Compass Reversal

[adkfun]

So I am getting ready for a week long trip and going over my gear to make sure everything is in good working order. While checking out my compass, I noticed it changed polarity - North needle indicator now points South. Kinda freaked me out. I took it outside, walked around, still pointed to the South. I have had this compass now for about 5 years.

I think I may have put it on top of my phone holster which has a magnetic catch. That's the only thing I can think of that may have caused the reversal. I only put it on top for a few seconds, but it seems like that is the culprit.

Has this ever happened to anyone else? Glad I noticed it before I left. It would have been worse if had happened while on the trail. I could see myself heading off in the wrong direction. Until now I would have always trusted my compass to point me in the right direction.

Just reminds me that the most important piece of hardware is one's brain.

[kayakrski]

What kind of compass is it?

[adkfun]

The compass is made by Brunton. I don't know the model number. It is a small baseplate model. It's pretty much equivalent to this one. It had served me well for a long time.

[Wldrns]

Has this ever happened to anyone else?

Yes it has happened to me twice. I have a collection of about 20 medium and high quality orienteering compasses that I sometimes use for teaching, though they are just as good for using in the field. The problem is I often carry them to where ever I teach all together in close contact to one another. If they get banged around, it is possible for one to affect another enough to reverse polarity, especially if the compasses are arranged such that the needles are not free to rotate. So I use those 2 reversed compasses as teaching aids too, to show it can happen and it really is not the end of the world.

No one should blindly trust compass alone when setting out. Always check it at the trailhead with man-made (e.g. the road) and natural terrain clues you already know for direction. I always carry two compasses anyway, sometimes a third smaller one tucked away (but never close to each other in the field). If your compass is reversed and you know it, it is still useable (though I would certainly replace it). By the way, it is not possible for a compass to read anything other than correct magnetic, or reversed magnetic. Intermediate values due to compass malfunction are not physically possible. Yea, yea, there are magnetic anomalies that give intermediate values which alter the local magnetic field and the local direction of earth's magnetic field (the compass needle always aligns along the magnetic field), but those are rare and wont make sense when combined with terrain following.

[Willie]

Fyi

Information from Silva regarding the cause and "cure" for reversed polarity.

[redhawk]

Please take note the argument that "the compass is always accurate" is not in fact correct.

if you are in a place with a high concentration of iron ore for instance, your compass is useless.

[Wldrns]

Please take note the argument that "the compass is always accurate" is not in fact correct.

if you are in a place with a high concentration of iron ore for instance, your compass is useless.

Depends on your definition of "accurate". The compass needle will always align with the local magnetic field and from that standpoint is always accurate. We expect and hope that the field direction is the one that our declination tables have smoothly accounted for. True enough, if there is a local magnetic deposit, this may not be the case. In the few rare locations where it has happened to me, I could readily tell from other natural terrain direction clues that the compass needle was not pointing where I expected. But in a short distance the compass returned to its "normal" expected direction reading. I'm convinced that some of the old timer's tales I've heard (in locations not known for magnetic deposits) of their "compass being wrong" or "there's iron buried there" were no more than cases of their own navigation confusion rather than compass error or magnetic anomaly.

[redhawk]

Depends on your definition of "accurate". The compass needle will always align with the local magnetic field and from that standpoint is always accurate. We expect and hope that the field direction is the one that our declination tables have smoothly accounted for. True enough, if there is a local magnetic deposit, this may not be the case. In the few rare locations where it has happened to me, I could readily tell from other natural terrain direction clues that the compass needle was not pointing where I expected. But in a short distance the compass returned to its "normal" expected direction reading. I'm convinced that some of the old timer's tales I've heard (in locations not known for magnetic deposits) of their "compass being wrong" or "there's iron buried there" were no more than cases of their own navigation confusion rather than compass error or magnetic anamoly.

My definition of "accurate" is that the compass will always point to magnetic North at all times under all circumstances.

Since there are instances that there are not, there are times when a compass alone may not help you.

