Leader Resource 1: Super-easy Supermagnet Compass
By Windell H. Oskay, from the Evil Mad Scientist website, used with permission.
This compass requires a rare earth magnet, preferably a neodymium disk magnet. The magnet used in this example is part number ZD4 from K and J Magnets, Inc. It is one inch in diameter by a quarter-inch thick, and made of N42 grade (strong) neodymium-iron-boron, and costs about $5. Note: The magnet shown on the web page is very strong, and should not be used by foruth and fifth graders without supervision. On its own, it is perfectly safe, even for youngsters. It becomes hazardous when there are ferromagnetic materials nearby. Keep scissors somewhere else. Even worse, if you get two of these magnets close together, they can leap at each other with violent force. A small finger caught between them could get broken.
When I was a kid, I read in a science book about how to make a directional compass. You magnetize a sewing needle and balance it on a cork floating in a bowl of water. Even today, this is the standard story. For example, the website How Stuff Works still says that this is how to make a compass. (There are a lot of otherexamples, too.) It turns out that it's a whole lot easier than that. All you need is a really good magnet.
I stumbled across this quite by accident last week. For reasons that are — believe it or not — genuinely not related to compasses, I was attempting to balance a magnet on a little foam tray floating in a pan of water. However, the magnet wasn't behaving itself. It kept pulling its little boat to the edge of the pan of water. None-too-careful inspection of the situation revealed that there was a ten-inch, steel chef's knife sitting on the kitchen counter where I had set the pan of water. (Doh! If you've spent any time playing with strong magnets, you may also have had some of those close calls where your magnet and the nearest sharp knife go flying at each other. It only takes a couple of those experiences to give you a sinking feeling every time that you see a magnet and a ferromagnetic pointy thing within a foot of each other.) Moving the knife away stopped the magnet from drifting to the edge, but it still was turning on its own. Of course, that's when it dawned on me that I was looking at . . . a compass!
I had read a few times about making a compass with a supermagnet. I had seen it on Bill Beaty's Science Hobbyist site, as well as at Forcefield. Come to think of it, I had actually once tried one variation of this, which is to hang neodymium magnets on a string and watch them turn to align themselves. It works, but a string is actually a poor torsion bearing and the result is roughly as uninspiring as the floating-sewing-needle version. Since I myself was trained in the slowly moving-magnetized-sewing-needle school of compass design, I didn't think that making compasses was sufficiently interesting to spend more time on. But, that's the difference between seeing it and just reading about it.
Ready to build one? Let's get started:
The first thing that you will need is a rare earth magnet, preferably a neodymium disk magnet.
Last updated on Thursday, June 12, 2008.
