Why is plus on top?

[Paul Schreiber]

I'd guess because a + voltage means a "higher potential" than -, and
higher is on top. I doubt is has to do with 'flowing' like a river down
from north (European map-maker bias!)

I suppose a broader question: how has it decided what a "higher potential"
means?
(ie "above" or "below" ground).

I always thought it was originated from chemical plating experiments (Volta,
etc).

Paul S.