Introduction: Diamagnetic Levitation Experiment
Magnetic levitation is really fun to experiment with, and can make a great science fair project. However, it's hard to get dramatic results--the problem goes back to Earnshaw's theorem from 1842. He proved mathematically that any electrical or magnetic levitation would require an extra axis of stabilization...a force from a different direction.
You can show this with ring magnets. Put them on a stick, and they do indeed float over each other. However, the stick is required, that's your extra axis of stabilization--remove it, and the magnets simply flip over and jump to each other, no longer floating. The phenomenon of diamagnetism provides a simple, inexpensive solution to this problem and gives fairly dramatic results.
Most people are familiar with ferromagnetic materials--iron, steel and permanent magnets. These materials are attracted by magnetic fields, and can keep their magnetism after exposure to the field. Diamagnetic materials include carbon-graphite, water, protein, DNA, wood, bismuth, silver, diamond and gold. When exposed to a magnetic field, these materials induce a weak magnetic field in the opposite direction. Superconductors are perfectly diamagnetic, but require liquid nitrogen to work--not something that most folks have a jug of in their fridge!
Diamagnetism is the key to this experiment, and provides the extra stabilizing force needed. We'll be using carbon-graphite and bismuth because they have the strongest diamagnetic effect.
Step 1: Materials Needed
This entire project should cost you less than US$20. You may not be able to find all of them locally, but I've provided links and Google search suggestions for any that you might have to order off the internet.
carbon-graphite blocks, 1.5 inches square by 1/4 inch thick, quantity 2 -- carbon-graphite is commonly used in motor and alternator brushes. However, these are generally too small, and some samples are more diamagnetic than others. It's best to look for 'pyrolytic' carbon-graphite, or for samples that are known to be highly diamagnetic by the seller. They should cost under US$7 each.
small rare-earth magnet to levitate -- This magnet should be of neodymium-iron-boron (NdFeB) composition and of at least N40 grade. Cube or disc shapes work well. This magnet should be very small for best results--the one we will use here for the demonstration is a 1/8 inch cube of N40 grade. These magnets cost less than US$1 each.
large magnet for lifting -- This magnet counteracts the force of gravity to allow the week diamagnetic force to work, and does not have to be very powerful. We used a 1.5 inch diameter by 3/4 inch thick ferrite (ceramic) disc magnet, which cost about US$2.
experiment stand -- we built ours out of wood, but it could be as simple as playing cards stacked over platic cassette tape cases. The key here is fine adjustment -- the threaded rod allows very fine adjustments of the distance between the lifting magnet and the graphite plates.
Step 2: Build the Experiment Stand
We built ours out of wood. You could actually build something very simple out of cassette tape cases with playing cards to adjust the height -- but the threaded rod at the top that controls the height of the lifting magnet over the carbon blocks allows the fine adjustments you'll need to make the experiment successfully -- it takes very twitchy adjustments to get the tiny magnet to float.
We put our stand together with glue, and that's important--there must be no ferrous materials like screws or nails anywhere near the tiny levitating magnet and carbon blocks. The threaded steel adjustment screw on top is not a problem--it's only near the big lifting magnet. That's also why the adjustment rod for the upper carbon block (at right in the photo) is also made of wood--you could also use threaded brass rod here, but not steel. We didn't have brass available, so we used wood that's press fit.
The entire stand should end up about a foot tall, and the distance needed between the lifting magnet and the carbon blocks will vary depending on which magnets you use--but it will be between 3 and 5 inches.
Step 3: Levitate!
Now you can give it a try!
Using the threaded rod, move the lifting magnet up as far away from the carbon blocks as you can. Make sure the stand is as level as you can get it on the table top--shim it up to make it level.
Don't worry about the upper carbon block, swing it out of the way for now. Clean off any metal particles that have stuck to the tiny magnet with sticky tape. Place the tiny NdFeB magnet on the bottom carbon block, and slowly thread the lifting magnet closer to the NdFeB magnet. As you get closer, you'll see it start to move and try to lift. If you get the lifting magnet too close--the tiny magnet will fly up and stick to it. You might want to pad the bottom of the lifting magnet with leather or cloth, as it's possible to break a tiny NdFeB magnet this way!
It will take you some time -- the adjustments are very tiny and twitchy. But eventually you'll be able to acheive a tiny amount of levitation (photo).
Step 4: Use the Second Plate
Now slide the second plate in place, and try the experiment again with the tiny magnet between the two carbon plates. Since it's repelling the tiny magnet now from the top also, you should be able to achieve about twice the levitation....between 1/16 inch and 1/32 inch is excellent.
The more powerful your tiny NdFeB magnet, the higher the levitation. And, the more diamagnetic the carbon-graphite you used, the higher the levitation. If your tiny magnet is too large, it will be difficult--stick with a magnet that's 1/8 inch or smaller.
Step 5: What's Happening Here?
Now take a soda straw and gently blow on the edge of the levitating magnet. It will spin at a fast rate, and will keep spinning for many minutes! You can watch a video of that here:
Spinning magnet video
What exactly is happening here?
Diamagnetic forces are VERY tiny compared to ferromagnetic forces. The lifting magnet is simply used to counteract the force of gravity--the gravitational forces on the tiny levitating magnet are far too much for the diamagnetic force. But, once these are eliminated, you can see the diamagnetic forces at work. The tiny magnet puts a magnetic field into the carbon-graphite plates, which sends back the same field in the opposite direction, repelling the magnet and making it float.
Step 6: Variations
There are other variations of this experiment you can do, too. You can take a tiny chip of carbon-graphite and make it levitate on a stack of NdFeB magnets, with no lifting magnet. We used a stack of those same 1/8 NdFeB cubes, aligned like this:
We cut out the chip with a jeweler's saw, and sanded it down very thin on sandpaper.
Another option is to use bismuth, which is also extremely diamagnetic. You can get it in bismuth shotgun shells (sold where lead pellets have been banned for hunting). We melted the bismuth down into a puck. It works just as well as the carbon, but is more expensive. It's shown in the photo.
Step 7: Sources
FULL DISCLOSURE -- I work for a magnet company! And we do sell all the stuff for performing this experiment. But there are many other sources to get the carbon graphite blocks, lifting magnets and levitating magnets. Also, some places sell magnetic levitation stands already assembled. But, isn't it more fun to build your own? Especially if you are doing a science fair project!
You can get all this stuff from us, and see our complete experiments pages about this project, here:
Or, just search Google for:
For an even more incredible experiment, you can do all this with superconductors. You can get a simple superconductor levitation kit from us for about $40, and you'll just have to get a thermos full of liquid nitrogen....not usually very difficult. BUT, somewhat dangerous! Be sure to read up on handling liquid nitrogen before attempting any superconductor experiments.