The type of magnetic field established is determined by the method used to magnetize the specimen. Being able to magnetize the part in two direction is important because the best detection of defects occurs when the lines of magnetic force are established at right angles to the longest dimensions of the defect. This orientation creates the largest disruption of the magnetic field within the part and the greatest flux leakage at the surface of the part. As can be seen in the image, if the magnetic field is parallel to the defect, the field will see little disruption and no flux leakage field will be produced.
An orientation of 45 to 90 degrees between the magnetic field and the defect is necessary to form an indication. Since defects may occur in various and unknown directions, each part is normally magnetized in two directions at right angles to each other. If the component below is considered, it is known that passing current through the part from end to end will establish a circular magnetic field that will be 90degrees to the direction of the current. Therefore, defects that have a significant dimension in the direction of the current (longitudinal defects) should be detectable. Alternately, transverse -type defects will not be detectable with circular magnetization.