DIRECT-CURRENT TYPES

You can build a dc electromagnet by taking a large iron or steel bolt (such as a stove bolt) and wrapping a couple of hundred turns of wire around it. These items are available in almost any 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 38 inch in diameter and several inches long. You must use insulated or enameled wire, preferably made of solid, soft copper. “Bell wire” works well.
Be sure that all the wire turns go in the same direction. A large 6-V “lantern battery” can provide plenty of dc to operate the electromagnet. Never leave the coil connected to the battery for more than a few seconds at a time. And do not—repeat, do not—use an automotive battery for this experiment. The near-short-circuit produced by an electromagnet can cause the acid from such a battery to violently boil out, and this acid is dangerous stuff.

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 should see evidence of this if you do the preceding experiment with a large enough bolt and enough turns of wire. Another difference between an electromagnet and a permanent magnet is the fact that in an electromagnet, the magnetic field exists only as long as the coil carries current. When the power source is removed, the magnetic field collapses. In some cases, a small amount of residual magnetism remains in the core, but this is much weaker than the magnetism generated when current flows in the coil.