Compass turning error

I got Pete's explanation, and it's close to my own mental way of remembering
this.

The metaphor that helps me to visualize the N/S compass turning error in the
northern hemisphere is a string with one end tied to the back side of the
compass ring, behind where "S" (South) is painted, and the other tied to the
north pole. When I'm near the world's equator, this string pulls the
compass in the direction it's supposed to go (it always keeps the side with
"S" painted on it as close to the north as possible since the string is
parallel to the ground at that point), and there's no turning error.
However, as I approach the north pole, the string begins to pull more and
more downward, toward the ground (the pole itself). Using this metaphor, I
can "see" the effect that the string has on the compass; as I bank, the
string will pull the side of the compass on which "S" is written downward,
toward the earth. The rotation of the compass on its axis (the turning
error) then becomes apparent as I follow this metaphor through by
visualization.


"Casey Wilson" wrote in message
...
Okay, I know the compass swings into a turning error when initiating
a
turn from any direction other than East or West and that the amount of
turning error is greatest when starting from either North or South. I
even
know a couple of the cute acronyms like UNOS (Undershoot North - Overshoot
South) to aid in rolling out on the correct heading.

My questions a

What is the mechanics for causing the turning error in the first place?

Why doesn't the effect happen when starting from East or West, yet
develops
as the turn progresses to the North or South?