T.O. 33B-1-13-63.1.6.3 ElectricityandMagnetism.Electric current can be used to create or induce magnetic fields in parts made of ferromagnetic materials. Magneticlines of force are always aligned at right angles (90°) to the direction of electric current flow. It is possible to controlthe direction of the magnetic field by controlling the direction of the magnetizing current. This makes it possible toinduce magnetic lines of force so that they intercept defects at right angles.3.1.6.4 MagneticAttraction.Magnetic attraction can be explained using the concept of flux lines or lines of force. Each flux line forms a closedcontinuous loop, which is never broken. For a circularly magnetized object, the flux lines are wholly contained in theobject (ideal case). No external magnetic poles are present and therefore there is no attraction for other ferromagneticobjects. For a longitudinally magnetized object, the flux lines leave and enter at magnetic poles. The flux lines alwaysleave a ferromagnetic material at right angles to the surface. They always seek the path of least resistance, i.e.maximum permeability and minimum distance. When a piece of soft iron is placed in a magnetic field it will developmagnetic poles. These poles will be attracted to the poles of the magnetic object that created the initial field. As itapproaches closer to the source original field, more flux lines will flow through the piece of iron, thus creating strongermagnetic poles and further increasing the attraction. This concentrates the lines of flux into the easily traversed (highpermeability) iron path rather than the alternative low permeability air paths. This is magnetic attraction and is thereason magnetic particles concentrate at leakage fields. The leakage field is established across an air gap of relativitylow permeability at the discontinuity. Since they offer a higher permeability path for the flux lines, the magneticparticles are drawn to the discontinuity and bridge the air gap to the extent possible.3.1.6.5 EffectsOfFluxDirection.The magnetic field must be in a favorable direction, with respect to a discontinuity, to produce an indication. When theflux lines are parallel to a linear discontinuity, the indications formed will be weak. The best results are obtained whenthe flux lines are perpendicular (at right angles) to the discontinuity. Note: When an electrical magnetizing current isused, the best indications are produced when the path of the magnetizing current is parallel to the discontinuity.3.1.6.6 CircularMagnetization.A circular magnetic field always surrounds a current carrying conductor, such as a wire or a bar (see Figure 3-8). Thedirection of the magnetic lines of force (magnetic field) is always at right angles to the direction of the magnetizingcurrent. An easy way to remember the direction of magnetic lines of force around a conductor is to imagine that youare grasping the conductor with your right hand, so that the extended thumb points parallel to the electric current flow.The fingers then point in the direction of the magnetic lines of force. Conversely, if the fingers point in the direction ofcurrent flow, the extended thumb points in the direction of the magnetic lines of force. This is called the right handrule.Figure 3-8. Magnetic Field Surrounding an Electrical Conductor.3.1.6.6.1Since metals are conductors of electricity, an electric current passing through a metallic part creates a magnetic field asshown in Figure 3-9. The magnetic lines of force are at right angles to the direction of the current. This type ofmagnetization is called circular magnetization because the lines of force, which represent the direction of the magneticfield, are circular within the part.
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