Magnetism

English

We have all played with magnets. A pair of magnets by itself makes a wonderful toy. Today's magnets are even better than the best ones I remember playing with as a child. At toy stores and Radio Shack you can get flexible magnetic strips of plastic that can be cut into shapes with scissors. You can also get cheap and brittle ceramic magnets, stronger Alnico magnets, and even the new super strong rare-earth magnets. These are made of neodymium-iron-boron or samarium-cobalt, and are very powerful.

Through mail-order surplus houses you can get large neodymium-iron-boron magnets of incredible strength. For about five dollars each you can get magnets that will hold paperback books onto your refrigerator, or drag each other around a two-inch thick table, one on top of the table and one hidden underneath. I once entertained my guests and several waiters at a restaurant by mysteriously moving the stainless flatware around the table. People are not used to the effects of powerful magnets. They are amazed even when they can see what you are doing.

Because of their high strength-to-weight ratios, neodymium-iron-boron magnets seem to be little affected by gravity. Small ones can be placed on either side of a nose and will stay there until the wearer laughs so hard they slide upwards against gravity and snap together. Temporary earrings are also popular. Handle larger magnets with care, since they will pinch hard enough to cause blisters if they are separated only by small bits of skin. They are also easily capable of erasing the magnetically stored information on credit cards, computer floppy disks, and cassette tapes, so take care when selecting a pocket for them.

If you are showing magnets to a very young person for the first time, there are several tricks you will not want to forget. Use some large cheap ceramic magnets from Radio Shack or a toy store so they will not be easily lost or swallowed. Show how they attract and repel each other. At Radio Shack you can get some donut-shaped magnets that can slide over a pencil. The child can stack several so they each repel, forming a magic spring.

Put some cheap magnets into some clear plastic bags and drag the bags through some playground sand, or the sand at a beach. Show how the magnets attract the black iron ore out of the sand. Show how to sprinkle the ore on paper with a magnet underneath, to create arcing lines of powder that trace out the lines of force of the magnet. (The plastic bags keep the ore off the magnets, keeping them clean and making it easier to remove the powder. If the powder does get onto the magnets, use some sticky tape to remove it.)

A magnetic compass placed near the arcs of powder will align itself with them, following their curve as you move it around. Several magnets under the paper will create interesting shapes in the iron ore above. Paper clips, pins, keychains, and other bits of ferrous metal will alter the shape of the ore as they become temporary magnets under the influence of the permanent magnets.

Stroke a bit of iron or steel (such as a pin or a screwdriver) across the magnet. Show how this makes a new magnet, and how the new magnet can attract other bits of steel or the iron ore. By placing the new magnet under the paper, you can show with the ore that this magnet is not as powerful as the original. Heat up the pin or screwdriver to show how heat destroys the magnetism.

Place two magnets under the paper, separated by a paper clip or other small bit of iron. Make sure each magnet touches the paper clip. Now when you sprinkle the ore on the paper, it forms a much smaller arc of powder. This shows the effect of a magnetic flux concentrator (the paper clip) which narrows (and thus strengthens) the force between the two magnets.

These simple observations form the backdrop for the rest of this section. There we will show some effects that are not so simple or obvious, and yet are simple extensions of the properties of magnets that we all learned as children.