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has become much more prevalent is magnetic storage. The origins of magnetic storage date back to 1878, when the principles were described by American engineer Oberlin Smith (1840–1926). The first working device, however, came 20 years later in 1898 and was built by Danish inventor Valdemar Poulsen (1869–1942). Poulsen's telegraphone was originally intended as a device to record telephone messages when the person receiving the call wasn't at home. He employed an electromagnet—that ubiquitous device we've already encountered in the telegraph—to record sound along a moving length of steel wire. The electromagnet magnetizes the wire proportional to the ups and downs of the waveform of the sound. The magnetized wire can then induce a current to the same degree as it's moved along the coils of wire in the electromagnet. The electromagnet used for storing and reading is known as a head, regardless of the type of magnetic medium it's used with.

In 1928, Austrian inventor Fritz Pfleumer patented a magnetic recording device based on long lengths of paper tape that had been coated with iron particles using a technology originally designed for creating metallic bands on cigarettes. The paper was soon replaced with a stronger cellulose acetate base, and one of the most enduring and well-known of all recording media was born. Reels of magnetic tape—now conveniently packaged in plastic cassettes—still provide an extremely popular medium for recording and playing back music and video.

The first commercial tape system for recording digital computer data was introduced by Remington Rand in 1950. At the time, a reel of half-inch tape could store a few megabytes of data. In the early days of home computers, people adapted common cassette tape recorders to save information. Small programs stored the contents of a block of memory to tape and later read it back from tape into memory. The first IBM PCs had a connector for cassette tape storage. Tape remains a popular medium today, particularly for long-term archiving. Tape, however, isn't an ideal medium because moving quickly to an arbitrary spot on the tape isn't possible. It's usually necessary to fast-forward or rewind, and that takes time.

A medium geometrically more conducive to fast access is the disk. The disk itself is spun around its center while one or more heads attached to arms can be moved from the outside of the disk to the inside. Any area on the disk can be accessed very quickly.

For recording sounds, the magnetic disk actually predates the magnetic tape. For storing computer data, however, the first disk drive was invented at IBM in 1956. The Random Access Method of Accounting and Control (RAMAC) contained 50 metal disks 2 feet in diameter and could store 5 megabytes of data.

Since then, disks have become much smaller and of higher capacity. Disks are generally categorized as floppy disks (also called diskettes) or hard disks (also called fixed disks). Floppy disks are single sheets of coated plastic inside a protective casing made of cardboard or (more recently) plastic. (A plastic casing prevents the diskette from bending, so the diskette is no longer quite as floppy as the older ones, but it's still referred to as a floppy disk.) Floppy disks must be physically inserted by a person into a floppy disk drive, which is the component attached to the computer that writes to and reads from the floppy disk. Early floppy disks were 8 inches in diameter. The first IBM PC used 5 ¼-inch floppy disks; today the most common format is 3.5 inches in diameter. That floppy disks can be removed from the disk drive allows them to be used for transferring data from one computer to another. Diskettes are also still an important distribution medium of commercial software.

A hard disk usually contains multiple metal disks permanently built into the drive. Hard disks are generally faster than floppy disks and can store more data. But the disks themselves can't be removed.

The surface of a disk is divided into concentric rings called tracks. Each track is divided like slices of a pie into sectors.