The oldest head design is also the simplest conceptually. A ferrite head is a
U-shaped iron core wrapped with electrical windings to create the read/write head--almost
a classical electromagnet, but very small. (The name "ferrite" comes from the
iron of the core.) The result of this design is much like a child's U-shaped magnet, with
each end representing one of the poles, north and south. When writing, the current in the
coil creates a polarized magnetic field in the gap between the poles of the core, which
magnetizes the surface of the platter where the head is located. When the direction of the
current is reversed, the opposite polarity magnetic field is created. For reading, the
process is reversed: the head is passed over the magnetic fields and a current of one
direction or another is induced in the windings, depending on the polarity of the magnetic
field. See here for more general details on how the read/write heads work.
|
Extreme closeup view of a ferrite read/write head from
a mid-1980s Seagate ST-251, one of
the most popular drives of its era. The big black object is not actually the head, but the
slider.
The head is at the end of the slider, wrapped with the coil that magnetizes it for
writing, or is
magnetized during a read. If you look closely you can actually see the gap in the core,
though it is very small. The blue feed wire runs back to the circuits that control the
head. |
Ferrite heads suffer from being large and cumbersome, which means they must ride at a
relatively great distance from the platter and must have reasonably large and strong
magnetic fields. Their design prevents their use with modern, very-high-density hard disk
media, and they are now obsolete and no longer used. They are typically encountered in PC
hard disks under 50 MB in size.
Next: Metal-In-Gap (MIG) Heads
