The particles can be red or black oxide, or they can be coated with a substance,
which fluoresces brilliantly under ultra-violet illumination (black light). The object is to
present as great a contrast as possible between the crack indication and the material
background.
The technique not only detects those defects which are not normally visible to the
unaided eye, but also renders easily visible those defects which would otherwise
require close scrutiny of the surface.
There are many methods of generating magnetic flux in the test piece, the most
simple one being the application of a permanent magnet to the surface, but this
method cannot be controlled accurately because of indifferent surface contact and
deterioration in magnetic strength.
Modern equipments generate the magnetic field electrically either directly or
indirectly