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