Alternating and direct currents are used in demagnetizing aircraft parts after magnetic particle inspection. Although
direct current can be used for demagnetization, alternating current demagnetization has been found to be more
convenient. Since alternating current does not penetrate very deeply below the surface of magnetic materials, some
parts may be difficult to demagnetize completely using alternating current. This is particularly true with large heavy
parts, and may also be the case with parts of unusual shape. Direct current can be used to demagnetize if there is
provision for current decay or reduction and a means for reversing the direction of the current. Demagnetization
accomplished in this manner with direct current is the most complete and effective possible.
To demagnetize with direct current, the part is placed in a coil connected to a source of direct current. The current is
adjusted to a value at least as great as that used to magnetize the part and a shot of current is given at this initial value.
The direction of the current is then reversed the value reduced, and a shot of current given at the new value. This
process of reversing and reducing the current is continued until a very low value is reached. The part is now effectively
Paragraph 18.104.22.168 indicates that parts with a circular field do not have magnetic poles. This lack of measurable poles,
providing no discontinuities are present, makes it impossible to check the magnitude of residual circular magnetization
with the conventional residual field indicator. A common and recommended practice on aircraft parts is to magnetize
the part longitudinally after it has been circularly magnetized. The difficult to measure circular field is then replaced
by an easy to measure longitudinal field.
Separate AC Coil. The most common and convenient method of demagnetizing small to moderate sized parts is by
passing them through an open tunnel-type coil, through which alternating current at line frequency (usually 50 to 60
hertz) is passing. Another practice is to pass the 50 or 60 hertz AC through a coil with the part inside the coil, and
gradually reduce the current to zero. In the first case, the reduction of the strength of the reversing field is obtained by
withdrawal of the part axially from the coil (or the coil from the part) and for some distance beyond the end of the coil
(or part) along that axial line. In the second case, the gradual decay of the current in the coil accomplishes the same
results. This method of demagnetization is particularly suitable for large numbers of relatively small parts.
Stationary MPI Unit.
Stationary magnetic particle testing equipment often has demagnetization capabilities. If so equipped, AC current may
be passed directly through the part or through the coil on the magnetizing unit. For demagnetization of parts, the
alternating current is reduced to zero automatically by built-in means of step-down switches or variable transformers
for older equipment, or solid state devices for newer equipment. The step-down feature permits the demagnetization of
parts without removal from the magnetizing equipment. This procedure is more effective on long, circularly
magnetized parts than the separate coil method, but does not overcome the lack of penetration due to skin effect unless
frequencies much lower than 60 hertz are used.
Demagnetizing by the direct current reversing step-down feature is essentially identical in principle to the AC method.
Modern stationary DC magnetizing equipment usually incorporates this capability. The use of DC current permits a
more even and complete penetration of even large cross sections. The DC current flows in one direction for a short
time, it then is slightly reduced in magnitude, and completely reversed in direction. The process of automatically
reversing and reducing the current is continued until the current reaches zero and the part is effectively demagnetized.
This method of demagnetizing is very effective although it does require large, heavy equipment. It is especially
effective in removing circular fields when the current can be passed through the part and works well with a central
conductor, when applicable. Small parts can be placed in a standard coil and larger parts can be cable-wrapped for
their full length, as induction loss is not present with DC.