I've recently had two compasses switch polarity, probably because I'm not keeping them away from my phone. Lacking the skill for a DIY approach, I sent them off to Silva who've re-polarised them for free: address below:
I am sorry to hear your compass has depolarised. This has happened due to it being near a magnetic/electrical field or ferrous metal that has polarised the needle. Unfortunately this is getting more and more common in the world we live in today as we carry a lot of items that emit such fields as mobile phones, GPS’s, car keys, car speakers etc.
Our compass repolarisation service is offered for free. Please post your compass along with details of your name, address and telephone number to the following address:
Susy Kerr, Sales Co-Ordinator
Silva Ltd
8 Easter Inch Court, Easter Inch Industrial Estate
Bathgate, EH48 2FJ