And the globe says that it’s “powered by light”, so presumably there’s some unobstrusive solar cells on it.
here’s another cool “toy”
This is actually a pretty good system, why don’t they make them this way?
Are good diamagnetic materials just too expensive, or is the Neodynium magnet the issue? The only experience I have with diamagnetism is superconductors and liquid oxygen.
ETA: My mistake LOX is paramagnetic.
In case you aren’t familiar with the del notation (the upside down triangle) it’s just the symbol for the three dimensional second derivative, and it’s called the Laplacian.
In order to have a minimum in the potential the second derivative must be positive, so when del[sup]2[/sup] = 0 you can’t have a minimum and therefore you can’t have a stable electrostatic equilibrium.
Of course this has to be proved because there are little problems like x[sup]4[/sup] at x =0.
Everything is.
Well, technically yes, but some things are also paramagnetic or ferromagnetic, which more than cancels out any diamagnetism.
>This is actually a pretty good system, why don’t they make them this way?
They do. That’s how I could buy one. But the graphite is quite expensive, maybe as expensive as silver - well, expensive anyway. I found web sites explaining how to build these things for one’sself, including sources for the graphite, and these references say the graphite can’t be used in greater thicknesses because the effect stops getting greater once the graphite is thickner than a mm or two. This doesn’t seem correct to me. It’s not like diamagnetism on the surface of the graphite is going to shield the deeper graphite from the magnet’s field. I’ve toyed with trying to model it using a finite element package, but haven’t gotten around to it.
A related adventure: feeling the magnetic repulsion with a bar of bismuth, which is a leadlike metal used in “nontoxic” birdshot and fishing weights, among other things. I got a few 3/4" by 3/4" cylindrical neodymium iron boron magnets and arranged them along a heavy bar of very low carbon steel, with their faces alternating N S N S, and set this thing on the floor with the exposed faces pointing sideways. Then I hung a 1/2" diameter by 12" long bismuth bar on thread nearby. I find I can push this bar about an inch or so out of plumb using the magnets (and there is also a time dependent effect from eddy currents, but I am talking here about the steady state). I can just barely feel the forces involved - so, playing with magnets in my basement lets me feel diamagnetism, and it isn’t just an abstraction anymore.
I think I found the same website you did here. I like where he levitates the pencil lead. I wonder how diamagnetic nanotubes are. More importantly, I wonder if metallic nanotubes are more diamagnetic than the semiconducting ones making a potential way to separate them. This would be an extremely valuable thing to do.
Bismuth is primarily paramagnetic.
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This seems to contradict some statements in this thread. Or is that just advertising hype?
No, it’s not stable. It’s metastable, and that is enhanced by the gyroscopic effect of the top spinning. It can go for quite a while, but you can tell that it’s easily perturbed.
Are you kidding? Physicists probably were the people who started using lasers as cat toys!
>Bismuth is primarily paramagnetic.
“Diamagnetic materials have a relative magnetic permeability that is less than 1, thus a magnetic susceptibility which is less than 0, and are therefore repelled by magnetic fields. However, since diamagnetism is such a weak property its effects are not observable in every-day life. For example, the magnetic susceptibility of diamagnets such as water is = −9.05×10−6. The most strongly diamagnetic material is bismuth, = −1.66×10−4 , although pyrolytic graphite may have a susceptibility of = −4.00×10−4 in one plane.”
>This seems to contradict some statements in this thread. Or is that just advertising hype?
It is a surprise to me. Does it really explain anything to say that it is metastable? I mean, metastability is stability, with the additional consideration that there are other much more stable states not too far away, so it would be easily to bump the system into one of those other more stable states.
This will sound like some kind of sneaky hedge, and I am afraid I don’t have anything better at the moment, but there is one new wrinkle in the Levitron. Since you have to spin it to make it work, maybe it is spending the energy of the spin to create stability. A spinning device in a magnetic field can generate electrical power, like the batteries they are so proud to avoid. Or perhaps the spin is getting around the problem in some other way. Gyroscopes will translate a torque in one axis to a movement around a different axis, which is otherwise unusual. I understand the purpose of the spin is to keep the top pointed the correct way, but couldn’t you do that with a very long and lightweight beam, like a long delicate straw, sticking out the bottom, like the keel weight of a sailboat? You’d have to add a hole to the base, but wouldn’t that let you achieve what they claim to achieve, without the spinning?
If made nearly balanced it will remain that, nearly balanced. Also any flaw to how it is set in the field will quickly be multiplied until the system fails. Spinning averages this out, so to speak, and it takes longer to collapse. Also, it can spin for a long time due to the lack of friction (this is the part that looks cool too). But to spin at a slow speed, more of a rotation for effect rather than for function, brings us back to the problems of balance being multiplied until it topples.
An interesting concept… and if combined with super efficient LEDs…
I have one–a little one. (A bit smaller than a tennis ball.) It works as advertised, except that I have cats. (Speaking of cat toys…)
It’s kind of tricky to set up, but once set up, if a person didn’t have cats–or children, I have both–I think it would probably stay in place.
I would have come in to say how many volts, or whatever, the adapter has, except it’s plugged in behind a bookcase. Somewhere I have the instructions.
There are two magnets, and they are still magnets when it’s not plugged in, but it can’t be suspended between them, it sticks to either the top or the bottom.
The Levitron only works when spinning in a narrow range (20-35 rps). It is completely unstable above 35-40 or below 18 rps, and eventually fails due to air resistance. Experiments in a vacuum have kept it running for up to 30 min, and cheating by using invisible jets of air from an external source have managed to keep one going for days.
Despite the advertising hype on their “front page”, the company acknowledges that it doesn’t violate Earnshaw’s Theorem (and provides a good description the mechanisms behind the device) on their “Physics of LEVITRON” page.
I doubt you’d want a levitating globe that “fell off its stand” every 30 seconds or whenever you tried to slow it down from an unreadable blur.
“Metastable” levitation is a far cry from stable levitation, when the nearby stable states are “lying inert on your desk”
Oops – I also meant to say that the “spin” simply restrains the Levitron’s motion through a gyroscopic effect (which “cheats” by temporarily keeping the magnet from flipping over)
Its easy to create the illusion of stable levitation, if you restrict the motion of the object, e.g.the floating pen linked above, is really resting on its point on the solid end wall. Look carefully, and you’ll see that the photo has been tilted to conceal this fact. It is not actually levitating horizontally.
To those who find the Leviton still meets their definition of “stable levitation”, I’d suggest stacking Neodymum disk magnets in a close-fitting clear plastic tube. They’ll float quite nicely. The main difference is that you can see the tube, but not the Levitron’s angular momentum.
Also, I believe it is possible to work around Earnshaw’s theorem by using multiple magnets. At least that’s what my crude lunchtime experiments with paper, tape, and a stack of N45 disks suggest. I taped 4 magnets around a pencil ~1.5" down from the end, and another magnet on the end (all Ns facing “out”), and made a “cup” by taping a magnet to the center and each end of a “cross” of tape, then folding it onto a long narrow “cup” (all Ns facing in), reinforced with more tape.
The “cup” levitates, and if I inverted the assembly I think the pencil would, too – had I thought far enough ahead to use a short pencil stub. I suggest this as an afternoon project, for those with the time and magnets. I have a feeling I’ll be refining the design myself this evening.
IIRC, Earnshaw’s theorem only applies to a single magnet being levitated.
Not so: It applies to any static configuration. Your cup either had some other forces acting on it, or was just very slowly coming unbalanced.