The enduring attraction of magnets

What do all these items have in common: doorbells, compasses, electric guitars, computer disks, VCRs, credit cards, airbags, and refrigerators?

Answer: They all need magnets to work.

You may play with toy magnets of your own, small bar or horseshoe magnets, or even ones that come in strips that you can cut with scissors. But you might be surprised at how many magnets there are around you, and how they are used.

Do you have magnets on your refrigerator to hold notes or lists? Magnets may also be used to keep the fridge door closed. Or the doors on the kitchen cupboards. Electric can-openers use a magnet to lift the lid away from the can.

How do videotapes, audio tapes, and computer disks store information? Billions of tiny magnets on the tape or disk turn in one direction or another in patterns that can be decoded by the VCR, tape recorder, or computer disk reader.

When a clerk swipes a credit card through a reader, the machine quickly reads information stored in tiny magnets on a wide black strip on the back of the card. This is why tapes, computer disks, and credit cards should never be placed near magnets. Their magnetic information might be rearranged and destroyed.

These tiny magnets can be moved around on disks and tapes because of an important property of all magnets. Every magnet has two poles, called north and south poles. Put two magnets near each other and they will either pull toward or push away from each other. Poles are attracted to their opposites. A north pole will pull toward a south pole. Two north poles or two south poles push away from each other.

When a computer stores information on a disk or hard drive, or when you use a cassette recorder to tape a song, both use magnets to change the way billions of tiny magnets (actually powdered metal) are positioned. (The diagram shows how sound is recorded by a tape recorder. The same principle is used to store images on a videocassette or data on a floppy disk.)

Another very useful function of magnets is in electric motors. These motors could not work without magnets. When electric current goes through coils made of certain metals, magnetic fields are created. A metal bar inside the coil becomes a very strong magnet, called on electromagnet. By changing the strength of these fields, one magnet can make another one move or spin. This movement can be used to spin socks in the clothes washer, move your remote-controlled toy car, or power a locomotive.

Electricity can be used to create very powerful magnets. Have you ever seen a crane pick up a car using a large magnet? Electric current turns the large metal disk at the end of the crane into a powerful magnet so that it can pick up the car. To set the car down, the crane operator switches off the electricity. Then the disk is no longer a magnet and the car drops.

ELECTROMAGNETS are called temporary magnets, because they stay magnetic only while electricity is flowing. Permanent magnets are magnetic without electricity, though they can lose their magnetism by being heated or even dropped.

Magnetite is a naturally magnetic rock. These rocks, also called lodestones, were the first magnets humans encountered. People used to think that lightning made the rock magnetic, since electricity and magnetism are so closely related. Now scientists say that the crystalline structure of the mineral - an important iron ore - creates its magnetism.

Magnets only attract certain metals, such as iron, cobalt, and nickel. This comes in handy in many ways. Recycling plants use magnets to separate metal cans from other garbage. Workers with special magnetic grippers on their hands and feet can walk up the sides of metal tanks and bridges. Maglev trains (for "magnetic levitation") are lifted off the tracks and moved forward by powerful electromagnets. Magnets vibrate the cones of speakers attached to stereos or electric guitars. The cones vibrate the air and create sound.

Magnetic sensors tell a car's airbags when to deploy. Electromagnets pull the strikers that make your doorbell ring. Some popular magic tricks are done with magnets. Scientists think animals and birds may use the earth's magnetic field to direct them when they migrate, and lead homing pigeons home.

Electricity is generated by magnets rotated to create electric fields. So you can thank magnets for the electricity that lights and powers your home. From small toys to giant generators to the earth itself, magnets are at work all around you.