Mike Clarke wrote:

I don't pretend to understand the theory of cyclone extractors

Well, I do (pretend, that is); the air circulation route is shaped so
that it swirls round circularwise in (I think) an increasingly tight
spiral. Being light, air can do this; being denser, even dust can't
manage to turn the corners fast enough, so the suspended solid stuff
gets thrown out to the edges of the cylinder, which it hits and (now
away from the strongest circulation) falls to the bottom.

James Dyson observed this principle in use in an industrial application
(cyclone extraction's been used there for decades, AIUI), and thought
it'd work OK scaled down to domestic-vac proportions. Whether that
counts as brilliant innovation or bleedin' obvious is a matter of
opinion - though by the bizarrely trivial standards of patents, it
counted as 'novel' enough for Dyson to patent the idea in the
domestic-vac application...

Stefek