The reason we use this test is to find small defects in objects before they become bigger defects and cause serious problems. In magnetic particle inspection, a magnet or electrical current is used to establish a magnetic field in the object.

Iron filings are then dusted on to the surface of the object. The filings should align along the magnetic lines of force. If a crack or other defect is present, the magnetic lines of force will be disrupted and the magnetic particles will cluster along the edges of the flaw.