T.O. 33B-1-1
3-108
magnetic field. They are true indications but, since there is no unintentional interruption of the material, they do not
affect the usefulness of the part. It is important that the inspector know how and why these non-relevant indications are
formed and where they can occur.
3.7.5.2
Classes.
3.7.5.2.1
Magnetic Writing.
This is a condition caused by a piece of steel rubbing against another piece of steel that has been magnetized. Since
either or both pieces contain some residual magnetism the rubbing or touching creates magnetic poles at the points of
contact. These local magnetic poles are usually in the form of a line or scrawl, and for this reason the effect is referred
to as magnetic writing. In Figure 3-69 the part in the top view is magnetized with a circular field. If another part
made of magnetic material is rubbed against or comes into contact with the magnetized part, as in the second view, a
weak field will be induced into the smaller part. After the smaller part has been removed, the circular field in the
original part will be altered or distorted to some extent as shown in the bottom view. Since there is no force to change
the direction of the altered field, there will be some leakage at the point of distortion that will attract magnetic particles.
Figure 3-69. Creation of Magnetic Writing
3.7.5.2.2
Longitudinal Magnetization.
When a part is longitudinally magnetized in a coil, there are always magnetic poles at the ends of the piece. Magnetic
material such as chips or magnetic powder or paste will be attracted to these poles. The same situation occurs when a
yoke is used to create a magnetic field; poles are induced on the part in the areas where the yoke touches the part.
3.7.5.2.3
Cold Working.
Cold working consists of changing the size or shape of a metal part without raising its temperature before working.
When a bent nail is straightened by a carpenter with a hammer, the nail is being cold worked. Cold working usually
causes a change in the permeability of the metal where the change in size or shape occurs. The boundary of the area of
changed permeability may attract magnetic particles when the part is magnetized.
3.7.5.2.4
Hard or Soft Spots.
If there are areas of a part which have a different degree of hardness than the remainder of the part, these areas will
usually have a different permeability. When a part with such areas of different permeability is inspected with magnetic
