With the magnetization of specimen the magnetic flux (magnetic lines of force) are for the most part inside the ferromagnetic material. The magnetic field is composed of magnetic flux and are introduced into the specimen. Any flaw in the specimen interrupts the flow of magnetic flux, some of these lines must exit the specimen and re-enter it. Opposite magnetic poles is formed by the exit and re-entry of these lines. The formed magnetic poles attract minute particles when sprinkled over the specimen to generate a visual indication which is nearly the shape and size of the flaw. Cause of this build up of particle is the abrupt change in permeability.
The particles of the magnet can be applied as powder or in more general as liquid suspension commonly known as magnetic. To ensure accurate detection, linear flaws like crater crack must be in favor of the relation to the magnetic field direction. For easy detection, the colour of the surface of the specimen must be in high contrast to the colour of the magnetic particles. The flux density should be oriented 90 degree to the discontinuity to assure maximum sensitivity. However, it is completely possible to detect flaw which spreads up to ± 45 degree to the direction of the magnetic lines of force. It is essential to note that, owing to better sensitivity, when the discontinuity is at 90 degree to the flux lines, the magnetic field should be introduced towards several varied directions if the orientations of the possible flaw are not known.