Explore Lenz’s Law with Gravity-Defying Magnet Tricks

Drill a ⅛” guide hole into the center of one end of the dowel, and drive the 1¼” screw into the hole until about ½” remains sticking out, as in Figure B.

Cut your file card to make a rectangle 6″×2⅜” as in Figure C. Fold the trimmed card in half (Figure D) and add tape to one edge (Figure E). Fold the tape over and open the card out as a tube (Figure F), and you should be able to slip it over the dowel (Figure G). If it won’t fit, make another copy that is a fraction wider.

Now you need two circles of corrugated cardboard, shown in Figure H. To make each of them, first cut the outer edge, 2″ in diameter, then draw around the end of the dowel in the center of the circle, and cut along that line. Punch a tiny hole in one circle, using a thumbtack or pushpin.

With the dowel inside the tube that you made from the file card, push the circles of cardboard over the tube and glue them ½” apart as in Figure I.

Now you need 30-gauge magnet wire. You can find it on eBay. Thread 4″ of the wire through the little hole that you punched in one cardboard circle and tape it to the tube to stop it from waving around. Clamp a pen or pencil so that it is almost horizontal (sloping up a little) and place the spool of magnet wire on it. Wind a few turns around the card on the dowel, as in Figure J.

Tighten the chuck of your drill onto the head of the screw sticking out from the end of the dowel (Figure K), and you can use the drill to spin the dowel, taking up magnet wire from its spool.

You need about 600 turns of the 30-gauge magnet wire. If you put a piece of tape on the chuck of the drill, this will help you to count its rotations. Alternatively, if you have an accurate scale, weigh the spool of wire from time to time, and stop when you have transferred 1 ounce of wire from it onto the coil (Figure L).

Don’t wind the coil too tightly. You’ll need to pull the dowel out (Figure M).

Use very fine sandpaper to remove insulation from the ends of the magnet wire. Attach an alligator patch cord to each end, and use a meter to check the resistance of the coil. It should be around 20Ω. If you can’t measure it, or you get an intermittent value, you probably didn’t remove the insulation entirely.

Allow your magnet to cling to the head of the screw in the end of the dowel, as in Figure N. Now for the big moment. Clip an LED between the patch cords, and push the dowel rapidly in and out of the tube so that the magnet energizes the coil (Figure O). You should see the LED flash.

You’ve just demonstrated the way in which almost all electricity is generated in the world. The exceptions are electrochemical methods (batteries) and photovoltaic methods (solar panels). If no one had ever figured out that a magnet can induce electricity in a coil, civilization as we know it would not exist.

Do you see how this relates to the Drifting Magnet Mystery? You can think of the aluminum tube as being like a very long coil with only one turn. Do you suppose that a moving magnet induces current in the aluminum? Tape a couple of pieces of hookup wire to the tube, spaced about 1″ apart, and use a meter to measure millivolts. When you drop the magnet through the tube, or push it through, your meter should briefly measure a couple of millivolts.

In fact, the moving magnet induces an electrical flow known as eddy currents inside the aluminum. According to Lenz’s Law, these currents create their own magnetic fields, which oppose the magnet.

But this still isn’t the whole story. Remember that electricity creates a small amount of heat when it flows through a conductor. Where is this heat energy coming from? The magnet has potential energy, depending on its distance from the center of the Earth. As it loses potential energy by falling, the energy creates a tiny amount of heat in the aluminum. The first law of thermodynamics tells us that energy cannot be created or destroyed, and the Drifting Magnet Mystery demonstrates this.

You can make the Drifting Magnet Mystery into a magic trick. All you need is a piece of metal rod that has not been magnetized but looks identical to the neodymium magnet. McMaster-Carr, or many other online metals vendors, should sell you a 1-foot length of 11/16″ rod.

Cut a section of rod equal in length to the magnet using a hacksaw or (easier) an abrasive disc in a handheld circular saw. Rub with fine sandpaper, and polish it. I’ll call this the fake magnet.

Hold the real magnet in one hand, and keep the fake hidden in your other hand. Drop the real magnet through the tube a couple of times to demonstrate the drifting effect, then give the tube to a friend, quietly substituting the fake magnet. Needless to say, it will fall straight through.

I can think of a couple of enhancements. Figure P shows how I cut a slot in a piece of tube, using a hacksaw. This allows everyone to see the magnet as it descends.

Another option is buy a piece of ½” aluminum channel from a hardware store. You should find that the magnet fits into the channel with a tiny amount of clearance, so that it can roll without getting stuck, as in Figure Q. It won’t move as slowly as it does through a tube, but eddy currents still prevent it from rolling as fast as you would expect.

If anyone asks you how this trick works, you can demonstrate the coil and the LED. Alternatively, just tell them that it demonstrates the first law of thermodynamics. That’s what I told the audience at Maker Faire, although some people seemed a bit skeptical.

For more about magnetism and electricity, see my book Make: Electronics. And for a how-to guide on workshop tools, from saws to screwdrivers, you could look at my new book, Make: Tools.