An earbud contains a membrane made out of thin plastic. In the center of the membrane is a coil of wire called the voice coil. The coil fits in a cylindrical slot in a permanent magnet. The center rod of the magnet is one pole, the outside tube is the other, resulting in a magnetic field perpendicular to the wire. When there is a current through the wire the Lorentz force on the wire pushes the membrane in and out. The membrane exerts forces on the air molecules producing the longitudinal waves constituting sound. (Refer to the Sound chapter for more information.)

Magnets are used in the compact motors that turn the disks in the CD or DVD drive and that move the laser that reads the disk to the correct position. Motors rotate the disks in a hard drive. The arm on which the read/write head is mounted is rotated to the correct portion of the hard drive disk has a coil of wire on it in a magnetic field. When there is a current through the wire the force moves the arm to the correct position.

The disk itself is often made of aluminum coated with an extremely thin (10-20 nanometers) film of magnetic material that is divided into sub-micrometer thick regions that are perpendicular to the surface of the disk. Each region is magnetized one way to represent a “1” and another way to represent at “0.” A tiny coil in the read/write head carries the current that magnetizes the regions. The state of the magnet is read using mag-netorestriction where the magnetic field causes a change in resistance of a very thin wire in a coil in the read/write head.