MHD I: Demonstrate Magnetohydrodynamic Propulsion in a Minute

Surprisingly enough, a working example of this futuristic drive system is quite easy to build. Assuming that you've got the materials handy, you can build one in about a minute. You'll need a strong magnet, two pieces of thick copper wire, a little dish of warm water, salt and pepper, and a regular battery.

Use thick copper wire here because thin wire will get very hot and burn your fingers. The best type of magnet to use is a neodymium disk magnet, but just about any strong magnet will work. You can get good magnets from these sources. Note that magnets from hard drives are not the best choice because they usually have both poles on the same face of the magnet.

Why salt and pepper? MHD propulsion relies on conducting current through water. Water is normally a poor conductor, but salt water is a great conductor. Adding the final dash of pepper makes it so that you can see when the water moves. =)

Important safety tip: You're dealing with electricity and salt water; use some common sense. Small quantities of potentially poisonous and/or explosive gasses may bubble out of the water; don't try and scale this up unless you really know what you're doing. Besides that, copper wires can have pointy ends and magnets can pinch you.

Michael Faraday's original 1821 homopolar motor was somewhere between these two examples. Rather than using a rotating magnet, he used a pool of liquid mercury (an excellent conductor) sitting atop a magnet. In his Popular Science column this month, Theodore Gray recreated that original motor, using a genuine pool of mercury. Among the different designs that have evolved from that original are those (like the one with the screw) that rotate a mechanical object, but also the magnetohydrodynamic version, which can propel a ship through the ocean.

Sure, you say, but how does it work? Let's back up for a moment and first look at how the (rigid) mechanical homopolar motor works. For the sake of concreteness, we'll use the example of the motor that we wrote about previously.

A simple, "rigid" homopolar motor. Electrical current flows in the direction indicated by the purple arrows: from the bottom end of the battery, down the screw, through the magnet, up the wire, and back to the top of the battery.

(By the way, I made these drawings in POV-Ray. Someday, I'll put the rest of my tutorial on line.)

For the currents that flow in the screw and wire, the forces are minimal since the current direction is parallel to that of the magnetic field. However, there is a substantial force on the electric charges that flow horizontally through the magnet, at 90 degrees to the (vertical) magnetic field orientation. Thus, a force (green arrow) is generated in the third direction, which gives rise to the magnet spinning.