The 3/4" thick ring magnet works just as well, but tends to shatter if
allowed to slam with other supermagnets. But but but $2 each!
Supermagnetman
3/4" x 3/8" thick ring neo magnet $1.90
If you can manage to
place a TV camera on the top end of the pipe and set it for macro
close-focus, the whole lecture-hall will be able to see the strange
effect.
If you cover the copper tube with green "magnet viewing film", you'll be
able to see the leakage fields which get through the copper as the magnet
settles slowly downwards. It's disturbing to watch, since it makes the
solid copper appear almost transparent, as if you can see the 3D magnet
down inside the tube. (If the copper was a perfect conductor,
the eddy currents would create a perfect shielding effect and the green
film would show nothing. But then the
drag force would vanish.)
Cool your copper tube with dry ice or liquid nitrogen. Handle it with
oven mits. Will the magnet drop slower, because the copper resistance is
much lower? Or will it drop faster because the copper is sheilding itself
and not letting the magnetic field enter the metal?
NOTICE: Aluminum pipe works too (the magnet travels a bit faster than with
copper), but aluminum pipe is MUCH MUCH CHEAPER. I obtained my copper pipe
from ALASKAN COPPER AND BRASS, Seattle WA, 1-800-552-7661. (They
have a $100 minimum order.) Also try Online Metals, and Southern Copper
[links below]. The exact diameter isn't too important, but the pipe
should have a VERY thick wall, 3/16" or 1/4" thick if possible, and the
inner diameter should be large enough to allow a 1/2" neodymium disk to
have large, obvious clearance all around it as it falls (approx. 3/4" ID.
pipe, but larger is better if you want to use larger magnets). For
Alaskan Copper, ask for "schedule 40" pipe with 3/16" wall, then pick an
inner diameter that clears your supermagnet with at least 1/8" space all
around it. The "schedule 80" pipe is 1/4" wall thickness, even better,
but half the time they don't have the right size. If they don't stock the
exact dimensions, see if they have anything that's close. [ P. Ledlie
found another supplier: "Copper and Brass Sales" at 503-254-2600, $65/ft
for pipe that's 1" ID. WHEW! EXPENSIVE! It should be more like $10-$20
per foot. ]
INSIDE-OUT MAGNET TUBE
In the previous demo, a magnet falls down through a tube. To reverse this
effect, build a long stack of neodymium magnets which have their repelling
poles stacked adjacent, like NS SN NS SN NS. To make it easy, buy
ring-magnets like those linked below and stack them on a wood dowel.
This will create a pole pattern like: N-S-N-S-N-S-N.
(Use crazy glue, or stack the rings on a brass or nylon bolt)
Ring magnets: www.supermagnetman.net
Now drop a small segment of copper pipe down your magnet stack. Find
thick copper rings as 1" plumbing adapters, or just saw off a
1" long ring heavy of copper pipe. It will
fall sloooowly along the magnet stack. Each pole of the magnet induces a
ring of large electric
current in the copper. Do the same with a copper washer. Now try it
again, but this time cut a slot in the pipe or washer which breaks the
electric circuit. It falls faster. Yet it still drags (why?!!) Try a
plastic pipe (perhaps painted copper-color) and the pipe isn't slowed at
all and falls very fast.
EVEN SLOWER VISIBLE HOVER
Do you have liquid nitrogen available? Or even dry ice? If so, then you
can do the new demo that TAP-L physics people are all playing with. Get a
slab of copper, a plate 1/2" thick and several inches wide. Cool it in
liquid nitrogen (or instead make your own cryo-liquid using rubbing
alcohol and dry ice.) It helps if you put some grease on the bottom of
the copper to prevent the liquid nitrogen from boiling off.
Once the plate is cold and frosty, pull it out, lay it horizontal, and
drop a large NIB cylinder face down onto the plate. It will settle
sloooowly. (It helps if you spin the magnet to keep it falling vertical.)
Uh. Tongue being attracted to frosty metal. No. Noooo. Must resist.
Ahhn. AAAHHHnnnnn! Tleash thake ne tll a hosthithal innediately.
INVISIBLE GEAR TEETH
Carve a lump of wood or plastic into a 3" cylinder, drill four shallow
3/4" holes in the cylinder surface, and glue four 3/4"-dia. neodymium
magnet slugs in the holes. Stick the assembly on a drill bit (even use a
hand-powered drill) so you can spin it.