My point it that the statement "your compass is always right" is incorrect. Therfore no person should ever make that statement, as I have heard or seen some people do in the past. Not necessarily on this forum although there was an instance here or maybe on the adkhighpeaksforum where that statement was made by a self titled "compass purist" who was pooh poohing gps use.

Hawk

[Neil]

Does this mean that compasses should not be stored together, as in a drawer one beside the other?

Or displayed in stores on a rack one behind the other?

Quite a challenge if you own 30 compasses.

[Wldrns]

Does this mean that compasses should not be stored together, as in a drawer one beside the other?

That's generally how mine or stored, in a drawer or piled in a briefcase with teaching materials. As I said, 2 of them have been reversed that way. I think the problem arises when the needles are not free to rotate (such as when other than horizontal) and external magnetic fields are present (such as from immediately adjacent compasses), especially if there is mechanical banging around. But I always check my field compass and spare(s) before departing on a backcountry trip, and then don't store them together in my pack or pocket. If they are not reversed at the beginning, then I give them no mechanism to reverse while I am out there.

[Willie]

Is there a physicist in the house?

Anecdotal evidence aside, can anyone explain, in layman terms, the mechanism for a compass needle polarity reversal and the forces required for such a reversal?

[Adk Keith]

Anecdotal evidence aside, can anyone explain, in layman terms, the mechanism for a compass needle polarity reversal and the forces required for such a reversal?

As long as I can use the word abracadabra!

[Wldrns]

Anecdotal evidence aside, can anyone explain, in layman terms, the mechanism for a compass needle polarity reversal and the forces required for such a reversal?

Ok, here's a simplified explanation... But you have to understand the mechanism for generating magnetism in the first place. The USGS has a nice web page FAQ on the topic.

It glosses over magnetism fundamentals a bit, but if you look at the first FAQ on "what is a magnetic field", you will see it is generated by moving electric charges. You know atoms have moving electrons orbiting around a nucleus. That simple atomic unit of a moving electric charge constitutes a very tiny magnet. In most materials the orientation of the atoms is random enough so that each tiny microscopic magnetic field doesn't go very far before it gets canceled by one from another direction. But in certain kinds of materials (such as iron, cobalt, nickel), you can fairly easily force a majority of the atoms to align themselves in the same direction and stay there, producing a macroscopic magnet - see "what is a permanent magnet". However, sometimes all it takes is a counter-acting force from another strong magnet (such as another compass in close proximity) to cause misalignment of the atomic magnets, or even reversal of direction. This misalignment can be assisted by heat (which makes movement at the atomic level easier), or even mechanical shock (banging it around). Does this help?

[Willie]

[S]ometimes all it takes is a counter-acting force from another strong magnet (such as another compass in close proximity) to cause misalignment of the atomic magnets, or even reversal of direction. This misalignment can be assisted by heat (which makes movement at the atomic level easier), or even mechanical shock (banging it around). Does this help?

No. Thanks anyway.

[redhawk]

Ok, here's a simplified explanation... But you have to understand the mechanism for generating magnetism in the first place. The USGS has a nice web page FAQ on the topic.

It glosses over magnetism fundamentals a bit, but if you look at the first FAQ on "what is a magnetic field", you will see it is generated by moving electric charges. You know atoms have moving electrons orbiting around a nucleus. That simple atomic unit of a moving electric charge constitutes a very tiny magnet. In most materials the orientation of the atoms is random enough so that each tiny microscopic magnetic field doesn't go very far before it gets canceled by one from another direction. But in certain kinds of materials (such as iron, cobalt, nickel), you can fairly easily force a majority of the atoms to align themselves in the same direction and stay there, producing a macroscopic magnet - see "what is a permanent magnet". However, sometimes all it takes is a counter-acting force from another strong magnet (such as another compass in close proximity) to cause misalignment of the atomic magnets, or even reversal of direction. This misalignment can be assisted by heat (which makes movement at the atomic level easier), or even mechanical shock (banging it around). Does this help?

If that's the simple definition I'm glad that all i need to know about a compass is that the needle points to North.

Well. most of the time anyway.

Hawk