Magnetic particle examination (MPI) is usable only on materials having ferromagnetic properties, principally low alloy steels and heat-treatable stainless steels. It is a sensitive non-destructive method for detecting surface and near-surface cracks and discontinuities. MPI is used in two general types and forms: Visible/fluorescent and dry/wet particle. Fluorescent wet particles offer sensitivities up to 1000 times that of the visible dry methods. However they both have their use in industry as one method looks for gross discontinuities while the other seeks out minute defects.