How to use a compass – How to fix a reversed compass
In the August 2018 TRAIL magazine we were asked how to use a compass, and specifically reversing a polarised compass, or how to fix a reversed compass – same thing.
How to use a compass- how to fix a reversed compass
What can I do if my compass reverses polarity? Is it fixable? How do I fix a reversed compass?
We have had a few people arriving on courses and when we ask them to check their compass before heading on the hill, their compass is reversed. Car speakers are the prime suspects, but any magnetic or metal object can interfere with compasses so it’s best to keep them apart.
Check your compass by using your map oriented to features on the ground to see where grid north is, in the UK this will be very close to the direction of magnetic north and where your compass red needle should be pointing to. You can use the sun, stars or moon to also get an idea which way is north.
One easy way is to get a friend to indicate Magnetic North’s direction with their compass (keeping at least 500mm apart from yours) and see if yours matches. Check neither of you are near metal, under pylons and so on before you check. The red needle should point to Magnetic North.
If your White South needle is pointing in the opposite direction to your friends i.e. your White needle is pointing to Magnetic North, your compass has reverse polarised.
I would always cross reference with ground features though in case your friend has had their compass next to the speakers in the car door pocket too!
At the Ultimate Navigation School we recommend you carry a spare compass in your rucksack and always sense check a bearing with the map and ground features. Don’t try to navigate with a reversed compass as they tend to be a few degrees out all the time, even if you do remember to use it the opposite way to normal.
The Silva compass in the big picture is reversed but it also has a big bubble interfering with the needle. Even if I fix the polarity this compass is unserviceable as the bubble is so big it is interfering with the needle. Small bubbles are ok for a short while until you can replace the compass.
When you are back home you can send it to the manufacturer (Silva, Suunto, etc) for correction and this should maintain any current warranty, and some stores do it for you there and then as well. However, if you wish to try to correct it yourself, Strong Magnets such as the 10×10 mm Neodymium Disc round rare earth magnet, work well and are only a few quid to buy online.
Follow the simple instructions below to fix your compass.
Correcting the reversed polarity or reversing a polarised compass (same thing) –
- Hold the compass flat and horizontal with the needle floating freely
- Take you Neodymium magnet and using the SOUTH end of the magnet, flick it outwards along the NORTH red needle of your compass.
- Repeat until your compass red needle points to Magnetic North again.
- Test the accuracy of your compass against another working compass (keeping them 500mm apart).