The principle consists of creating an intense magnetic flux in a material that must therefore be ferromagnetic. The resulting magnetic flux is deflected by a defect in its path. This deflection, also called the leakage field, can be observed by applying an ink with ferromagnetic particles (colored or fluorescent).

The observation of the deflection in natural light or via the UV lamp gives a characteristic signature of the defect.

In addition to the limitation to ferromagnetic materials, magnetic particle inspection offers the advantage over penetrant testing, faster processing and observation of indications that are open but also deeper (not open on the surface).