Subject: How does the ball float in those car compasses?

How does the ball float in those car compasses?
Do you know those compasses for cars in which a small ball is
suspended in a liquid and freely rotates?  How does that ball
maintain perfect equilibrium, so that it neither scrapes on
the top of the inner surface (by being too bouyant), nor scrapes
on the bottom inner surface (by being too heavy)?  It seems that
the tolerances these floating ball-within-a-balls are constructed
are pretty small -- about 1 millimeter -- and the liquid itself
is not viscous at all.  So how are these balls built with such
precision?

Re:How does the ball float in those car compasses?