Reverse Polarity in Compasses

By Nigel Williams, Glenmore Lodge, Revised Jul 2016

Reverse polarity is where the magnetism in the compass needle becomes permanently reversed so the red end of the needle points south instead of north. This is different to the magnetic needle being temporarily deviated a little when near a metal object or weak magnet and correcting itself as soon as it is moved away. It can affect any brand of compass.

Working in the outdoor industry for 40 years, I had never come across a reversed polarity compass until about 8 years ago. Since then I have personally seen about 30 and heard of quite a few more cases, at least 3 of which ended in a mountain rescue call out for competent navigators. This problem is unlikely to be caused by proximity to ordinary metal, a penknife for instance (how would the military survive with all their armoured vehicles and weaponry?). I have a box of about 20 assorted compasses all mixed together, they are strongly affected by each others magnetic needle yet once separated they have never reversed as their magnetism is actually quite weak. So what is going on?

Could it be 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 the rucksack over many years without effect. Pass these items by your compass, it hardly affects the needle, in fact less effect than another compass would. So that does not explain the sudden increase of incidents. Mobile phone batteries are different being rechargeable, but remove it from your phone and put it by your compass – little or no effect, now try the mobile phone without the battery – it has a strong effect from a magnet in the speaker system. This also illustrates that whether the phone is switched on or off is irrelevant.

Mountain rescuers and sea kayakers carry radios with a speaker with a strong magnet in it which will cause a change in polarity if left rubbing together in a rucksack or hatch regardless of whether switched on or having a battery in. For a sea kayaker in poor visibility this could be a very serious situation as they may have no other features to support their navigation decisions. Walky talky devices will do the same thing (I used one recently in the field to correct a reversed polarity compass).

The guilty culprit seems to be anything with a speaker system in it. Additionally some phone cases have a small magnet in that puts the phone into a hibernate mode, that too will change needle polarity. An experiment of stroking one of these devices on a compass reversed the polarity after just 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 days walk. The needle can be reversed back by again repeatedly stroking a magnet along it. However there is no guarantee 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 will do it for you, however they do warn against leaving compasses near metal objects in the instruction leaflet so they may not feel obliged to offer a replacement.

It appears that a few outdoor clothing manufacturers are producing clothing items with magnets in. Fingerless gloves 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 rucsac lid or pocket with a compass you have another potential to reverse polarity. Also came across a belt on a mountain trouser that has a magnet in the plastic buckle to help secure it together.

A conscious effort is required to keep the compass isolated from other gadgets, from phones and radios, to digital cameras, GPS, avalanche transceivers and SPOT devices. 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 challenge for MR personnel with suggestions that transceivers can be affected by phones as well, they are often carrying a phone, a radio, a transceiver and a compass!

If you suspect your compass has reversed then compare it with one that you may have on your GPS, phone, or watch, or think of other natural signs such as the expected wind direction or position of the sun.

A reverse polarity compass could have a life threatening consequence; try to treat it as a delicate scientific instrument.