A couple of weeks ago, we visited the The Exploratorium, where we saw this giant homopolar motor, labeled “Daisy Dyno.” This is a classic demonstration of a homopolar motor. There’s a giant permanent magnet. In its jaws sits a big copper disk that is free to spin. A low-voltage but high current power supply is provided, where the positive end is hooked to the bearing in the center of the copper disk.

To run the motor, you touch the loose lead from the negative end of the power supply to the edge of the copper disk. A neat little arrow shows you exactly where to touch. The electric current flows from the center of the disk to that point of contact, which is in the direction perpendicular to the magnetic field, which creates a force in the correct direction to cause the disk to spin. In order to help the electric currents move in a fairly straight line between the edge of the disk to the center, the disk has a lot of radial slits cut through it, giving the disk the appearance of a daisy (hence the name) when it’s at rest. In the photo here, it’s moving pretty quickly.

We, of course, are quite fond of motors and magnets and things that spin, and have (so far) written up three under-one-minute science projects that are related: How to build a homopolar motor, how to make the version that spins water instead of a metal disk, and how to make a super-simple directional compass.