3. 0 PRINCIPLES OF STORING INFORM TION
3.1 M GNETIC THEORY
magnetic material is one that can be polarized or partially polarized when
placed under the influence of a magnetic field. For every magnetic material, a B-H
curve can be plotted showing the resultant flux densities in the material when placed
in the influence of a varying magnetizing force. If a magnetizing force of ampere-
turns (NI) is slowly increased in the positive direction, the resultant flux density in
a magnetic medium would at first increase rapidly and then slowly attain a steady value
of flux as shown in Figure 2. The phenomenon of attaining a steady value of flux
density is designated as saturation of the magnetic material. When the ampere-turns
are slowly reduced, the flux density also decreases, but at a different rate. This
phenomenon is known as the hysteresis effect. When the magnetizing force is again
zero, the flux density is not equal to zero, but is equal to some positive value, shown
in Figure 2 as point . The amount of magnetic flux remaining when NI is equal to
zero, as denoted by the distance -O in Figure 2, is the residual magnetism in the
magnetic circuit.
If the ampere-turns are reversed by a reversal in current and the current mag
nitude is increased in the negative direction until saturation is again reached and then
once again returned to zero, a negative resultant flux remains (point B, Figure 2).
Thus, by employing suitable circuit techniques, a flux pattern of either positive or
negative polarity can be impressed on a magnetic material.
The schematic sketch in Figure 3a illustrates the storing of bits of information.
The magnetic circuit consists of the laminated ring, the air gap, a shim and the mag
netic oxide on the tape. The ring is made of mu metal. Mu metal has a large mu *
and low retentivity **. The gap has a mu of 1. The oxide has a mu of approximately
7-9, but has a high retentivity in comparison to the rest of the circuit. The half
mil gap is used to force the magnetic lines of force away from the head and into the
magnetic material on the tape.
♦Permeability is the measure of the ability of a magnetic material to con
duct lines of flux. It is designated by the Greek letter mu (u) and is numerically
equal to the ratio of the flux density to the magnetizing force, u = B *■ H. The
mu of air = 1.
♦♦Retentivity is the capacity of a magnetic material to retain magnetism
after the magnetizing force has been removed.
3.2 NRZI SYSTEM
The NRZI (non-return to zero) system of recording binary information on tape is
a system in which the tape is continuously saturated in either the plus or minus direc
tion. change in saturation polarity is called a "one." No change within a given
period of time is called a "zero."
The following is a simplified description of how the NRZI systems function.
The process by which information is stored is called "writing" and the process of
detecting stored information is called "reading."
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