MT is done by inducing a magnetic field in a ferromagnetic material and dusting the surface with iron particles (either dry or suspended in a liquid). Surface imperfections will distort the magnetic field and concentrate the iron particles near such discontinuities, thus indicating their presence.

The part is magnetized. Finely milled iron particles coated with a dye pigment are then applied to the specimen. These particles are attracted to magnetic flux leakage fields and will cluster to form an indication directly over the discontinuity. This indication can be visually detected under proper lighting conditions.

Either alternating current or direct current may be used to magnetize the material. Alternating currents, because of the skin effect, produce fields which are confined to the surface of the metal. Consequently, alternating currents are best suited to locating defects which are open to the surface. Direct current produces magnetic fields which penetrate into the metal. Consequently, direct currents are best to locate subsurface defects.

The finely dived magnetic particles used for magnetic particle testing may be either in a dry powder or suspended in liquid. For refinery work, dry powders are usually used. Recently, wet fluorescent magnetic particle testing has been used more frequently. If the defect is a surface flaw or lies sufficiently close to the surface, the powder is attracted and held by the leakage flux to form a visible indication of the location and extent of the defect. Surface defects usually produce patterns which are sharp. Subsurface defects usually produce less sharply defined patterns because the powder is less tightly held. Experienced operators can usually evaluate the severity of the flaw by the powder pattern.