When spun near a metal plate, this device acts as an electromagnetic
friction drive, or a gear with invisible teeth! Hold a Cu or Al plate
near it, and it will push the plate along without touching. Support the
spinning cylinder horizontally over a table top with a 1/2" space between
table and cylinder, then stick various nonferrous metal plates under it.
They will be launched forward.
I made a "top" from a 1/4" x 6" aluminum disk with a plastic rod
through the center. With practice, one can spin this top up to
high speed WITHOUT TOUCHING IT by using the above magnet "gear teeth"
cylinder mounted on a power drill.
ELECTROMAGNETIC LEVITATION PLATE
Attach a large Cu or Al disk to a motor or power drill. Affix this
assembly under a plastic or wooden plate for safety, with the spinning
metal disk parallel and almost touching the plastic plate. Run it up to
speed and hold a neodymium magnet over the plate. You will feel both
magnetic drag forces AND repulsion forces. Don't drop the magnet or it
will be hurled across the room!
Tie your neodyimum magnet to a string, a piece of tape, etc, and affix the
other end of the string or tape to the plastic plate so the string stops
the drag but not the repulsion. THE MAGNET WILL LEVITATE! Watch out
though, if the string lets loose, the EM drag will fling your brittle NIB
magnet.
Clue: there is a way to levitate a bar magnet above two spinning metal
objects which requires no strings. Can you figure it out? See my
Maglev page elsewhere on this site, under DOING IT MECHANICALLY.
TWO-DIMENSIONAL "FLUID"
Place several
disk-magnets in a slippery bowl so they repel each other. They form
patterns. It's almost as if the magnets were the "atoms" of a gas or
liquid. We can bring out this behavior even more strongly. Clamp a large
number of tiny disk magnets between two glass plates. Put spacers around
the edges of the plates so that the magnets can slide around. Now place
the sandwitch on edge. The magnets lift up and "crystallize" in a hex
array! But they can also slide around. WIth enough magnets, the region
of "fluid" will exhibit surface waves (and also sound waves within the
bulk "liquid", and even cavity resonances!)
It might help to
lubricate the glass surface a bit. Also, place a row of fixed disks all
around the edges in order to form the walls of a two-dimensional "beaker"
which repels the "fluid" within.
GIANT VISIBLE "AUDIO TAPE"
Obtain some green "magnet viewing" film. Place it on the back of a rubber
refridgerator magnet sheet (such as a magnetic business card.) The green
film shows that the rubber magnet has a pattern of NSNSN stripes. This
increases the overall attraction for metal. Now slide a large neodymium
magnet across the rubber sheet, then "view" it again with the green film.
Hey! The neodymium magnet ERASED THE POLE PATTERN! It's now all one
north pole (or perhaps an S pole.) Don't worry, you can
put it back again by sweeping small neo. magnets across the "erased" area.
Or better yet, why not erase the entire rubber magnet... then write a
secret message upon it? Or try drawing various NSNS patterns and see
which one attracts a ferrous plate most strongly. Or try drawing a
pattern of very fine parallel stripes (using a tiny neo. magnet fragment),
then sweep the magnetic head from a tape recorder across it. Will it make
noises?
Hey, why not put the rubber magnet sheet on a rotating cylinder,
and magnetize the sheet with a pattern of parallel stripes. Spin the
cylinder, hold a small NIB magnet near it, and the magnet should vibrate.
Put the tiny magnet on a sounding-board, and the spinning cylinder should
create a "magnetic siren". Try various patterns of stripes to make
various noises.
FAKE MONOPOLE
Have you ever magnetized a nail by stroking it with a magnet? Well,
neodymium magnets are so powerful that you can magnitize A BAR MAGNET by
stroking it with a neodymium slug. A stack of NIB cylinders works great
for doing this. Teachers can strengthen their dead classroom bar magnets.
You can use a small piece of NIB magnet to write your name on the face of
a large ceramic loudspeaker magnet (then use iron filings or a "magnaview
sheet" to see the writing.) You can mess with your teacher's mind by
reversing the poles of one of those classroom magnets so the "N" label is
actually south, and vice versa. Teachers can mess with students' minds by
making a "monopole" magnet: use one end of your NIB stack and stroke a
classroom bar magnet from the center and outwards. Do this to both ends
and both sides of the bar, always starting from the center of the bar
magnet and stroking towards the end, and always using the same pole of the
NIB magnet stack. The result will be a bar magnet which is marked "N" and
"S", but in fact both ends will be North, and there will be a big fuzzy
South pole in the center (or vice versa, depending on how you held the NIB
stack). If you wave your "monopole" magnet near a compass, the compass
will show that both ends are the same! (Maybe you'll want to make another
one which has two "S" poles. Maybe you'll want to paint "N" on both ends
of your physics-joke magnet!)
