This non-destructive testing method is accomplished by inducing a magnetic field in a ferromagnetic material and then applying either dry or wet iron particles to the surface of the test piece.

When a surface or subsurface discontinuity interrupts the magnetic field induced in the part, part of the field is forced out in the air above the discontinuity, forming a leakage field. Iron particles will form an outline of the leakage field revealing its location, size and shape.