The magnetic flux leakage creates a magnetic field gradient in the vicinity of the feature,
which in turn locally attracts a cluster of the magnetic particles. The nature of the magnetic fines
is such that they are intended to visually contrast with the background color of the sample. This
is done by either mherent coloration of the particle or through the use of a coating of some sort
on them. Therefore, the inspector does not see a crack per se but rather a somewhat larger area
of visually contrasting material