Make a Ferromagnetic Surface Magnetic
Of course, its not really magic, it’s the imposition of a magnetic field to a ferromagnetic (that means made of iron and is magnetizable, in most cases) surface and applying some tiny iron particles to the field. Any discontinuities in the magnetic field disturb the field and the particles, when applied, will ‘fill in’ the discontinuity and allow the magnetic field to become a lot less disturbed. The excess particles are removed from the surface and the collected particles now show the defect and make it visible.
Magnetic particle inspection is primarily a surface defect detection method but can be used to detect sub-surface anomalies too. In this manner, magnetic particle inspection is like the dye penetrant method in that both are methods that enhance contrast during a visual inspection making defects more visible.
A limitation to magnetic particle inspection is that the part to be inspected must be able to conduct the magnetic field and the test must be conducted twice, in two directions essentially perpendicular to each other.