DIAMAGNETIC WATER
Place a neodymium magnet in a shallow dish. Fill the dish with water so
the magnet is completely covered (about .5cm water above the magnet).
Bounce light from a distant small source off the water surface and onto a
wall or screen. (sunlight works well.) You will see a uniform oval
projected spot of sunlight reflection from the water surface. In the
projected oval of light, right at the location of the submerged magnet,
you'll find a small bright splotch.
The bright splotch is caused by a concave dimple in
the water surface. The magnet repels the water slightly, which creates
the concavity. Try using less water so there is just 1mm between the
magnet face and the surface. This gives a bigger effect, but some people
might then suspect that surface tension plays a role. Try looking down
into the bowl so you see the reflection of the ceiling. If you move your
head back and forth, you will detect a small distortion at the location of
the magnet. C. Brown suggests placing the bowl in front of a screened
window, then looking at the reflection of the wire mesh within the bowl.
The distorted water surface will cause Moire' patterns to be seen.
Add more water so there is 1cm between the surface and the magnet, and the
dimple will be *very* shallow and subtle. But it will still be
detectable. Repeat the experiment by using a thin plastic dish with the
magnet BELOW the dish (no surface tension effects possible!) Idea: use a
large number of magnets to write some initials, perhaps cast them into an
epoxy block so their faces occupy the same plane. When this is submerged,
the magnetic field will spell out the initials in the reflected sunlight.
A sort of "chinese magic mirror" image, with the "mirror" being the
surface distortion created by magnetism. (Note: put the bowl on a sturdy
countertop to stop the water ripples caused by floor vibrations.)
Neodymium magnets will repel ice cubes slightly. Hang an ice cube from a
thread, and let it stretch the thread and cease spinning. Now bring your
NIB magnet stack near the side of one end of the cube. You can push it so
that it slowly turns. Cast a long icecube (or use an icicle in winter),
which increases the lever arm and makes the effect more obvious.
DIAMAGNETIC WATER II
Haven't tried this one (inspired by John Shurner.) Levitate a ball of oil
submerged in an alcohol/water mixture adjusted for neutral density as
follows. Fill a jar with rubbing alcohol, put in a small drip of oil
(which promptly sinks.) Next, add small amounts of water to the alcohol
and mix slowly (don't break up the oil drop.) Repeat until the oil drop
neither sinks nor rises. (This is achieved with only a small percent
water in the alcohol.) Use a pipette to add many more oil droplets, so
the jar is full of them. Now approach the jar with an NIB magnet. Does
it attract the oil? Or, does it repel the water which causes oil to
approach the magnet? John S. may have replicated the Finnish Antigravity
experiment: he finds that an HTSC disk chilled in liquid nitrogen causes
the oil balls to "convect" when the jar is held above the superconductor.
But does this occur even if electrical shielding and thermal shielding is
provided?)
DIAMAGNETIC EVERYTHING
Hang a small object from a long thread. Bring an NIB magnet near it. If
the material is diamagnetic, the object will be very slightly repelled.
If the object is paramagnetic, the object will be very slightly attracted.
DIAMAGNETIC LEVITATION
Use a really huge NIB magnet to lift a tiny NIB magnet from below.
The small magnet will float in the air.
Adjust the distance so the small NIB magnet is *barely* attracted upwards,
then place a piece of diamagnetic material immediately below the small
magnet. The large magnet will cancel gravity, but an "energy well" will
be formed because the small magnet is weakly repelled upwards by the
diamagnetic material. The small magnet will hang in space. Earnshaw's
theorem is apparantly violated! There is a
photo of it in SCIENCE NEWS, 7/24/99, and a paper on it in NATURE,
7/22/99, author A.K. Geim. The authors use graphite plates (and even
human fingers!). Bismuth slabs give a similar effect.
Complete info about all this is now on John Lahr's
page. There's also a new build-it article on the
SCITOYS diy site. Buy pyro-grown graphite plates from SCITOYS catalog
.