Magnetic recording

A recording head converts an electrical signal into a magnetic field that changes the state of a nearby recording medium. In tape, magnetic particles in a coating are aligned to encode a signal; in disk storage, information is represented by patterns of magnetization in a thin magnetic layer. Playback reverses the process: a read head senses magnetic variations and converts them into an electrical signal. In digital systems, the patterns are interpreted as encoded bits, rather than as a direct representation of sound or images.

Magnetic tape is a flexible strip with a substrate and a recording layer containing magnetic particles held in a binder. Tape moves past a recording or playback head, making it suitable for sequentially accessing recorded material. Hard disk drives use rigid, rotating platters coated with magnetic material. A movable head writes or reads data at positions on the platter, allowing data to be accessed without winding through the entire medium. The specific magnetic materials and recording formats have changed as engineers have sought greater capacity, reliability and speed.

IBM introduced the 305 RAMAC system in 1956, using the Model 350 disk storage unit for random-access storage. The Computer History Museum records that its first commercial Model 350 shipment went to Zellerbach Paper in June 1956. It stored data on fifty 24-inch disks and held five million six-bit characters, equivalent to about 3.75 megabytes. Later development increased storage density and changed how information was magnetized: perpendicular magnetic recording, in which recorded magnetization is oriented perpendicular to the medium’s plane, became a commercial hard-disk technology in the 2000s after earlier research and development.

Magnetic recording depends on the recording pattern remaining stable and distinguishable from neighboring patterns. In hard drives, Yale materials-science teaching material explains that very small or closely packed domains can become harder to read and may be affected by heat, noise or neighboring magnetic fields. Magnetic tape also faces physical and chemical deterioration: preservation guidance from the Northeast Document Conservation Center describes binder degradation and loss or alteration of magnetic information as concerns for collections. Appropriate storage, handling and, when needed, professional preservation or migration help protect recordings.