medium is saturated by applying a unidirectional field (i.e., DC powered or by employing a permanent magnet). A degausser is a device that can generate a magnetic field for degaussing magnetic storage media. The magnetic field needed for degaussing magnetic data storage media is a powerful one that normal magnets cannot easily achieve and maintain. Irreversible damage to some media types Many forms of generic magnetic storage media can be reused after degaussing, including reel-to-reel audio tape, VHS videocassettes, and floppy disks. These older media types are simply a raw medium which are overwritten with fresh new patterns, created by fixed-alignment read/write heads. For certain forms of computer data storage, however, such as modern hard disk drives and some tape drives, degaussing renders the magnetic media completely unusable and damages the storage system. This is due to the devices having an infinitely variable read/write head positioning mechanism which relies on special servo control data (e.g. Gray Code) that is meant to be permanently recorded onto the magnetic media. This servo data is written onto the media a single time at the factory using special-purpose servo writing hardware. The servo patterns are normally never overwritten by the device for any reason and are used to precisely position the read/write heads over data tracks on the media, to compensate for sudden jarring device movements, thermal expansion, or changes in orientation. Degaussing indiscriminately removes not only the stored data but also the servo control data, and without the servo data the device is no longer able to determine where data is to be read or written on the magnetic medium. The servo data must be rewritten to become usable again; with modern hard drives, this is generally not possible without manufacturer-specific and often model-specific service equipment. Audio tape recorders In reel-to-reel and compact cassette audio tape recorders, remnant magnetic fields will over time gather on metal parts such as guide posts tape heads. These are points that come into contact with the magnetic tape. The remnant fields can cause an increase in audible background noise during playback. Cheap, handheld consumer degaussers can significantly reduce this effect. Types of degaussers Degaussers range in size from small ones used in offices for erasing magnetic data storage devices to industrial-size degaussers for use on piping, ships, submarines, and other large-sized items, equipment to vehicles. Rating and categorizing degaussers depends on the strength of the magnetic field