Inductance core fans:

[snip]
PS. interesting how pot cores have very little inductance change with
changing gausse.
[snip]

Most, if not all, pot cores used for "precision" inductors and transformers
come in matched pairs with a very accurately ground narrow air gap between
the two center posts. The Al (or alpha from some mfgs) is accurately set by
the width of the air gap as ground during manufacture. The air gap also
forms a large part of the overall magnetic path (air having a much larger
reluctance than the ferrite). Such pot cores for precision inductors
usually have an adjustable slug (set with a non magnetic screwdriver) to
allow the finally assembled inductor to be set to an "exact" value.

And so... pot cores intended for use in making precision inductors (as in
filters or delay equalizers) or precision transformer applications exhibit
little change in inductance over a wide range of conditions simply because
of the air gap.

--
Pete k1po
Indialantic By-the-Sea, FL