In the first figure the magnetized metal has no crack and there only two poles that is north pole and south pole. And in second figure the magnetized metal has a crack and at the crack point there creates another north and south pole for the magnetic flux leakage. S N S N NS Magnetic Flux Line Fig.1: Magnetized Metal with no crack Fig.2: Magnetized Metal with crack CrackMagnetic Flux Leakage Basic Principle of MPT