/ NEW ARTICLE: Reverse Polarity - a Warning to Compass Users

This topic has been archived, and won't accept reply postings.
UKC Articles - on 12 Apr 2013
Winter Navigation, 2 kbAre navigation errors 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 of the MCofS investigates.

Read more at http://www.ukclimbing.com/articles/page.php?id=5385
Giz - on 12 Apr 2013
In reply to UKC Articles: I had a compass do this. I reckoned it was one of those torches that charges when you shake it. I had one in my rucsac. I don't any more and I don't let my compass near anything with a battery.
AlH - on 12 Apr 2013
Calder - on 12 Apr 2013
In reply to AlH: I like the second one more.
tony on 12 Apr 2013
In reply to Calder:
> (In reply to AlH) I like the second one more.

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 ...
Calder - on 12 Apr 2013
In reply to tony:
> (In reply to Calder)
> [...]
>
> The caption is particularly useful.

I hadn't noticed it. But yes!
Mr. K - on 12 Apr 2013
In reply to UKC Articles: Isn't reversing the polarity something the engine room on the Enterprise used to do to get them out of a tight spot?
Lukeva - on 12 Apr 2013
In reply to Giz:
> (In reply to UKC Articles) I had a compass do this. I reckoned it was one of those torches that charges when you shake it. I had one in my rucsac. I don't any more and I don't let my compass near anything with a battery.

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
cammi - on 12 Apr 2013
In reply to UKC Articles:
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 :-)
Sean Kelly - on 12 Apr 2013
In reply to UKC Articles: This has possibly happened to me in the Drummocter hills when descending from the summit in a white-out, and went down in exactly the opposite direction from that intended. Realised my mistake and went back to the top and did precisisely the same again. I couldn't believe that the compass was at fault so returned to the summit and descended in the opposite direction and found my track from the way up. I just thought I was having a 'moment' but this probably explains all. Recently walking on Aonach Mor in total white-out, my compass was spinning wildly and finally realised that the compass was actually broken so it has now been ditched. Let's hope that the replacement works better.
Roberttaylor - on 16 Apr 2013
In reply to UKC Articles: Worth noting that holding the compass in the same hand as your ice axe is a pretty poor idea.
tom_in_edinburgh - on 16 Apr 2013

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.
saz_b - on 22 Apr 2013
Had a Silva compass do this last year - I even talked myself into thinking the sun rose from the west and set in the east haha!

Quick convo with Silva and I posted it back and they repolarised this for me for free within a week - great service.
mh554 on 29 Apr 2013
This happened to my compass this winter in a whiteout on Helvellyn. Fortunately I knew roughly where I was, and mates had another. I sent the compass to Silva who fixed it FREE and returned in about 10 days! Great service Silva!!
p3tersen on 13 Jun 2013
Two very separate things are being conflated here.

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.
]
p3tersen on 13 Jun 2013
In reply to Giz:

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.
Offwidth - on 13 Jun 2013
In reply to p3tersen:

A small bar magnet will reverse the polarity of come compasses, they are not 10,000 gauss more like 100.
tom_in_edinburgh - on 13 Jun 2013
In reply to p3tersen:
> Two very separate things are being conflated here.
>
> 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.
Offwidth - on 13 Jun 2013
In reply to tom_in_edinburgh:

For white-outs on the Ben I'll stick with a compass (at least as a backup)
ads.ukclimbing.com
Doug on 13 Jun 2013
In reply to p3tersen: A friend a Silva compass that reversed its polarity (ie the red end ended up pointing south) after being left next to a speaker stack during a concert - presumably there were fairly strong magnetic fields

We kept it as a curiosity, and did once lend it to someone for an orienteering competition :-)

This topic has been archived, and won't accept reply postings.