February 2022 - Compasses and magnetism

by Nigel Williams

Having a compass and knowing how to use it will have a big impact on your navigation. As technology develops, so does interference with the magnetic needle of a compass. In this blog, Nigel Williams discusses the working principles of the compass and what can affect its magnetism.

A compass needle is a weak permanent magnet strip of metal mounted on a pin allowing it to swivel. The needle is drawn to align along the earths' magnetic fields, with the red end pointing to the north. For us in the UK this is simple as the magnetic field lines across the UK run quite accurately north south. The magnetic field lines are constantly moving as the north magnetic pole moves due to the swirling of the earths' metallic molten inner core. Currently they appear to be accelerating westwards at a rate of around 1 degree every 6 years. That creates what we refer to as "magnetic variation" or "magnetic declination" which is the difference between the grid lines on the map which cannot be changed, and earths' magnetic field in the UK. In 1976 magnetic north was about 8.5 degrees east of the grid lines, decreasing westwards about 0.5º over 5 years. It continued to decrease to zero degrees at Greenwich in 2019 and has now headed West by about 0.5º. There are a couple of degrees variation between the most easterly and westerly points of the UK.

Every time we take a "grid bearing" off the map using the grid lines we should currently be adjusting the compass dial to subtract around 0.5º to match it to the ground, changing a "grid bearing" to a "magnetic bearing" that we can follow. This is reversed if one takes a bearing on an object and then brings that back onto the map. However, the compass dial is usually marked in 2 degree increments and almost impossible to adjust for anything less than that.

If it sounds complicated do not worry, in the UK we can forget it for probably the next 25-30 years when it starts to get to around 5 degrees or more, because according the compass manufacturers the devices can be up to 2.5º inaccurate. Like it or not, taking a bearing on the map involves several steps where we may build in small alignment errors of just a degree to two, and that is before we try to walk on it which is likely to add a few more degrees of error. These minor errors don't necessarily compound each other, but there is a level of inaccuracy that we have to accept, and it is unlikely to be relevant for quite a few years for all but the most demanding navigational challenges. Novices certainly do not even need to know about magnetic variation for their use of a compass.

If you put a compass next to a piece of ferrous metal, such as a penknife, the red end of the needle may be drawn towards or repelled by it. This is referred to as causing "magnetic deviation". But as soon as the compass moves a few inches away it settles accurately again. If we have a strong magnet and gently rub it around on a compass capsule it can cause the north and south ends of the needle to swap permanently, reversing everything you do with a compass by 180 degrees. This is serious and can affect any brand of compass and it has caused a few mountain rescue call outs for competent navigators.

Working in the military and outdoor industry for 40 years, I had never come across a reversed polarity compass until about 15 years ago. Since then I have personally seen about 60 and heard of many more cases. Like many hill walkers my compass had always been in a pocket or lid of the rucksack, often along with a head torch, spare batteries, penknife, GPS etc without any issues. The problem began around the time we started carrying smart phones on the hill. They are the main cause of reverse polarity because there is a strong magnet in the speaker system. The phone shape rather matches the long baseplate compasses and it is easy to put them together in the same pocket where they happily rub along together as we walk. It is difficult to say how long this needs to happen to result in the needle changing polarity. I quite often inadvertently end up with them together for maybe 30 minutes in a pocket without a problem, but if one really stroked the phone along the compass needle with the aim of changing the polarity it could probably be done in 5 minutes. I have reversed one in the field to correct it in that time but there is no guarantee as to how strongly the north end has been re-magnetised. The manufacturers are now well aware of the issue and returns departments will usually correct the problem free of charge. I have had a box of compasses for decades, and although the compasses influence each other when together, as soon as they are separated they behave correctly.

Small walkie talkie radios, mountain rescue radios and VHF radios for sea kayakers all have speakers in them, and some cameras may also have speakers all reliant on fairly strong magnets. Just for clarity, these magnets do not require a current. You can switch your phone off and throw the battery away. The magnet is still there functioning the same as your magnetic compass needle.

Some phone cases also have magnet closures in them. More recently outdoor equipment manufacturers have been using magnets in belts, chin straps, drinking tube clips 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. These are unlikely to cause the needle to change polarity as they are not likely to rub together for an extended period, but they will deviate the needle significantly if the compass is held and used near them.

The best advice is to avoid clothing items with magnets in fastenings if you are expecting to be doing some serious navigation. Check your compass around your devices including Spot emergency devices and avalanche transceivers so you understand the implications, and register a safe distance to avoid interference with the needle. Keep compass and phone in very different pockets. Carry a spare small compass or ensure there are 2 compasses in a group. Or the culprit, the phone itself, is a backup. Many have a compass which usually needs an initial calibration. The OS locate app is useful for this. Often the realisation that something is wrong is that you were expecting to go uphill but the compass is pointing you downhill or sending you back the way you have come.

If you are unlucky enough to find your compass has reversed polarity, simply trying to use it knowing the white end is now pointing north doesn't really work. In fact, the first hint that something might be wrong is that the needle doesn't seem to float and settle normally. This is because when the compass is operating normally the north end of the needle wants to dip to towards the ground. To get the needle to float level in the capsule the white end is about 1mm longer and therefore heavier to counterbalance this dip. If the white heavier end also becomes magnetised as the north end then the dip become significant and the needle ends up scraping the floor of the capsule and fails to settle accurately.

All this starts to question the old adage "always trust your compass". In reality we need to be aware of the potential for interference with the magnetic needle in the modern world. Your compass, and the skills to use it, can have a life preserving impact. Treat it as a valued and delicate scientific instrument.

Browse the full range of navigation aids and compasses
Read more on compass skills: June 2021 Intermediate Navigation & Compass Skills
Read more on compass bearings: June 2021 Intermediate Navigation & Compass Skills




Return to the Navigation Blog