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Magnetic Compasses

Discussion in 'Back to Basics' started by ghrit, May 11, 2008.

  1. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    How many still use magnetic compasses? How many have you thrown away because they seemed inaccurate and you couldn’t rely on the needle to tell you what you wanted to know?

    Being a founding member of the Ancient, Archaic, Anachronistic Woods Wanders (AAAWW) I don’t have and wouldn’t know how to use anything more modern than a magnetic compass and map. I’ve learned a few things along the way about the care and feeding of magnetic compasses.

    Start with a fact. Magnetic compasses point to magnetic north, reliably and with no fanfare. Even those little ones you pin on your lapel will do what they just naturally do, that is, point north.

    As usual, there are some IF’s involved. It cannot be emphasized enough, maintenance comes high on the list of reliability enhancers for anything that you want to rely on, and compasses are no different. What’s to maintain on a compass, or asked another way, what can go wrong that will take you east when west is what you had in mind?

    There are only two items that compasses depend on. The bearing that carries the needle, and the magnetism that the needle has when it leaves the factory. El Cheapo compasses have rather crude bearings that will exert a drag on the needle, and thus point close to, but not exactly north, simply because of the drag. That affects accuracy to a degree, but if not abused by jarring and completely ignoring them, they will function. Better quality instruments will have a jeweled bearing that reduces drag to a minimum, and shows the way to a much better degree of accuracy or higher precision. Usually these instruments have a device that will lift the needle off the bearing when not actually in use. Use it, your north won’t wander too far, and drag on the bearing will not happen as fast.

    In very good instruments, needle motion is dampened by a fluid in the case. There is little advantage in this other than assisting the needle to settle down when you set up to take a reading, and in moving situations (while walking, for example) it actually can prevent taking an average of the swinging needle. The downside is that they are less hardy, the cases can leak and lose the damping fluid (which sometimes also is a corrosion preventative for the bearings.) Except in very high precision (surveying, for example) the utility is debatable.

    Second important item is the magnetism that is on the needle. Wanna get your needle to wander? Store the compass such that the needle is pointed reverse, that is, have the north end of the needle pointed south. You can do this by storing it on its side, if there is no needle lifter, or point it anyway you want with the lifter engaged. Your magnetic field will go away, the poor thing will become confused, and in turn do it to you.

    More sources of inaccuracy result from outside influences. One of the worst things I ever did when orienteering was stick a KaBar in the ground and set the compass on the butt. Nice and stable platform, I could let it settle down and get a good reading. Yep, I took off 40 degrees off course. Set or hold your compass close to a magnetic material, your rifle for example, and you get something other than what you want. Cars are worse. You can, if you have a mind to, compensate for the metals in a car, but that requires a proper binnacle (look it up) with magnetic material masses that can be adjusted to compensate for the steel in the car. But it is cheaper by far to walk away from the car (50 feet or more) and take your reading.

    Maintaining the needle’s magnetism is easy. Take the compass and set it on the windowsill away from all the metal toys, release the lifter if it has one and walk away. The earth’s magnetism will help keep (and maybe realign, strengthen) the magnetic domains in the materials. If you, as I do, have a habit of leaving a compass in your vehicle, get two compasses and rotate them periodically. Nothing screws up a magnetic field like jarring, vibration and jerking the field around without letting the needle settle in the “right” direction. And of course, don’t drop or throw them around; the bearings will last longer even in the El Cheapo cup bearings.

    Just for the fun of it, take your favorite Monkey Gun out in the middle of the back yard, stick the bayonet in the ground and take a few compass readings right next to it. Step away and read it again. Then a bit further, and do it again. You should see the needle move more and more to the north, if the compass is decent quality. When it no longer looks like it is moving, you have established the distance you need to be from the rifle. ‘Course, you might like to leave the heavy knife and 1911 back with the Monkey Gun, too --

  2. Quigley_Sharps

    Quigley_Sharps The Badministrator Administrator Founding Member

    A magnetic compass can also be used to check the direction of the magnetic field around a wire with a current flowing through it.
    and mineral spirits is the most common fluid in them.
  3. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    True for DC. AC will scramble the needle magnetism. Are your wind machines generating DC then inverting?
  4. RouteClearance

