Ideas for magnetometer torsion thread

The Cavendish balance used a quartz thread, according to my physics book. Where you could obtain a quartz thread is way beyond me.

John S

BTW, you do realize you need your setup to be where air currents cannot affect it or, better, in a vacuum. Yes?

For any instrument like this, you will have to let the suspension reach equilibrium. This could take weeks, but once it is there, it should be stable. I think the nylon fishing line has real promise, although it could possibly have some stresses built into the filament when it was extruded that will respond to temperature changes.

To get rid of the atmospheric effects, you may have to suspend it in an evacuated vessel.

Jon

Oh, one other thing, you can get extremely fine steel "music" wire, way finer than guitar strings, and these might also work well.

Jon

You have to learn to milk a spider!

You could take a quartz rod (maybe ~1mm diameter), and warm it ;-) in the middle, then draw it out to a thread. Might take a few tries to get it just right, but if it should be possible. MP of quartz is not that high.

I would avoid nylon- there are lots of stresses in plastics, those long molecules squirm around, and it's horribly hygroscopic. Makes a good hygrometer though.

Or, how about cannibalizing a taut band meter movement? They use a thin metal ribbon. You could remove the hairsprings.

P.S.

Here's one way to do it described in detail ca. 1917!

(also how to make your own laughing gas)

Copper magnet wire. I doubt that it's ideal, but you can get it ready- made in a bunch of different sizes.

Like Spehro said -- nylon absorbs (and reacts to) water. It'll react to vacuum, too -- probably by drying out, getting brittle, and breaking.

If your goal is a home-built, really accurate magnetometer, you may want to investigate NMR. You can, apparently, make inductors by wrapping wire around bottles of water, and measure magnetic field strengths by measuring the inductance of the resulting mess. There's some magic with the protons interacting with the magnetic field that I don't remember. In fact, there are any number of details that I don't recall about this, other than I read it in a magazine -- probably Circuit Cellar, and probably over four years ago.

It's something to Google for, if you get tired of looking for limp strings.

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If my memory is correct? there was a magnetometer article in Scientific American that used inductors over bottles filled with water. Mikek

He says it is so simple, too, and then talks about a fair number of difficulties.

I think most folks have done the soft glass drawn out to a thin fiber in chem labs. But I've never considered using a catapult!

...

Then look at page 107 of the same book:

Look to the lower right corner where there is a section titled "Formation of the Earth's Surface" and a little blurb about Dr. Wegener! 1912 was the year he first published on this, I think. Might be wrong, though.

Jon

?

Jon

You will also have to shield the device from seismic disturbances, once you get over all the other problems.

Personally I would be going for the NMR method, or maybe start looking at SQUIDs. At least then you are only seeing magnetics, and not all the other possible effects.

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Adrian Jansen           adrianjansen at internode dot on dot net
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cut

I would try thin copper wire, if you stretch it with a bit of force, you get a very straight wire, and if thin enough it should act as a suitable spring wire.

Optical fiber! And, that might be excellent for this purpose. Just strip off the jacket, and maybe dissolve the plastic cladding in solvent.

Jon

Thanks for the suggestions. To summarize, the ideas so far are:

1) Wait for the nylon thread to achieve equilibrium. I might have to do that if other things fail, although with thermal changes it might keep oscillating and never reach complete equilibrium. Still, if the drift gets down to some acceptable level, why not...

2) Use quartz thread. This would be an interesting project by itself. I'm afraid it might turn out to be way too fragile - the device is currently a tabletop experiment with no permanent housing (yet), but drawing a quartz wire is an interesting thing to try by itself.

3) Music wire, or even using very thin magnet wire as an easier alternative. I think I'll try this first. I found some extremely thin "straight" (as opposed to rolled up) music wires. One thing though - any suggestions about the right diameter? I found a 0.008" diameter steel music wire, but according to my guesstimates (which might be wildly off the mark) this will snap if I hang about 20 gramms off it. Any diameter recommendations/considerations?

4) Milking a spider - good idea, considering that I own a spider farm already (my patio), :-)

5) Cannibalize a taut band meter movement - very good idea, thanks. If I give up on the current design (which is already built and working, with the exception of the wire) I might do just so.

6) Water bottle NMR - I'll have to look into that. Sounds promising, although it will mean rebuilding almost everything from scratch.

7) Building a SQUID magnetometer, having the advantage to exclude other influences (such as seismic activity which, considering I live in California, is quite frequent) - good idea too, just maybe a bit too much for my limited technical abilities (I have to think about liquid nitrogen cooling, etc.); also, I kind of like it when my magnetometer registers a quake, :-)

Thanks for the ideas. If you have more, please share them - I'll keep monitoring this thread.

Regards,

- Alex

Spehro beat me to it. Quartz fiber is the gold standard, at least as of the turn of the last century :-). Procedure (same technique as what Sphero linked) described in this book:

4562

Heat a the center of a quartz rod red-hot in a bunsen burner flame, pull.

Other materials like copper and plastic have much higher internal friction, and, worse, hysteresis. Quartz is simply superb.

A glass fiber might be almost as good. Jon Elson mentioned optical fibers--that's a good idea. Or, possibly, a strand from fiberglass cloth, or an insulation fiber?

-- Cheers, James Arthur

Messy mess of solvent, and perhaps some terminology problems, too.

The cladding and the core are both glass (with the rare exception of actual quartz, which is not a standard item in the typical uses, but easy enough to find for laboratory UV uses.) Anything that will dissolve or strip off glass fiber is more jacket - the cladding and core do not separate. ie, 250um "bare" fiber is not (or hardly ever, anyway) bare fiber, it's 125um fiber in a 250um OD jacket, which is easily removed with a fiber stripper rather than mucking about with a lot of needless VOCs and nasty remaining waste. Rub the stripped fiber with an alcohol pad 'til it squeaks and Bob's your uncle. Also note that 125um bare fiber breaks a LOT easier than 250 um coated fiber, so handle with care. Chunks long enough for any sort of balance you might care to make are easy to come by as installation waste if you find an installer (who would also have the right strippers...) but if you want actual quartz you'll either have to order it special or know someone that works with it in a lab environment and generates scraps.

In less esoteric terms, those are 1/8 and 1/4 mm.

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Do your calculations again. ASTM A228 lists the yield strength for music wire to be 230000 to 399000 psi. 0.008" diameter works out to 50*10^-6 square inches, for a minimum strength of over 11 pounds.

If anything, you're going to be disappointed by how stiff it is.

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magnetogram_2011_09_...

dp/0917914562

Optical fiber comes coated with plastic these days, so you'd have to strip it somehow (heat?)

Go to a hobby shop that caters to model airplane builders and get some fiberglass cloth; you should be able to tease out single strands from that.

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