# Another use for rare earth magnets

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Another fun use for rare earth magnets which gets a rise out of people who haven't seen it before is this recipe:

Ingredients:

1 cylindrical rare earth magnet about 1/2" diameter by 1/2" long. 1 piece of 1/2" copper water pipe about a foot long.

Holding the pipe vertical, drop the magnet into it and watch how long it takes it to ooze all the way down through it.

The hard part is finding a "simple" way to explain the effect to non-techies.

Jeff

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Thanks Ken

I'm going to borrow that concept for a travel readout on my lathe and maybe on my mill till i can replace the vernier travel indicators (Or splurge on a DRO)

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Good idea, and terrific photo of the bicycle.

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Thank you!

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How long does it take to 'ooze'?

Wes

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Mine takes about 5 seconds to drop through about 10 inches of pipe.

BTW, the priciple which makes for the slow decent is that when the magnet moves with respect to the pipe it's magnetic field induces a dc electric current flowing in a circle in the metal of the pipe. That current creates its own magnetic field which exerts a opposing force on the magnet.

Since the induced current and the opposing force on the magnet only occur when the magnet is moving, a balance of forces establishes the steady speed of magnet's decent.

It's akin to the "regenerative braking" used on hybrid cars, except the car's electric motor windings are a lot more complex that the single shorted turn created by the pipe.

Much of the current created when running the car's electric motors as generators when braking gets put back into its battery, while with the magnet and pipe, the current is all wasted in imperceptible resistance heating of the pipe.

At least that's the way I understand it...

Jeff

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Another way to put this is that as magnet induces current in the pipe, the current wastes energy by heating the pipe slightly. That energy has to come from somewhere, which if from the mechanical energy of the magnet. That's why it slows down -- the mechanical energy is converted to heat.

i
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Thanks for the reply and explaination. Got a tip on where to get one of these magnets?

Wes

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Computer hard drives have them. Find some old drives, take them apart, and look for them in the mechanism that swings the recording/reading head across the disc. I have one large magnet from a big old drive that is arc-shaped and has a steel flux plate across it. Getting that plate off is fun, and putting it back on can pinch your fingers hard enough to raise blisters. I use it to demonstrate the magnetic drag phenomenon to my Aircraft Systems class by running a strip of aluminum through it. They need to understand how a magnetic- drag tachometer works (magnet spinning inside an aluminum cup; the magnet is rotated by the tach cable and the cup drives the needle). Newer drives have smaller magnets that are still really handy.

Dan

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By the way. These rare earth magnets are very useful for hanging tools, car keys, as well as papers on the refrigerator. Be careful around small children. If a child swallows two magnets, they can pinch their intestines. Toy manufacturers mostly stopped making toys with magnets that can be swallowed, at least that's what I heard.

i
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The effect is even more pronounced if the "pipe" is a coil of heavy wire, and shorted. More EMF can be generated by the multiple turns than by just one.

We used to do this trick with the really heavy head-positioner mechanisms from old SMD removable disk drives.

The magnet weighed about twenty pounds. The coil of the positioner was around 100 turns of about - oh - ten gauge square magnet wire. If you shorted the coil, the slider (weighing 'bout another pound) would take about a half-minute to fall six inches. It was truly eerie, looking almost like it was viscous-damped.

LLoyd

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Got any dead disk drives? all but the oldest have a pair of these to provide the magnetic field for the pivoting servo to drive the arms with the heads.

Be careful taking them apart, however. Aside from typically needing a range of Torx drivers for various parts of the drive, and needing to peel off the circular "Warranty void if seal is broken or removed" stickers to get to some of them, the real problem comes when trying to separate the two magnets -- each with a steel backing plate. They are typically only held together by the magnetic field, and apart by the structure -- or sometimes with spacers on pins (all stainless steel, of course). If you slip while prying them apart, you are likely to get a blood blister from the pinch. If you select a screwdriver to pry them apart -- go for a stainless steel or a bronze screwdriver, so the screwdriver does not become a permanent part of the assembly. I keep my collection of them stored on the side of a steel relay rack, and remove them by sliding them to the edge. (Yes, I've had a lot of disk drives die over the years. :-)

The newer and faster the drives are, the stronger the magnets.

Enjoy, DoN.

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I woud just buy them on eBay, they are inexpensive.

igor

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[ ... ]

Yep -- but you can also get some interesting bearings, and a collection of metric screws of good quality from the drives while you re about it. And even use the platters for some decorative purpose. (Maybe you could figure out something to launch them, with a sheet of paper glued over the center hole, to use as shotgun targets. :-)

Enjoy, DoN.

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Thanks,

Have some on the way.

Wes

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Always thought they'd be great for a Tesla turbine. Mike in BC

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I found them for sale at Michaels (an large arts and craft store).

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Yeah, that's good. But technically, it's backwards.

The loss due to resistance is why it falls at all, not why it slows down. If you do the same experiment without resistance, the magnet hovers without falling:

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When you add resistance, it causes the energy holding the magnet to slowly be converted to heat which allows the magnet to fall. So the heat loss is why it falls, not why it it slows down.

By talking about the heat loss, you have still failed to explain why the magnet is coupled to the pipe in the first place since the copper pipe is non magnetic or why a magnet will hover above a superconductor without falling at all. I don't know of any easy ways to explain the effect to someone that doesn't already understand electromagnetism.

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