/ NEW ARTICLE: Reverse Polarity - a Warning to Compass Users
Read more at http://www.ukclimbing.com/articles/page.php?id=5385
The caption is particularly useful.
A friend of mine had his compass buggered up by being in a rucksack with a mobile phone. I do always carry a spare compass, but I tend to keep them in the same place if I'm not using one, so if one goes wrong, the other one probably will too.
Just as well I've got my GPS ...
> The caption is particularly useful.
I hadn't noticed it. But yes!
So should I not keep mine with my head torch... cuz I do.
I already doubt my compass from time to time and always tell myself i'm wrong it's right
My Recta compass turned around in front of my eyes on my ML assessment. Never fear, I thought, I will get my Silva compass out. I took it out of my bag and it too spun around in front of me. The only thing I could think of that could have caused it was that I was stood next to a steelworks. The Assessor didn't believe me that both had gone, and when he saw it was the case he said it was because my Recta compass was cheap. Surely the Silva one would have been fine if that was the case.
Good to know about re- magnetising though :-)
I just tried holding a compass progressively closer to the phone on my desk and it swings about like crazy at about 10cm. Maybe we need to re-evaluate how accurate bearings from compasses actually are in the normal situation where people are carrying phones, keys and money and possibly metal climbing gear.
For example, lots of places give instructions for getting off the Ben involving following bearings of 231 degrees and being sure to compensate for 2 degrees magnetic deviation which tacitly assumes you can actually measure bearings to 1 or 2 degree accuracy with a compass.
Quick convo with Silva and I posted it back and they repolarised this for me for free within a week - great service.
This first is interference, where a local magnetic field temporarily deflects the needle to give an incorrect bearing. This is easy because the magnetic field of the earth is only about 0.3 Gauss, which consumer electronics or large lumps of metal (including structures or possibly even geological features) can certainly produce at close range. Once the impinging field is removed the compass will read correctly. This effect is not 'depolarization' in any sense and the UKC article is wrong to refer to it as such.
The second possible problem is actual demagnetization of the compass needle, which one could properly call 'depolarization'. This is a permanent* change but because of the physics of ferromagnetic materials it requires far, far stronger fields. For a steel compass needle, something on the order of 10,000 Gauss! There is zero chance of a shop anti-theft system** or your car keys permanently reversing a compass in this way. If your compass was permanently damaged, that wasn't what did it.
It's vastly more likely that the first effect - interference from local fields - was responsible for all these reports of wonky compass readings.
* "Permanent" is not 100% correct, since the change is reversible by another strong field. "Persistent" might be more accurate.
** I refer here to the sensor loops at the door of the store. The devices embedded in the checkout counter, used to remove the tags, may produce much stronger fields if you sat a compass right on top of one. So, um, don't.
A shake-to-recharge flashlight has a large permanent magnet sliding back and forth inside the thing - that's how they work - and if your compass was damaged it was the permanent magnet that did it. It's physically impossible for a normal battery-powered flashlight to have any permanent effect on a compass.
A small bar magnet will reverse the polarity of come compasses, they are not 10,000 gauss more like 100.
> This first is interference, where a local magnetic field temporarily deflects the needle to give an incorrect bearing. This is easy because the magnetic field of the earth is only about 0.3 Gauss, which consumer electronics or large lumps of metal (including structures or possibly even geological features) can certainly produce at close range.
Exactly. And based on this I would say that all the verbiage that you get in the outdoor press about measuring bearings to within a couple of degrees when coming off the Ben and correcting for magnetic vs true north is unhelpful. Rather than worrying about 1% measurement errors from true vs magnetic north they should be telling folk to take their cellphone out their coat pocket (because of the speaker magnets) and get it as far away as possible from the compass. Or better yet keep the compass in the pack and just use the GPS on the phone.
For white-outs on the Ben I'll stick with a compass (at least as a backup)
We kept it as a curiosity, and did once lend it to someone for an orienteering competition :-)
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