Reverse Polarity - a Warning to Compass Users

by Heather Morning - MCofS Apr/2013
This article has been read 11,262 times

A substantial number of mountain rescue callouts in the UK result either directly or partially from a navigation error. Is this always the fault of the individual navigator - or might there sometimes be a problem with the compass, a tool we tend to take for granted? Heather Morning, Mountain Safety Advisor at the MCofS, tackles the little-known subject of reverse polarity.

Picture the scene. You are out on the hill alone, ticking off that remote Munro. Unexpectedly the mist descends and you find yourself in poor visibility. Undeterred, you continue onwards and upwards. Navigation is easy; you are ascending a defined ridge which leads onto a broad summit plateau which gently rises to the summit over a shattered boulder field. Descent, however, is a little more challenging. The underfoot conditions mean that the normal baggers' path has not developed, and a compass bearing is required to locate the descent ridge.

+Winter Navigation, 31 kbHope the compass works. Photo: Sandy Paterson

As usual, your compass is sensibly attached to the zip in the chest pocket of your waterproof jacket - the same pocket where you put your mobile phone. Taking a bearing to the top of the ridge, you start heading down. Alarm bells start to ring as the ground ahead seems to be dropping far more steeply than you remembered on the ascent. Suddenly out of the mist, the ground drops dramatically in front of you over what looks to be quite a significant crag. You stop to re-assess. But imagine if this had been winter and the ground was snow covered; how easy would it have been to walk right over the edge?

It would be easy to assume that it was you who had made the mistake, but in the scenario above the compass has been affected by the magnet in your mobile phone case and the north/south needle has been reversed. The result is a bearing that took you in completely the wrong direction.

In the February edition of Scottish Mountaineer magazine, Nigel Williams (Head of Training at Glenmore Lodge) wrote an article on this very issue. Readers were invited to respond with similar experiences, and stories came flooding in. Indeed this very thing has happened to me three times in the last 12 months. There may be an increasing trend here.

As we all know, a compass operates on the earth's magnetic field; so what is 'reversed polarity' and how do we protect ourselves from being a victim of it?

+Spot the difference, 112 kbSpot the difference
© Heather Morning, MCofS

The Cause

Magnetic fields exist around many items we commonly carry with us on the hill; mobiles & smart phones, magnets hidden inside mobile phone cases, avalanche transceivers, radios, personal locator beacons, GPS, cameras, car keys, small magnets on belt fastenings and even under-wired bras. Lurking unexpectedly en route to the hill are security loops as you enter outdoor and food stores, and loudspeakers in your car. Magnetically charged items are everywhere.

The Effect

The compass needle may just be very briefly, partially or even totally reversed when in close proximity to one of the above mentioned objects. If the needle becomes sluggish and slow to settle (it may appear to stick and be out of balance) then it has become partially reversed. If the 'north' arrow (usually red) is pointing to south instead of north, then your compass has become completely reversed.

Can the compass be used in its 'reversed polarity' state by simply using the 'south' instead of the 'north' arrow? In my personal experience the needle does not invert by exactly 180 degrees, but would appear to be approximately 10 degrees out. Therefore using it in this state is not reliable.

Prevention

A compass is a precise measuring instrument and should be treated accordingly. Your compass should be kept well away from all of the above mentioned items and well clear of magnets and magnetic fields which are associated with electrical circuits and ferrous metal objects.

Cure

Kevin Thomson, Marketing Manager at Silva Ltd says:

'It is possible to 'reverse' the reversed polarity using a strong magnet. This can be achieved by quickly flicking the magnet outwards along the 'north' end of the needle. Repeat vice-versa. Always ensure you compare with a compass that is known to be correct. Note: If you are unsure how to cure your Silva compass then Silva will always take back and re-magnetise your compass, no matter how old [it] is.'

What to do?

Golden Rule 1: Keep your compass in a separate location to any electronic gizmos you carry with you on the hill, and ensure when you are using it that it is held well away from your body.

Golden Rule 2: Ensure you read the contours on the map. You should have a very clear idea as to what should happen under your feet as you walk on your compass bearing. If this is not happening, the alarm bells should be ringing and you should re-evaluate.

Golden Rule 3: Always carry a spare compass in the event of malfunction, damage or loss.

Golden Rule 4: Try to get into the habit of checking that your compass is working correctly prior to leaving home.

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