T.O. 33B-1-1
3-2
b. When a surface or near-surface discontinuity interrupts the magnetic field in a ferromagnetic material,
some of the field is forced out into the air above the discontinuity resulting in a leakage field. The size
and strength of the leakage field depend on the size and proximity to the surface of the discontinuity.
The discontinuity is detected by the use of finely divided ferromagnetic particles that are applied to a
part’s surface and are attracted to the leakage field. This collection of particles indicates the presence
and location of the discontinuity.
3.1.5
Basic Terminology.
The following terms and definitions are basic to an understanding of the MPT method.
a. Coercive Force. The negative or reverse applied magnetizing force (H) necessary to reduce the residual
magnetizing force (B) to zero in a ferromagnetic material, after magnetic saturation has been achieved.
The magnitude and direction of this force are represented by the line OG in Figure 3-16.
b. Direct Contact Magnetization. Use of current passed through the part via contact heads or prods to
produce a magnetic field.
c. Ferromagnetic. A term that describes a material that exhibits both magnetic hysteresis and saturation,
and whose magnetic permeability is dependent on the magnetizing force present. In magnetic particle
testing, we are concerned only with ferromagnetic materials.
d. Field, Circular Magnetic. The magnetic field surrounding the flow of electric current. For magnetic
particle testing, this refers to current flow in a central conductor or the part itself. It is best
characterized as having no magnetic poles present.
e. Field, Longitudinal Magnetic. A magnetic field that results in magnetic poles. An example would be
the field that exists in a bar magnet.
f. Field, Magnetic. The term used to describe the volume within and surrounding either a magnetized
part or a current-carrying conductor wherein a magnetic force is exerted.
g. Field, Magnetic Leakage. The magnetic field outside of a part resulting from the presence of a
discontinuity, a change in magnetic permeability, or a change in the part’s cross-section.
h. Flux Density, Magnetic (B). The strength of a magnetic field, expressed in flux lines per unit cross-
sectional area.
i. Flux Lines or Lines of Force. A conceptual representation of magnetic flux that can be illustrated by
the line pattern produced when iron filings are sprinkled on paper laid over a permanent magnet
j. Hysteresis, Magnetic. The behavior of ferromagnetic materials that results from both the retentivity and
the coercive force being greater than zero.
k. Induced Current Magnetization. Use of current induced in a part to produce a magnetic field.
l. Magnetizing Current (I). The flow of either alternating or direct current used to induce magnetism into
the part being inspected.
m. Magnetizing Force (H). The magnetizing field applied to a ferromagnetic material to induce
magnetization.
n. Magnetic Permeability (m). The ease with which a ferromagnetic part can be magnetized. It is equal to
the ratio of the flux density (B) produced to the magnetizing force (H) inducing the magnetic field. It
