Hardware Reference
In-Depth Information
rotating electromagnets on a shaft past coils of stationary wire conductors, which consequently
generates large amounts of electrical current in those conductors. Because electromagnetism works
two ways, a motor can become a generator, and vice versa. When applied to magnetic storage
devices, this two-way operation of electromagnetism makes it possible to record data on a disk and
read that data back later. When recording, the head changes electrical impulses to magnetic fields,
and when reading, the head changes magnetic fields back into electrical impulses.
The read/write heads in a magnetic storage device are U-shaped pieces of conductive material, with
the ends of the U situated directly above (or next to) the surface of the actual data storage medium.
The U-shaped head is wrapped with coils or windings of conductive wire, through which an electric
current can flow (see
Figure 8.3
)
. When the drive logic passes a current through these coils, it
generates a magnetic field in the drive head. Reversing the polarity of the electric current also causes
the polarity of the generated field to change. In essence, the heads are electromagnets whose voltage
can be switched in polarity quickly.
Figure 8.3. A magnetic read/write head.
The disk or tape that constitutes the actual storage medium consists of some form of substrate material
(such as Mylar for floppy disks, or aluminum or glass for hard disks) on which a layer of
magnetizable material has been deposited. This material usually is a form of iron oxide with various
other elements added. Each of the individual magnetic particles on the storage medium has its own
magnetic field. When the medium is blank, the polarities of those magnetic fields are normally in a
state of random disarray. Because the fields of the individual particles point in random directions,
each tiny magnetic field is canceled out by one that points in the opposite direction; the cumulative
effect of this is a surface with no observable field polarity. With many randomly oriented fields, the
net effect is no observable unified field or polarity.
When a drive's read/write head generates a magnetic field (as when writing to a disk), the field
jumps the gap between the ends of the U shape. Because a magnetic field passes through a conductor
much more easily than through the air, the field bends outward from the gap in the head and actually
uses the adjacent storage medium as the path of least resistance to the other side of the gap. As the
field passes through the medium directly under the gap, it polarizes the magnetic particles it passes
through so they are aligned with the field. The field's polarity or direction—and, therefore, the
polarity or direction of the field induced in the magnetic medium—is based on the direction of the
flow of electric current through the coils. A change in the direction of the current flow produces a
change in the direction of the magnetic field. During the development of magnetic storage, the distance
between the read/write head and the media has decreased dramatically. This enables the gap to be
smaller and makes the size of the recorded magnetic domain smaller. The smaller the recorded
