Magnetic Head Definition

A magnetic head, often referred to as a read/write head or just a head, is a high precision electromagnet that writes data to and reads data from magnetic storage media, such as hard disk drives (HDDs), floppy disks or magnetic tape. It accomplishes this by converting electrical variations into magnetic variations and converting magnetic variations in the magnetic media back into electrical signals, respectively.

Double-sided floppy disk drives have two heads, one for each side of the disk. HDDs contain multiple heads, usually one for each side of each platter. A platter is a thin, high precision disk that is coated on both sides with a high precision magnetic material and which is used to store data. Modern HDDs typically contain multiple platters, all of which are mounted on a single shaft, in order to maximize the data storage surface in a given volume of space.

Each head in a HDD is mounted on the end of a small arm that can rotate from its other end sufficiently that the head can hover over any of the tracks on the platter with which it is associated. A track is any of the concentric circles on the magnetic media on a disk or platter over which one head passes while the head is stationary but the disk is spinning. All of these arms are mechanically linked at the pivot end so that all of heads move in unison and are always over the same corresponding tracks (referred to as a cylinder) on their respective platters.

A distinguishing characteristic of modern HDD technology as compared with tape drives is that the heads do not make physical contact with the magnetic media while it is in motion but, rather, float above it. The heads were in direct contact with the magnetic material in the earliest HDDs, but this design was changed due to the unacceptable wear that occurred to both the delicate heads and the magnetic coating.

When the platters are at rest, the spring steel of the head arms causes them to press against the platters in order to provide protection in the event of mechanical shock. But as the platters accelerate to their high operating speed, this speed causes air within the HDD to flow under the arms and lift them off the surfaces of the platters, using the same principle of lift that operates on aircraft wings and enables airplanes to fly.

The gap between the heads and the platters, referred to as the floating height, flying height or head gap, is less than a millionth of an inch. It must be maintained for proper operation, and even very small particles of dust can interfere with it. Thus, HDDs are assembled in cleanrooms similar to those in which heads and semiconductor devices are produced.

A head crash is the situation in which one or more heads make physical contact with the magnetic coating on the platters while they are spinning. Although the platters usually have a protective layer on their surfaces that allows them to tolerate a certain amount of contact before it becomes a problem, head crashes can still result in loss of data, damage to the heads, damage to the magnetic coating, or all three. The most common causes are microscopic particles becoming stuck in the head gap and mechanical shock to the HDD while it is in operation.

Created April 23, 2006.
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