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iPhones and Reverse Polarity in Compasses


Working in the outdoor industry for 35 years, I had never come across a reversed polarity compass until about 4 years ago. Since then I have personally seen at least half a dozen and heard of a good few other cases, one of which ended in a mountain rescue call out. Reverse polarity is not likely to be caused by proximity to ordinary metal such as a penknife, for instance (how would the military survive with all their armoured vehicles?). Whilst I have a box of about 20 assorted compasses all mixed together that are strongly affected by each others’ magnetic needles, once separated they are fine. So what is going on?


After a little research it appears that the guilty culprit maybe the iPhone, Blackberry generation of mobile phones and in some cases - those with a leather pouch that have a small magnet in that puts the phone into a hibernate mode. An experimented of stroking one of these devices on a compass reversed the polarity after a few minutes. It is not hard to envisage this happening quite by accident with a phone and compass lying together in the lid of a rucksack or jacket pocket on a day’s walk. The needle can be reversed back by again repeatedly stroking a magnet along the needle. This just illustrates the power of a magnet on a compass needle (I do however worry as to the strength of the re-magnetised needle and how easily it might reverse again). The compass manufacturers would not support you sorting this problem yourself. They do warn against leaving compasses near metal objects in the instruction leaflet so they may not feel obliged to offer a replacement.

Could it be the batteries? Many of us will have repeatedly had AA or AAA batteries in a head torch or GPS for instance adjacent to our compass in our rucksacks over many years without effect. Pass your head torch or GPS by your compass: it hardly affects the needle - in fact less so than another compass would. So that does not explain the sudden increase of incidents. Mobile batteries are different being rechargeable, but remove it from your phone and put it by your compass: again, little or no effect; now try the mobile phone without the battery - it has a strong effect (I suspect from a magnet in the speaker system). This also illustrates that whether the phone is switched on or off is irrelevant.


I have recently come across a few manufacturers producing fingerless gloves but with a flap that can be pulled over the finger ends that turns them into mittens. When not required the flap is held in place on the back of the hand with a magnet! Potentially a worrying thought if you are compass in hand trying to follow a bearing. However if these mitts are stuffed into the rucksack lid or pocket with a compass you have another potential to reverse polarity.

We need to make a much more conscious effort to keep our compass isolated from modern devices, from phones to digital cameras, GPS, avalanche transceivers and SPOT location devices, additionally mountain rescue folk carry radios which again with a speaker have a very strong influence on the needle. Try experimenting with all your devices near your compass so that you are aware of the ones that have a significant affect and keep them apart whether using them or storing them. A reverse polarity compass could have a life threatening consequence: that’s why I treat mine as I would any vital scientific instrument.


Nigel Williams


Glenmore Lodge