[artimaeis]

Magnetic induction damping compasses have traditionally used a flat plate under the needle in order to arrest the motion of the needle. This component is not transparent. By removing the plate and adding the ring, you can see through the face, providing the benefits of a liquid damped compass without the possibility of a bubble forming.

[K0balt]

Interesting, maybe new for pocket compasses. I had a marine plotting compass that used a massive copper cylindrical housing, with a sapphire glass bottom and window. It was very well damped. It was made in the 1940s, presumably when yachts were mostly wooden. (More modern boats would usually need significant compensation) or maybe it wasn’t for marine use? But anyway, it was a great plotting compass that I used extensively on my little fiberglass weekender sloop. Better than the westmarine garbage mounted on the cabin bulkhead by a long shot.

[ZiiS]

Lots of liquid damped compasses do not have a transparent base. The liquid is very good at protecting the needle (induction compasses often use a lock), prevents condensation, stabilises temperature, and is noncompressible for diving. Induction compasses tend to be used for fast reading whilst off-level, so tend only be useful for sighting compasses. TBH I am not sure even map compasses grain a lot from transparent dials, it is more that they are making the baseplate and top from transparent plastic and have no need to make the bottom from something else.

[jacquesm]

That's because the magnetic needle's orientation will only induce meaningful flux in a cross section large enough for it to have any damping effect. That braking effect is more or less proportional to the number of fieldlines cut and diminishes (from memory) to the cube of the size of the air gap.