    RouteClearance Monkey+++ Site Supporter

    All I have ever used is the military lensactic compasses, GPS's are fine for backup, but they are more fragile than a lensatic, and never let technology replace your own skills and techniques.
  5. Seacowboys

    Seacowboys Senior Member Founding Member

    What , nothing on correcting for deviation and variation? Nothing on all that useful information related to a compass that is printed on maps...maybe it is more useful than nothing, to have a general idea where magnetic north is, but that isn't very helpful for real navigation over longer distances.
  6. Tango3

    Tango3 Aimless wanderer

    good start... bad storing on it side ? hmmm never thought of that, I like my lensatic.Though general direction (over cast days) has usually been close enough...
  7. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Nothing on that yet, maybe later start a thread on map reading to go with it. Get a USGS topo of Minnesota and have a look at the corrections you need to apply near iron deposits. Even then, they may not be up to date at the rate the magnetic pole is moving. You are right, real north and magnetic north ain't in the same place.

    Hm. Is that movement a cause or effect of global warming?[coffee2]
  8. Seawolf1090

    Seawolf1090 Adventure Riding Monkey Founding Member

    I have compasses in my car and all pack gear. I used to have one ont he bike, but the Thumper grenaded two of them - I gave up. Besides, the vibrations of that single-pumper made it buzz when running. I use my GPS when on the road or woods-running.
    However, the GPS can and will fail (as will anything electronic) when it's really needed!
    I do sometimes wear a pin-on ball compass on my riding jacket sleeve as a back-up. all I really need is a basic N-S-E-W direction, not Nth-degree precision.
    For the pack gear, I have the flat Sunnto compasses, that are very compact and easy to carry. They aren't as quirky as the cheap lensatics.

    When I had the compass on the bike, I did find that placement was critical - same in the car. Any ferrous metal close to it will throw it off.
    The bike's tachometer would really send it spinning! Powerful E-fields coming from that gadget! I had cobbled a mount on my windshield - but it couldn't stand up to the vibrations.
  9. vegasrandall

    vegasrandall Monkey+++

    I got a brunton pocket transit on ebay,this is the ultimate pocket tool for surveying,navigation,caculating heights of objects.the new ones are pricey but used ones can be found on ebay
  10. Tackleberry

    Tackleberry Krieg Hündchen

    I carry an old military compass in my work car.

    Seacowboys is right about figuring in deviation. It can take you way off course if you are not careful.
  11. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Agree, and deviation is more significant now than it was as recently as 30 years back. The earth's magnetic poles are moving, and I haven't found an explanation for it. A compass is only part of the package for navigation, a current chart showing the amount of deviation is essential for anything more than a few miles; more so if visibility is reduced to the point where landmarks disappear. Out of sight of land, as SC knows, landmarks are scarce anyway --
  12. monkeyman

    monkeyman Monkey+++ Moderator Emeritus Founding Member

    If its more than a few miles, what about just haveing maps (or knowledge of the area in between) and useing the old time meathod of following water sheds and land marks? Kind of like if I was in New Orleans and had to get home, aside from any possibility of useing the roads/road maps, could simply find the Mississippi, follow it up stream (use the maps and general direction to cut across some horse shoe bends and to help show what tributary is what) to the Missouri river, follow that up river to the bridge (or whatever was left of it) at 50 HWY, hang a left and follow the road for 30 miles, hand a right and go 2 more miles. If had the better maps than a road atlas could go the whole distance by water ways untill was a lot closer then give directions by draws and hills could also bypass larger bends in the river by sighting high hills and such. Basicly the same way most of the pioneers found their ways around.

    Im not saying the other method cant be VERY useful, but not understanding real well how, with a good map or knowledge of the areas features (on land), useing landmarks/features wouldnt elimenate the need for much more than a basic ability to figure north from south (as in not needing to worry about 10-15 degrees of deviation for magnetic vs true north) to get from point A to B?

    I figure if I am missing something then would be good to figure it out since for me at least, heve never really considered a compass to be particularaly escential gear. Now I COULD see where a good GPS could come in to play to find EXACT spots that are unmarked, like say which square foot to dig to recover a cache, but dont figure compases would be much better than landmarks and x feet from this stone toward that tree or whatever. Have been known to be wrong though and if Im missing something let me know.
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