You learned in Section 3 that a current in a wire creates a magnetic field around the wire. By turning the current on and off, you can turn the magnetic field on and off. So by using an electric current to create a magnet, you produce a magnetic field that you can control.

The magnetic field around a current-carrying wire forms a cylinder around the wire. If the wire is twisted into a loop, the magnetic field lines become bunched up inside the loop. You can see this by looking at the iron filings in the photo below.

The strength of the magnetic field increases as the number of loops is increased. If the wire is bent into a second loop, the concentration of magnetic field lines within the loop is twice as great.

By winding a current-carrying wire into a coil you have strengthened the magnetic field in the center of the coil. The two ends of the coil act like poles. The iron filings around the loops of wire in the photo below line up much as they would around a bar magnet.

A current-carrying coil of wire with many loops is called a solenoid. A solenoid creates a magnetic field that can be turned on and off by switching the current on and off. The north and south poles change with the direction of the current.

If a ferromagnetic material such as iron is placed inside a solenoid, the magnetic field is increased. For example, when iron is placed within the solenoid’s magnetic field, it becomes a magnet as well. A solenoid with a ferromagnetic core is called an electromagnet.

The temporary magnetic field of an electromagnet is produced by the current in the wire and the magnetized core. The overall magnetic field can be hundreds or thousands of times stronger than the magnetic field produced by the current alone. An electromagnet is a strong magnet that can be turned on and off.

A solenoid is used to ring an alarm bell. When current flows through the circuit, the coil acts as a magnet. The strip of iron on the spring is attracted to the electromagnet and the clapper strikes the bell. At the same time, the spring opens the circuit and stops the current. The spring returns the clapper to its resting position.

There are a number of ways you can increase the strength of an electromagnet’s field. You can increase the current in the solenoid. You can add more loops of wire to the solenoid. You can wind the coils of the solenoid closer together. Also, you can increase the strength of an electromagnet by using a stronger ferromagnetic material for the core.

When you record information on audiotapes, videotapes, computer disks, or credit cards, you are using electromagnets. When you talk into a microphone, the vibrations of your voice are changed into an electric current that varies with your voice. That current is used in an electromagnet in the recording head of the tape recorder to produce a magnetic field. Since the current changes, the magnetic field of the electromagnet changes as well.

A recording tape consists of a plastic ribbon coated with a thin layer of metal powder. The metal particles of the powder are magnetized by the magnetic field of the electromagnet in the recording head. As the tape moves past the electromagnet, the metal particles are magnetized more or less by the electromagnet.