On parts with keyways, a circular magnetic field can also set up non-relevant indications as in Figure 3-71. Particle
accumulations may occur at A where there are leakage fields. A keyway on the inside of a hollow shaft may also create
indications on the outside as indicated at area B in Figure 3-72.
Figure 71. Concentration of Field in a Keyway
Figure 3-72. External Leakage Field created by an Internal Keyway
The gear and spline shown in Figure 3-73 were magnetized circularly by passing current through a central conductor.
The reduced cross section created by the spline ways constricts the magnetic lines of force and some of them break the
surface on the outside diameter. Particles gather where the magnetic lines of force break through the surface thereby