Two comments: I use arrays of compass needles in my lecture demonstrations.
We have about twenty-five cheap compasses and two arrays of compass needles
between Plexiglas sheets in our demonstration stores*. The night before I
use them I always select and remagnetize the needles I plan to use by the
one-magnet technique alluded to by John Mallinckrodt. It is very easy to do
and one just cannot trust compasses stored near magnets to retain their
polarity.
I want to recommend that you purchase some neodymium iron magnets from some
local source. They are strong and inexpensive. They can be purchased here
in three sizes from a local yuppie hardware store. The largest are half-
inch diameter by eigth-inch thick cylinders that cost C$1.40 (~US$1) each
in quantities of ten or more. They have to be played with to be appreciated,
but a pair will hold on to each other through the thickness of your finger!
I keep some of these stuck to my desk to demonstrate to students and I have
a stack at home also. Bill Beattie told me how to demonstrate the very weak
diamagnetism of water using them. They also make robust compasses.
Leigh
*I don't know where our cheap compasses came from, but they are in fully
transparent mountings with an "N" shaped cutout in one side of the needle
so they can be used on the overhead projector.