It used bubble memory cartridges, a sort of non-mechanical magnetic storage system. The effect is used in reading heads of magnetic storage media (the "GMR effect"). As with all magnetic storage, it is vulnerable to magnetic fields. Disk density is a capacity designation on magnetic storage, usually floppy disks. Home computers have long used magnetic storage devices. Magnetic storage is "non-volatile". Most magnetic storage devices use error correction. Sequential access devices are usually a form of magnetic storage or optical storage. Magnetoresistive RAM stores data in magnetic storage elements called magnetic tunnel junctions (MTJs). Beckmann designed the I/O boards and controllers for peripheral magnetic storage. Computers and now most audio and video magnetic storage devices record digital data. Early magnetic storage devices were designed to record analog audio signals. Magnetic storage uses different patterns of magnetization on a magnetically coated surface to store information. Magnetic storage was known from World War II and computer data storage in the 1950s.