Datasheet

strength required to switch the poles is the crystal’s “coercivity.” Oberlin Smith realized he

could draw magnetic patterns in material coated with millions of tiny magnetic particles by

arranging their poles under a coercive force. A magnetic “writer” creating varying

magnetic fields will leave a record of those variations in magnetic material pulled past the

writer, just as a moving pen writes on paper pulled under it in a lie detector. This remains

the basic design used today.

The writers are electromagnet heads that have a gap between their poles. As varying

amounts of current run through the head, a varying magnetic field spreads from the gap.

When blank magnetic tape is pulled across the head during recording, the billions of

microscopic magnetic particles that make up the pigment coating

magnetically rearrange

themselves to match the magnetic field above them as they pass below the gap in the

head (Figure 1). The particles do not move physically; they are frozen in position within

the coating. They merely rearrange the polarity of their magnetic poles to simulate the

field under the gap as they passed below as if the head were “magnetically

Magnetic “Printing”

Magnetic head

printing” a pattern in the pigment. When recorded tape is pulled across the head, the

magnetic pattern on the tape passing below the gap creates a small varying current in the

head that matches the original current that made the recording. The head now “reads” the

pattern on the magnetic coating. The pattern can be rewritten by recording over the

coating again. Erasing the pattern is easy: the head creates magnetic fields that change

too fast for the particles to react uniformly. Half the particles in any area are magnetized

The term “pigment” is used for tape-based media because the coating on the plastic film substrate

acts as “magnetic paint.” Magnetic particles make up 40-50% of the coating thickness and the rest

is resins, dispersing agents, and other chemicals. Gamma ferric oxide crystals were the first

common magnetic particles, and they gave the tape a distinctive color. The same type of oxide is

also used as the pigment in inexpensive, stable paint used for barns and railroad boxcars. That’s

why both are often a brownish red.

Substrate

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Recorded pattern

As the magnetic coating moves from right to left, the magnetic pulse

focused by the gap in the magnetic head are recorded in the coating.

Blank or erased track has no patte

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gap

Plastic film for tape or diskettes; glass or aluminum platter for hard drives.

Figure 1