In an electromagnet, the magnetic flux is concentrated in a ferromagnetic rod surrounded by a current-carrying coil.
Any electric current, or movement of charge carriers, produces a magnetic field. This field can become intense in a tightly coiled wire that has many turns and carries a large current. When a ferromagnetic core is placed inside the coil, the magnetic lines of flux are concentrated in the core, and the field strength in and near the core can become tremendous. This is the principle of an electromagnet (above figure). Electromagnets are almost always cylindrical in shape. Sometimes the cylinder is long and thin; in other cases it is short and fat. But whatever the ratio of diameter to length for the core, the principle is the same: the magnetic field produced by the current results in magnetization of the core.
You can build a dc electromagnet by taking a large bolt, such as a stove bolt, and wrapping a few dozen or a few hundred turns of wire around it. These items are available in any good hardware store. Be sure the bolt is made of ferromagnetic material. (If a permanent magnet sticks to the bolt, the bolt is ferromagnetic.) Ideally, the bolt should be at least 1 cm (approximately 3⁄8 in) in diameter and several inches long. You must use insulated wire, preferably made of solid, soft copper. “Bell wire” works well. Be sure all the wire turns go in the same direction. A large 6-V lantern battery can provide plenty of current to work the electromagnet. Never leave the coil connected to the battery for more than a few seconds at a time. And never use a car battery for this experiment! The acid can boil out of this type of battery, because the electromagnet places a heavy load on it.
Direct-current electromagnets have defined north and south poles, just like permanent magnets. The main difference is that an electromagnet can get much stronger than any permanent magnet. You will see evidence of this if you do the preceding experiment with a large enough bolt and enough turns of wire.
Do you get the idea that an electromagnet can be made far stronger if, rather than using a lantern battery for the current source, you plug the wires into a wall outlet? In theory, this is true. In prac- tice, you’ll blow the fuse or circuit breaker. Do not try this! The electrical circuits in some buildings are not adequately protected and it can create a fire hazard. Also, you can get a lethal shock from the utility mains.
Some electromagnets use ac, and these magnets will stick to ferromagnetic objects. But the polarity of the magnetic field reverses every time the direction of the current reverses. With conventional household ac in the United States, there are 120 fluctuations, or 60 complete north-to-south-to-north polarity changes (above figure), per second. If a permanent magnet, or a dc electromagnet, is brought near either “pole” of an ac electromagnet, there is no net force because the poles are alike half the time and opposite half the time, producing an equal amount of attractive and repulsive force. But if a piece of iron or steel is brought near a strong ac electromagnet, watch out! The attractive force will be powerful.