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I have a set of small magnetic spheres the size of ball bearings. When many of them are built into a cylinder such that they are hexagonally packed, there is no magnetic attraction radially (between the walls inside).

Cylinder of magnetic balls

That is, if I try to squish the cylinder,

getting squished by pencil

the walls do not stick together;

enter image description here

rather it easily resumes its cylindrical shape.

However, if I force a little ball inside, it is attracted to the walls,

ball inside cylinder

such that when I flatten it, it stays flattened.

staying squished on its own

What is the magnetic field doing in this situation? Why is there no apparent attraction (and even some resistance—though that may be frictional) radially?

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Could you add some diagrams/pics of the situation? It's really hard to understand how you are adding the little ball and all. – Manishearth Apr 21 '12 at 18:34
@Manishearth The only camera I have is my webcam, so the pictures are low quality. (Is there a way to display them smaller so the my post won't be so long without reducing the resolution?) I hope this clarifies the question and that someone has the mathematical know-how to figure out what is going on! This little things are fascinating in their behavior. – Kazark Apr 22 '12 at 0:03
I found this related post. Very interesting. As I got these as a gift I didn't know what they were called or where to get them. – Kazark Apr 22 '12 at 0:06
up vote 2 down vote accepted

The ball you forced inside is affecting the magnetic field.

Note that each of these magnets has a north and south pole--the poles are hemispherical. Also, these poled are perfectly aligned in the cylinder. enter image description here

In the first case, there are two forced acting here:

  • The force of attraction between neighboring magnets--this will resist the deformation and will try to get the cylinder back to its original state. This is because there are regions formed with a not-so-perfect attraction.

  • Force of attraction and repulsion between opposing elements. In certain cases, the opposing magnets will repel, and they will attract in other cases. So there will be a grid of attraction and repulsion formed when you squeeze the magnet, and the net effect will be zero. There will only be perfect attraction is you manage to skew it so that the "top" and "bottom" layers are hexagonally packed as well--pretty hard since there will be repulsion elsewhere. Most probably the resistance to squeezing manifests itself around halfway, and increases from there.

When you add a magnetizable ball, it aligns itself at a spot and becomes half a magnet as well.

enter image description here

Note that I've added it on the outside in the diagram, since I can't figure out how to add it to draw it on the inside. When the ball is inside, the same thing happens.

Now, this makes it much easier for the opposing sides to touch each other

enter image description here

The point is, even though the magnetized ball isn't that good a magnet, its still touching the sides. And this means that the magnetic attraction is large. On the other hand, whatever deformation repulsion is there is still pretty small, since you haven't deformed it that much yet. So, the attractive force wins, and it stays in place.

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+1 Wow, normally I would wait to accept to see if I got other answers but those diagrams swayed me. – Kazark Apr 22 '12 at 6:10

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