Had a few emails asking if reversing the polarity, to correct it, is possible at home. The answer is yes it is very straight forward to do and all you need is a small strong magnet, Neodymium Permanent Strong Rare Earth Magnets are ideal for this. You can buy a 10x10mm Cylinder Neodymium Magnets for £1.50, delivered.
To correct the reversed polarity of a compass:
1. Hold the compass so the needle is horizontal.
2. Take the Neodymium Magnet and using the south end of this magnet flick it outwards along the north end (red) of the needle.
3. Repeat until the compass needle points magnetic north again.
4. Compare the corrected compass with one that you know is working correctly, but do not put them side by side, they should be at least 500mm apart.