What is induced magnetism? it is a process by which the screws become a magnets. the magnet acts as a source of screw then screw becomes an induced magnetism and it has a capability to attract a small iron made materials thus it makes the screw becomes magnetic. remember that magnet can attract each other when the seeking polarities are opposite (where south to north and north to south )
-we would represent the screw a magnet-
so if the north pole suspends the first nail by attraction then the head of the screw most be the magnetic south pole (they must be opposite) so then the head screws seeking polarities is magnetic north pole.