When the magnetic field is interrupted by a discontinuity, some of the field is forced out into the air above the discontinuity, forming a leakage field. The leakage field will be stronger and more concentrated the closer the discontinuity is to the surface. The presence of a discontinuity is detected by the ferromagnetic particles applied over the surface. Some of these particles will be gathered and held by the leakage field. This magnetically held collection of particles forms an outline of the discontinuity and indicates its location, size, and shape.
Electric current is used to create or induce magnetic fields in magnetic materials.
The direction of the magnetic field can be altered, and it is controlled by the direction of the magnetizing current. The arrangement of the current paths is used to induce the magnetic lines of force so that they intercept and are as near as possible at right angles to the discontinuity. The magnetic field must be in a favorable direction to produce indications. When the flux lines are oriented in a direction parallel to a discontinuity, the indication will be weak or lacking. The best results are obtained when the flux lines are in a direction at right angles to the discontinuity.