just, once again, throwing out technical terms you think will impress people with your knowledge.
--
Jim Pennino

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On Mon, 3 May 2010 21:16:15 -0700 (PDT), Bret

Not clear what's the point of the different metals.
Do you mean "string" as in suspended in space? Exercise for the student: Compute the stress on the wire just due to gravity.
Now imagine you actually got this to (nominally) work. How are you going to allow for the change in load due to wind, rain, and dust? What happens when birds perch on it?
As it turns out, they've been using laser interferometry for just this purpose for decades.
Best regards,
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