The principle of the method is that the test point is magnetized to produce magnetic lines of force, or flux, in the material. If these lines of force meet a discontinuity, such as a crack, secondary magnetic poles are created at the faces of the crack. Where these secondary magnetic fields appear at the surface of the metal, they can be revealed by applying magnetic particles, as a powder, or in a liquid suspension, to the surface. The particles are attracted to the flux leakage and accumulate around the flaw, making it visible. The particles may be color contrast (visible), or fluorescent to increase their visibility under black light.