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Reverse Polarity in Compasses

Posted By Administration, 04 July 2018

Reverse Polarity in Compasses

Author: Nigel Williams



Reverse polarity is where the magnetism in the compass needle becomes permanently reversed such that 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 military and outdoor industry for 40 years, I had never come across a reversed polarity compass until about 10 years ago. Since then I have personally seen about 40 and heard of many more cases, at least 2 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 had normal 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 affect 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, it has little or no affect, now try the mobile phone without the battery - it has a strong affect 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 kayak 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 in sea fog. Walky talky devices will do the same thing (I used one recently in the field to correct a reversed polarity compass by stroking the speaker area of it along the needle). Most digital cameras also now have a speaker in.

The guilty culprit seems to be anything with a speaker 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.




Outdoor equipment manufacturers have recently been using magnets in belts, chin straps, drinking tube clips for rucksack water containers which keep the tube attached to the front of the sac and fingerless gloves with a fold over mitten piece which is attached out of the way to the back of the hand with a magnet. Potentially a worrying thought if you are compass in hand trying to follow a bearing. Gloves also get stuffed into pockets where a compasses may be.

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!

Trying to use a reversed walkers compass knowing the white end of the needle is pointing to the north is not really an option. The white end is about 1mm longer than the red end to counter the dip of the north (red end of the needle). Once polarity is reversed, not only is the north end now heavier it is also wanting to dip. This results in the end of the needle brushing against the compass capsule floor and not being stable or reliable.

Consider carrying a simple spare compass or have at least 2 in the party. If alone and 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.

The magnetism in a compass needle is very small and can be reversed if the needle can be held still by stroking the north pole of a good bar magnet repeatedly along the needle from the intended north to south of the needle.
However, there is no guarantee as to the strength of the re-magnetised needle and how easily it might reverse again. The compass manufacturers will usually rectify the problem if the compass is returned to them.

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

Tags:  Compass  Education  Land Navigation  Navigation On Foot 

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