Magnetic viewing film is a thin sheet holding fine particles suspended in oil. Laid on a magnet, the particles align with the field and the pole pattern appears as a visible image — light where the field runs one way, dark where it runs the other.
It is the standard way to answer three questions quickly: is this magnet coded at all, is the pattern the one that was designed, and is it centred on the part.
north-facing maxel south-facing maxel not magnetised
What you are looking for
A clean, symmetrical pattern with sharp boundaries. Smeared edges, an off-centre pattern or
missing regions mean the part was not written correctly.
Worth owning a sheet. It is inexpensive, it needs no power and no
calibration, and it turns an argument about whether a part is right into a thing you can look
at. Viewing film is in the catalogue.
What it will not tell you
Film shows the pattern, not the strength. It cannot tell you the holding force at a given gap, whether the material has been degraded by heat, or the grade. For those you need the datasheet and, for a critical application, measurement.