Magnetic particle testing is a widely established non destructive method for the detection of surface breaking flaws as well as sub-surface flaws with depths of 2-3 mm. The method is accomplished by inducing a magnetic field in ferromagnetic materials. The surface is coated with a white contrast paint and over this paint iron particles suspended in liquid are applied. Surface and near surface flaws produce magnetic poles and iron particles are attracted and concentrated, producing a visible indication of the defects on the surface of the material. The length of an indication can be accurately determined, but the depth cannot be determined with this method.