method of preserving sounds, pictures, and data in the form of electrical signals through the selective magnetization of portions of a magnetic material. The principle of magnetic recording was first demonstrated by the Danish engineer Valdemar Poulsen (Poulsen, Valdemar) in 1900, when he introduced a machine called the telegraphone that recorded speech magnetically on steel wire.

In the years following Poulsen's invention, devices using a wide variety of magnetic recording mediums have been developed by researchers in Germany, Great Britain, and the United States. Principal among them are magnetic tape and disk recorders, which are used not only to reproduce audio and video signals but also to store computer data and measurements from instruments employed in scientific and medical research. Other significant magnetic recording devices include magnetic drum, core, and bubble units designed specifically to provide auxiliary data storage for computer systems.

Magnetic tape devices. Magnetic tape provides a compact, economical means of preserving and reproducing varied forms of information. Recordings on tape can be played back immediately and are easily erased, permitting the tape to be reused many times without a loss in quality of recording. For these reasons, tape is the most widely used of the various magnetic recording mediums. It consists of a narrow plastic ribbon coated with fine particles of iron oxide or other readily magnetizable material. In recording on tape, an electrical signal passes through a recording head as the tape is drawn past, leaving a magnetic imprint on the tape's surface. When the recorded tape is drawn past the playback or reproducing head, a signal is induced that is the equivalent of the recorded signal. This signal is amplified to the intensity appropriate to the output equipment.

Tape speeds for sound recording vary from less than 2 inches (5 centimetres) per second to as much as 15 in. (37.5 cm) per second. Video signals occupy a much wider bandwidth than do audio signals and require a much higher relative speed between the tape and the head. Data recording requires even greater speeds. The tape transport of a data-storage unit of a high-performance digital computer, for example, must be able to move the tape past the head at a rate of 200 in. (500 cm) per second.

Magnetic tape was initially designed for sound recording. German engineers developed an audio tape recording machine called the magnetophone during World War II. U.S. and British researchers adopted the basic design of this device to create a magnetic tape recorder capable of high-quality sound reproduction in the late 1940s. Within a decade magnetic tape supplanted phonograph records for radio music programming. Prerecorded tapes in the form of cartridges and cassettes for sound systems in homes and automobiles were in widespread use by the late 1960s.

Related to the audio cassette recorder is a magnetic tape recording system that serves as a telephone answering device. Messages or instructions prerecorded on tape are reproduced automatically when a telephone user's number is dialed. The answering device then actuates the recording head, which records any messages that the caller wishes to leave.

In 1956 Charles P. Ginsburg and Ray Dolby of Ampex Corporation, a U.S. electronics firm, developed the first practical videotape recorder (video tape recorder). Their machine revolutionized television broadcasting; recorded shows virtually replaced live telecasts with a few exceptions, such as coverage of sports events. Almost all programs are videotaped during their original telecasts, and individual broadcasters then rerun the shows at times most suitable for their own viewers. An increasing number of videotape recorders are used for recording television broadcasts received in private homes. Many such units can produce home movies if connected to an accessory video camera. Commercially produced video cassettes of popular motion pictures also can be played on these recorders. See also videotape recorder (video tape recorder).

Magnetic tape was introduced as a data-storage medium in 1951, when it was used in the auxiliary memory of UNIVAC I, the first digital computer produced for commercial use. For about the next 10 years nearly all computers employed magnetic tape storage units. By the 1960s, however, magnetic disk and magnetic drum auxiliary memories began replacing the tape units in large-scale scientific and business data-processing systems that require extremely fast retrieval of stored information and programs. Magnetic tape devices, particularly those using cassettes, continue to be employed as a principal form of auxiliary memory in general-purpose minicomputers and microcomputers because of their low cost and great storage capacity. About 48,000 bits of information can be stored on one inch of tape.

Magnetic tape recorders have also been widely used to record measurements directly from laboratory instruments and detection devices carried aboard planetary probes. The readings are converted into electrical signals and recorded on tape, which can be played back by researchers for detailed analysis and comparison.

Magnetic disks are flat circular plates of metal or plastic, coated on both sides with iron oxide. Input signals, which may be audio, video, or data, are recorded on the surface of a disk as magnetic patterns or spots in spiral tracks by a recording head while the disk is rotated by a drive unit. The heads, which are also used to read the magnetic impressions on the disk, can be positioned anywhere on the disk with great precision. For computer data-storage applications, a collection of as many as 20 disks (called a disk pack) is mounted vertically on the spindle of a drive unit. The drive unit is equipped with multiple reading/writing heads.

These features give magnetic disk devices an advantage over tape recorders. A disk unit has the ability to read any given segment of an audio or video recording or block of data without having to pass over a major portion of its content sequentially; locating desired information on tape may take many minutes. In a magnetic disk unit, direct access to a precise track on a specific disk reduces retrieval time to a fraction of a second.

Magnetic disk technology was applied to data storage in 1962. The random accessibility of data stored in disk units made these devices particularly suitable for use as auxiliary memories in high-speed computer systems. Small, flexible plastic disks called floppy disks were developed during the 1970s. Although floppy disks cannot store as much information as conventional disks or retrieve data as rapidly, they are adequate for applications such as those involving minicomputers and microcomputers where low cost and ease of use are of primary importance.

Magnetic disk recording has various other uses. Office dictating machines and transcribing units utilize the process for storing spoken messages for later use. Magnetic disk technology has also facilitated and improved a method known as “instant replay” that is widely used in live telecasts, especially of sports events. This method involves the immediate re-showing of, for example, a crucial play in a football game during a live-action broadcast. Videotape recorders were initially used for instant replay, but they proved too cumbersome. In 1967 Ampex developed a special videodisk machine that made it possible to locate and replay a desired action in less than four seconds.

Such magnetic recording mediums as drums and ferrite cores have been used for data storage since the early 1950s. A more recent development is the magnetic bubble memory devised in the late 1970s at Bell Telephone Laboratories.

Auxiliary computer memories using a magnetic drum operate somewhat like tape and disk units. They store data in the form of magnetized spots in adjacent circular tracks on the surface of a metal cylinder. A single drum may carry from one to 200 tracks. Data are recorded and read by heads positioned near the surface of the drum as the drum rotates at about 3,000 revolutions per minute. Drums provide rapid, random access to stored information. They are able to retrieve information faster than tape and disk units, but cannot store as much data as either of them.

Core memories use hundreds of thousands of magnetizable ferrite cores that resemble tiny doughnuts. Through each of the cores run two or more wires, which carry electrical currents that magnetize the cores in either a clockwise or counterclockwise direction. Cores magnetized in one direction are said to represent 0, and those in the opposite direction to represent 1. The 0 and 1 correspond to the digits of the binary system, the basis for digital computer operations. Data are stored by magnetizing an array of cores in a particular combination of 0s and 1s. Core storage units allow extremely fast, random access to stored information. Unlike other magnetic memory devices that have to wait for tape reels to unwind or drums to rotate, retrieval is performed simply by sending electrical pulses to the specific array of cores holding the desired data. The pulses reverse the direction of magnetization in the cores, which includes output signals corresponding to the stored data.

The magnetic bubble memory is more economical to operate than mechanical tape, disk, or drum units and is considerably more compact. The device consists of a chip of synthetic garnet about the size of a matchbook. It stores data in tiny cylindrically shaped magnetic domains called bubbles that appear and disappear under the control of an electromagnetic field. The presence and absence of the bubbles represent information in binary form in much the same way as do the two states of magnetic cores. As each tiny garnet chip accommodates hundreds of thousands of binary digits, enormous amounts of data can be stored in a memory unit comprised of a small stack of these chips.