Date: Wed, 13 Nov 2002 10:15:33 -0800

Author: Paul Doherty

Subject: Re: mod on eddy current


At the Exploratorium I get the largest piece of scrap aluminum I can find
2" x 4" x 18"

Then mill a rectangular groove lengthwise down the center of the wide face.

Then I place a neodymium disk magnet into the groove hold the groove
vertically and drop the magnet.

It slowly falls in plain sight to the bottom of the groove.

Or you can simply sandwich the magnet between two quarter inch thick
aluminum plates. See the photo at

We also have a new exhibit named "Floating in Copper".
A neodymium disk 1/2 inch thick and 1 " in diameter sits between two flat
copper plates (each plate is 2 inches thick) The plates are sppaced 1"
apart. A neodymiium magnet (1" thick by 1" diameter) is held over the top
copper disk.

Using this setup the eddy currents slow down the motion of the lower magnet
so much that human eye-brain-hand feedback can "fly" the magnet. It's just
the right level of challenge to keep visitors interest.

Here's a sketch:

Paul Doherty

>Last night I did the standard Lenz's law demo (Cu pipe and Nd magnet), but
>with a small twist. I used the short thick Cu tube from Educational
>Innovations ( and one of those BIG $35 Arbor
>Scientific Nd magnets. MAN! That demo ruled! Just watching it from the
>top blew the students away. It is beautiful. Now, to find a long length of
>the thick Cu tubing.
>I know this has been mentioned before but, it's amazing how many students
>were stunned by it.
>Sam Sampere wrote:
>> I bought some of those cool NdFeB magnetic spheres for various demos.
>> Here's a way cooler way to show the Eddy current tube.
>> First, roll the ball down a long piece of flat plastic. It's not very
>> spectacular, you get just what you expect. Then roll it down a long
>> piece of copper. I'm using a piece of 1/8" x 8" x 36" OFHC. The ball
>> very slowly rolls down the new incline. Repeat this several times until
>> the copper is vertical. It's just too cool!
>> The sheet is much cheaper than the thick walled tube, and much lighter
>> too.
>> Sam