Magnetic storage medium used with computers. Floppy disks are made of flexible plastic coated with a magnetic material, and are enclosed in a hard plastic case. They are typically 3.5 in. (9 cm) in diameter. Data are arranged on their surfaces in concentric tracks. A disk is inserted in the computer's floppy disk drive, an assembly of magnetic heads and a mechanical device for rotating the disk for reading or writing purposes. A small electromagnet, called a magnetic head, writes a binary digit (1 or 0) onto the disk by magnetizing a tiny spot on the disk in different directions, and reads digits by detecting the magnetization direction of the spots. With the increasing use of e-mail attachments and other means to transfer files from computer to computer, the use of floppy disks has waned, though they are still widely used to keep second (backup) copies of valuable files.