Doing the impossible

 

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This is Generally Considered Impossible.

Sometimes, you just have to flaunt physics.  I was bored.  Its amazing what you can do with a little cash, some rare earth elements, and some wood.  See the entire contraption here.  See a video here if you still need more convincing (note: normally it just sits there, this video is after being blown on.  It does continue to spin for an amazing amount of time).  This particular magnet has been levitating since March 22, 2003 (update: 03/15/2005, its still floating nonstop).

The stable levitation of magnets is forbidden by Earnshaw’s theorem, which states that there is no stable and static configuration of levitating permanent magnets. (Earnshaw, S., On the nature of the molecular forces which regulate the constitution of the luminiferous ether., 1842, Trans. Camb. Phil. Soc., 7, pp 97-112.)  or to put it another way, that no stationary object made of magnets in a fixed configuration can be held in stable equilibrium by any combination of static magnetic or gravitational forces. Earnshaw’s theorem can be viewed as a consequence of the Maxwell equations, which do not allow the magnitude of a magnetic field in a free space to possess a maximum, as required for stable equilibrium.

A small rare earth magnet (NdFeB, an alloy of neodymium, iron and boron) is suspended above, in between 2 plates of a highly diamagnetic material, pyrolytic graphite. If the electrons in an atom (or a molecule) create equal numbers of north and south magnetic poles that cancel each other out, the atom has no magnetic poles. If you bring a magnet near such an atom, the magnetic field causes the electrons in the atom to move. Moving electrons create a magnetic field, and the field they create is opposite to the original magnetic field. The atoms move away from the magnet. Materials that act this way are called diamagnetic materials. Pyrolytic graphite is a synthetic material, made by a process called chemical vapor deposition. To make pyrolytic graphite, methane gas as low pressure (about 1 Torr) is heated to approximately 2000 degrees Celsius. Very slowly, (one thousandth of an inch per hour) a layer of graphite grows. The graphite made this way is very highly ordered, and the layers of carbon atoms form like a crystal of hexagonal sheets. These sheets lay on top one another like sheets of mica.

The effect is very weak, even in Pyrolytic graphite which is the most diamagnetic material available at room temperature. Pyrolytic graphite is more diamagnetic than bismuth, but only in the direction perpendicular to the sheets of carbon. In other directions, it is still diamagnetic, but not as good as bismuth. Bismuth is the next best diamagnetic material at room temperature in all directions.

For characterizing diamagnetic materials the magnetic susceptibility is used. Values of for some diamagnetic materials are given in the table on the right.

Material

Water

8.8

Bismuth metal

170

Graphite rod

160

Pyrolytic graphite (perpendicular axis)

450

Pyrolytic graphite (parallel axis)

85

 

 

The diamagnetic effect of the pyrolytic graphite alone is not enough to levitate the magnet, so some strong NdFeB magnets (pulled from some dead computer hard drives) are used above as a biasing or compensating magnet to carry most of the weight of the small magnet. Causing the small magnet above to levitate, thereby apparently violating Earnshaw’s theorem. There are not really exceptions to any theorem but there are ways around it which violate the assumptions. In Earnshaw's theorem the term "permanent magnet" is meant to specify ferromagnetism, and therefore does not apply to diamagnetics as they behave like "anti-magnets" Even though I am levitating a magnet, I am cheating a little by using diamagnetics for stabilization.

You can actually build something similar for hardly any money at all if you have access to dead computer parts and a sporting goods store.  The prototype of this unit was built from scrap wood, magnets from old dead hard drives in computers, and bismuth that was melted down from shot shells (You have just got to love the government for mandating non-toxic shot shells).  The shells are the most expensive part.

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This site was last updated 03/13/05