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Comment by IgorPartola | Hacker News Reader

Yup, it's pretty cool. This was one of the most impressive demo's my physics professor did in our freshman physics class. The point is that as the magnet is falling through the copper tube it creates an electric current which then creates a magnetic field in the opposite direction of the magnet's movement. In other words, this is the practical application of the Right Hand Rule [1].

Two fun facts about it: first, if you cut a small notch along the length of the tube, this will not happen as the current cannot go around the tube.

Second, imagine a superconducting tube with an extremely powerful magnet right in the center of it. Now, try to get the magnet out without cutting the tube.

[1] http://en.wikipedia.org/wiki/Right-hand_rule

Edit: Of course, there's nothing special about the tube being made of copper. Any conducting substance will do.

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You can also think of it as magnetic flux lines encountering "drag" as they pass through conductors.

The limiting behavior of this view is in type II superconductors, which entirely "pin" flux lines -- they present an effectively infinite resistance to the movement of flux lines. That's why a magnet levitates over such a superconductor. [1]

You can even image these "vortices" directly [2]

[1] http://en.wikipedia.org/wiki/Flux_pinning [2] http://www.youtube.com/watch?v=7_ZgiumS41Q

The levitation over a superconductor was the other cool demo that the professor did :).

Lucky kids. When I was in school, the only commercial superconductors were liquid Helium cooled Niobium-Titanium alloys, something not many classrooms could handle. ;-)

Luckily, when I got to programming, we already had zeroes and ones. Imagine our predecessors who had to do it with uppercase o's and lowercase l's...

lOl

Zeroes and Ones? Luxury! In my day we couldn't afford the Ones, we had to make do with just Zeros...

Ha! When I was in school, we had to push the electrons around by hand! You kids had it easy!

You guys had electrons? We had to use scotch tape on a bunch of quarks and give them a half spin and then push them.

Uphill. Both ways.

I'm declaring a winner here ... the only possible thing that could top that is "through three-foot deep snow".

I forget what type we used but it was cooled by liquid nitrogen. That prof. used lots of magnets and superconductors in his research, some cooled by Helium-3. Now that stuff is expensive.

The high-temperature ones were probably Yttrium-Barium-Copper-Oxides (YBCO) I've seen this demo done with them. The magnet's motion is almost undetectable, they appear suspended in the copper pipe until they start to warm up.

http://search.dilbert.com/comic/We%20Didn%27t%20Have%20Zeros (Sept 8, 1992 Dilbert)

You might wanna take a look at this video: http://www.youtube.com/watch?v=VyOtIsnG71U

Incredible. For anyone watching, make sure you at least go past 2:20 - that's when they pull out the magnetic rail.

> ... nothing special about the tube being made of copper. Any conducting substance will do.

Any non-magnetic conducting substance that is. Otherwise the magnet will just stick to the tube wall.

Could this be used as a kind of elevator tech? I'd love to ride one of these down a gigantic copper tube.

It'd be completely impractical, of course, but a whole lot of fun.

You could probably use eddy-current brakes as a safety system for elevators. Many roller coasters use eddy-current brakes.

It would take a hell of a lot of magnets to strap to a person to allow them to jump down a copper pipe though.

Yea my hs physics teacher demoed this and talked to us about possible applications for a while. i don't remember any but thank you wikipedia: http://en.wikipedia.org/wiki/Eddy_current#Applications

Another fun example of braking are the electrodynamic tethers [1], in which you (a satellite) are the copper tube and the entire earth is the magnet!

[1] http://en.wikipedia.org/wiki/Electrodynamic_tether

But it's possible to the same experiment with a battery and an electro-magnet, isn't it? (Not sure how the power-density of lithium-ion batteries would relate to floating 80 kgs down a tube... but seeing as people have demonstrated hover-boards, it should be doable) ? Assuming the purpose is a) float down tube, and b) SCIENCE! -- and not c) with rare earth magnets...

The inverse square law of force strength falloff would make it very hard to scale this linearly, that combined with the fact that weight increases with the cube of size means that the magnets involved would have to be incredibly strong.

Not impossibly strong mind you, if magnets can hold a hundred ton train car off the ground I am sure they could slow the fall of an elevator.

I'd not want to be inside that elevator with anything magnetic in my pocket, though.

> The inverse square law of force strength falloff ...

Because there aren't any magnetic monopoles, I believe in ordinary circumstances the relationship is inverse cube of distance. Reference: http://www.newton.dep.anl.gov/askasci/phy05/phy05629.htm

Yup, magnetic field falls off as 1/(distance3). That's why being 5 feet from an NMR magnet is bad news bears, while being 8 feet away is fine (numbers made up, do not test). I remember seeing signs all over the place at our labs at the uni, saying to not enter if you wear a pacemaker or if you are wearing a wrist watch.

Every elevator you've been in has been held up by a magnet. They're just not obvious inside the motor housing.

To be pedantic, they're usually held up by a counterweight or by hydraulics. They generally move by magnets, though.

> It'd be completely impractical, of course, but a whole lot of fun.

And at the end of the ride, you want to take a selfie and email it to all your friends but you can't because your phone is fried.

actually not true, modern phones use SSD, and don't really get affected by magnets in the way you might expect.

True, they would only get fried if they used electronic components.

If the elevator carries the magnets, the magnetic field relative to the phone is constant, thus no induced current and no fried components.

Even hard disks are not affected by external magnetic field in the way most people expect. Strong magnetic fields interfere with the operation of drive (seeking voice coil) but accidental erasure of data on platters by external magnetic field is relatively improbable.

Generally most of the damage caused by strong external magnetic fields to any electronics are caused by second order effects, i.e. something gets confused and causes data-destroying failure.

Wait so does that mean that, ironically, friction is what it allows magnets to fall through a tube that isn't superconducting?

Yes, pretty much, but you usually call it electrical resistance.

Technically, this has nothing to do with the Right Hand Rule. That is just a human convention to assign a positive and negative signs to a particular direction. Electromagnetism doesn't care about our silly human notations.

It does, however, exhibit the properties we use language to describe. There's a kernel of truth at the center of it.

semantics vs syntax - magnets don't have hands obviously, but the metaphor holds just the same.

There is no "technically..." nonsense! It's not a human convention either - it's an anthropomorphic metaphor.

Name another animal that uses that convention.

The point is that it's not a convention - it's a metaphor, as hnriot pointed out.

It's not a metaphor.

http://en.wikipedia.org/wiki/Metaphor

No one is comparing or likening charge to some other thing. We simply give opposing charge "directions" a name. Everyone uses the name. It's a convention.

The comparison that is being made is the direction your thumb points when your fingers wrap in a particular direction. It's not merely giving it a name - it's specifically likening the vector to the relative orientation of your fingers. That's why the comparison is memorable. If it were merely a name with nothing that it is compared to, it would not be memorable nor helpful.

As long as it isn't a magnetizable substance like iron.

Yeah I'm actually pretty surprised that this made it to the front page. Pretty novel and I thought more people would know about it (being a typical physics class demonstration). Cool nonetheless.

I would wrap an electromagnet around the magnet in the tube, then turn on current to create a field in the opposite direction. Hm... If I pulse the field quickly, maybe it would even remove itself :)

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