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Posted 07/19/2022 7:37 pm
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Can you describe in more detail your moving magnet test?
I have a rare earth magnet extracted from a computer hard disk but see no response in, to be fair, smaller silver coinage. It has to be my poor technique.
Can you describe in more detail your moving magnet test?
I have a rare earth magnet extracted from a computer hard disk but see no response in, to be fair, smaller silver coinage. It has to be my poor technique.
Dnas is describing the "eddy current effect". The effect isn't strictly diamagnetic - anything that conducts electricity will produce eddy currents. Some high speed trains and roller coasters use eddy current braking, and the tracks these train brakes use are normal railway tracks, made of paramagnetic steel. It's just that for testing coins with magnets, you'll only notice the eddy current effect in diamagnetic materials, because a paramagnetic object will simply grab onto the magnet and stick there permanently, rather than let go once the motion stops.
The eddy current effect becomes most noticeable when:
- the strength of the magnetic field is high;
- the purity of the silver is high;
- the coin is physically large;
- the distance between coin and magnet is small;
- the magnet and coin are moving at high speed relative to each other. Once the motion stops, the magnetic field stops.
If you just take a powerful magnet and sweep it quickly across a silver coin sitting on a benchtop, you should notice the effect - the coin will seem to "stick to" and travel with the magnet, but only while it is moving quickly. Do it slowly, and you won't see anything move.
Albert's four techniques are:
- Swinging magnet: a magnet is attached to a pendulum and allowed to swing. A suspected-to-be-silver coin is placed underneath the pendulum. Eddy currents from the coin will make the pendulum stop swinging.
- Suspended magnet: similar to the swinging magnet, only this time, you're keeping the pendulum still, and moving the coin underneath it. Eddy currents will grab onto the pendulum and make it start swinging.
- Sliding magnet: clamp the coin vertically, and allow the magnet to fall past it. Eddy currents will make the magnet fall slower than normal.
- Magnet slide: similar to the sliding magnet, but this time, it's the magnet that's clamped in place, and the coin is sliding past it. Eddy currents will make the coin fall slower than normal. This is perhaps the simplest to arrange, if you've got a bunch of identical magnets, as you can set up a chain of powerful magnets underneath a long strip of teflon or similar plastic, then rig this at an angle to slide the coin down.
All of these techniques require calibration: you need to take a coin you know is silver, another coin of the same size you know isn't silver, test them both and compare how they react to the magnet. Then take your "unknown" coin of the same size, and see if it behaves like the silver coin, or like the non-silver coin. Note that all these coins need to be about the same size; there's no point calibrating your magnet on silver dollar sized coins, then using it to test dimes. You can still use your magnet to test dimes, you just need to calibrate it with silver and non-silver dimes first.
All of these tests will work, but all of the eddy current devices I've seen are rather cumbersome to set up and use - not the sort of thing you can just bring along to a coin show or flea market.
You can even test coins sitting inside plastic slabs, so long as your calibration coins are also in similar slabs. But you can't test coins that are sitting in stapled 2x2s, because, well, staples are usually made of steel so they will try to stick to the magnet; this will interfere with the results.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis
