Magnetic Spinning Desk Toy

There was a vintage toy made in West Germany during the 1950s that consisted of two magnets, one hidden inside a ballerina and the other inside a mirror. If the mirror was moved close to the ballerina, she would start to pirouette. Amazingly this spinning motion was created without any gears, belts, motors, or batteries of any kind.

You can explore how this same spinning mechanism worked by constructing a simple desk toy from a few simple materials: magnets, tops of bottles, and paper. This would be a good STEAM activity for introducing the important physics concepts of magnetism, friction, center of gravity, and torque.

1. Lid of a spice jar - one with a rounded top
2. If you cannot find a suitable spice jar, then grab a 2 liter drinks bottle from the recycling
3. Magnets - neodymium is best but any strong set of magnets should work. It is preferable that they can easily be stacked on top of each other
4. Blue/white tack or some form of glue
5. Paper
6. Cardboard

If you have a spice jar with a rounded lid, then just detach it.

Alternatively, find a spherical part of an empty drinks bottle and mark a circle on it about 40 mm in diameter. Carefully cut this circle out. The 2 liter bottles work well for this. I tried smaller bottles but the top parts were far from spherical and the finished toy didn't spin quite so well as a consequence.

1. With the convex side of the spherical lid facing downwards, apply a small drop of glue or a ball of white/blue tack to the center of the concave side.
2. Attach a magnet to the center with the north-south poles oriented vertically. You can stack multiple magnets as long as you don't put so many that the lid topples over when pushed. If you are making more than one desk toy, make sure that the magnets are all oriented in the same direction e.g. north poles pointing upwards
3. Test the spinning mechanism - Hold another magnet parallel to the stack so that they repel each other. Tilt the magnet by 45 degrees to the vertical. Slowly bring it closer to the lid. The lid should start to spin.
4. If the lid doesn't spin, or only spins when the other magnet is very close and almost touching the lid, then try adding more magnets to both stacks.

Cut out cardboard or paper shapes to decorate the desk toy. There are only a few rules to follow:

1. Try to keep it more or less balanced - don't stick heavy cardboard shapes only on one side
2. Don't let parts drag on the ground. This risks hindering the spinning motion
3. Keep decorations short. Very tall shapes may make it too easy for the toy to topple over.

I've found that spiral shapes and simple paper cones yielded pleasing "barber pole"-like results.

Cut out a net pattern of a cube from cardboard. The cube net size should be chosen to accommodate the stack of magnets you were holding when testing the desk toy.

Glue the stack diagonally across the inner surface of one face of the cube. Glue the cube closed.

Bring the cube near the desk toy. The cube face with the magnet should be facing towards it. Note how the desk toy reacts.

1. Is it attracted or repelled by the cube?
2. Does it spin clockwise or anti clockwise?

Try holding the cube in different orientations. Mark the cube faces so that you can easily figure out how to hold it in order to get the desk toy to move in a particular direction.

Can you figure out how this toy works?

I myself am still trying to come up with a succinct and easy to understand explanation for how the rotational motion arises from simply pushing the magnets towards each other. Perhaps you can come up with a neat and tidy explanation.

Addenda

Steve Mould has an excellent explanation for how the two magnets interact to cause the toy to rotate. His video also features the ballerina with mirror that I mentioned in the introduction.