When the sample is magnetized and the magnetic lines of magnetic flux (force) are usually inside the ferromagnetic material. The magnetic field that is introduced into the specimen is made of magnetic lines of force. In the cases of defects where the flow of magnetic lines of force is interrupted, some of the lines exit and then re-enter into the specimen. This process of exit and re-entry creates the opposite magnetic poles. Whenever the minute particles are scattered onto the sample, these particles are attracted by the magnetic poles in order to develop a visual indication. The visual indication is approximating the shape as well as the size of the flaw. It is the sudden change in permeability, which causes the build up of particles.
The magnetic particles can be applied as powder or more commonly as liquid suspension usually known as magnetic. To be detected, linear flaws such as crater crack must be favorably oriented in relation to the direction of the magnetic field. The colour of the magnetic particles should be in good contract to the colour of the surface of the specimen for easy detection. For maximum sensitivity, the flux density should be oriented 90 degree to the discontinuity.
However, it is generally possible to detect flaw which lie up to i 45 degree to the direction of the flux lines. It is important to note that, because of the better sensitivity, when the discontinuity is at 90 degree to the lines of force, the magnetic field should be induced in several different directions when the possible flaw orientations are not known.