The basic idea behind an electromagnet is extremely simple: by running electric current through a wire you can create a magnetic field. By using this simple principle you can create all sorts of things, including motors, solenoids, read/write heads for disk and tape drives (cassette tape players, VCRs, floppy and hard disks in your compter, etc.), speakers, and so on. In this edition of How Stuff Works you will learn how electromagnets work. You will also have a chance to try several experiments with an electromagnet that you create yourself.

Before talking about electromagnets, let's talk about normal "permanent" magnets like the ones you have on your refrigerator and that you played with as a kid. Since you have played with magnets you know that all magnets have 2 ends, normally marked North and South. You know that magnets attract things made of steel or iron. You also know about the fundamental law of all magnets: Opposites attract and likes repel. So if you have 2 bar magnets with their ends marked North and South, then the North end of one magnet will attract the South end of the other. On the other hand, the North end of one magnet will repel the North end of the other (and similarly South will repel South). An electromagnet is the same way, except it is "temporary" - the magnetic field only exists when electric current is flowing.

An electromagnet starts with a battery (or some other source of power) and a wire. What a battery produces is electrons. If you look at a battery, say at a normal D cell from a flashlight, you can see there are two ends marked plus (+) and minus (-). Electrons collect at the negative end of the battery, and, if you let them, they will gladly flow to the positive end. The way you "let them" flow is with a wire. If you attach a wire directly between the positive and negative terminals of a D cell, 3 things will happen:

1. Electrons will flow from the negative side of the battery to the positive side as fast as they can.
2. The battery will drain fairly quickly (in a matter of several minutes). For that reason it is generally not a good idea to connect the two terminals of a battery to one another directly. Normally you connect some kind of load in the middle of the wire so the electrons can do useful work. The load might be a motor, a light bulb, a radio or whatever.
3. A small magnetic field is generated in the wire. It is this small magnetic field that is the basis of an electromagnet.

You probably knew about items 1 and 2 in this list, but item 3 might be a surprise to you. Yet this definitely happens in all wires carrying electricity. You can prove it to yourself with the following experiment. For the experiment you will need:

• An AA, C or D cell (a normal flashlight battery)
• A piece of wire (if you have no wire around the house, go buy a spool of insulated thin copper wire down at the local Radio Shack or hardware store. 4-strand telephone wire is perfect - cut the outer plastic sheeth and you will find four perfect wires within).
• A compass

The figure below shows the shape of the magnetic field around the wire. In this figure, imagine that you have cut the wire and are looking at it end-on. The green circle in the figure is the cross-section of wire itself. A circular magnetic field develops around the wire, as shown by the circular lines. The field weakens as you move away from the wire (so the lines are farther apart as they get farther from the wire). You can see that the field is perpendicular to the wire and the field has a direction that depends on which direction of the current flowing in the wire. The compass needle aligns itself with this field (perpendicular to the wire). If you flip the battery around and repeat the experiment, you will see that the compass needle aligns itself in the opposite direction.

Experiments to try

• What is the magnetic power of a single coil wrapped around a nail? Of 10 turns of wire? Of 100 turns? Experiment with different numbers of turns and see what happens. One way to measure and compare a magnet's "strength" is to see how many staples it can pick up.
• What difference does voltage make in the strength of an electromagnet? If you hook two batteries in series to get 3 volts, what does that do to the strength of the magnet? [Please do not try any more than 6 volts, and please do not use anything other than flashlight batteries. Please do not try house current coming from the wall in your house, as it can kill you. Please do not try a car battery, as its current can kill you as well.]
• What is the difference between an iron and an aluminum core for the magnet? For example, roll some aluminum foil in a tight roll and use it as the core for your magnet in place of the nail. What happens? What if you use a plastic core, like a pen?
• A solenoid is another form of electromagnet. It is an electromagnetic tube generally used to move a piece of metal linearly. Find a drinking straw or an old pen (and remove the ink tube). Also find a small nail (or a straightened paperclip) that will slide inside the tube easily. Wrap 100 turns of wire around the tube. Place the nail or paperclip at one end of the coil and then connect the coil to the battery. Notice how the nail moves? Solenoids are used in all sorts of places, especially locks. If you car has power locks they may operate using a solenoid. Another common thing to do with a solenoid is to replace the nail with a thin cylindrical permanent magnet. Then you can move the magnet in and out by changing the direction of the magnetic field in the solenoid. [Please be careful if you try placing a magnet in your solenoid, as the magnet can shoot out].
• You can look at a wire's magnetic field with iron filings. Buy some iron filings, or find your own iron filings by running a magnet through playground or beach sand. Put a light dusting of filings on a sheet of paper and place the paper over a magnet. Tap the paper lightly and the filings will align with the magnetic field and let you see its shape.

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