Reversed polarity on compass
This month I’m prompted to write about something that happened to a client on a recent navigation course – he discovered that he had reversed the polarity on his compass.. There may be many of you out there who know about this already, but probably a fair number who will still be taken by surprise, because this has simply never crossed your mind.
The magnetic needle on your compass can have its polarity reversed if it comes into contact with anything with a strong magnetic field. The obvious example would be a magnet, but who on earth carries a magnet out on a mountain walk? Well, surprisingly, an increasing number of us might unwittingly be carrying actual magnets with us – some manufacturers use magnets in their fastenings and closures, and a ‘notorious’ example is a magnetic attachment on the drinking tube of some hydration bladders.
However, there is a much more likely culprit – the 21st Century walker will never be far from something with all sorts of magnetic fields, namely their mobile phone. Chalk and cheese; oil and water; mobile phones and compasses. They don’t go together, as this article will explain.
Last weekend, a client had put his compass into the same pocket as his mobile phone and when he pulled the compass out to take a bearing it was quite clearly 180 degrees out from every other compass in the group.
So what can we do about it?
First, avoidance. Be aware of the potential problem and never put your mobile phone and your compass close together. This includes putting both into the same pocket of your clothing or in the lid of your rucksack. Watch out for the same place in different layers too – left breast pocket on an inner fleece and left breast pocket of the waterproof effectively bring the two together; as do the inner and outer compartments of the rucksack lid. The best way to separate them is, for example, having the compass in the left breast pocket and the phone in the right trouser pocket.
Next, spotting it has happened and dealing with it in the field. Always having more than one compass in the group is a good idea because a simple check of all compasses before a crucial leg of navigation will show them all pointing the same way. This also provides redundancy and allows you to use a good compass while temporarily discarding the reversed one. Having some awareness of how to find north through other methods is also helpful – the sun, the stars, and natural signs around you if these are available.
Finally, rectifying the problem when you are back home. There is some evidence that you can re-reverse the polarity by stroking the needle with a magnet again. However, there is no guarantee that this has been done fully, accurately and will be long-lasting so it is not recommended. This leaves you with two choices – return the compass to the manufacturer asking them to re-align it properly (they are usually happy to do so) or simply buy a new one.