Small magnet disc under keyboard... Purpose?

I have a iBook G4 and underneath the keyboard around the num lock key
and hiding a screwis a small disc magnet. I put in some memory
and didn't know which orientation to put the magnet back on top of the
screw...

Also what is it's purpose? iBook seems to work fine without it.

I know it is NOT the same magnet used by the sleep/reed switch,
which is near the right arrow/end key (lower right of keyboard).