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    Re: Test your magnetic compass.
    From: Gary LaPook
    Date: 2012 Nov 16, 01:19 -0800
    The 150 meters from power lines comes from my army field manual FM6-50, Field Artillery, for setting up my M2 aiming circle which contains a very sensitive compass, readable to 1/2 a mil, 1/12,800th of a circle. I could definitely see the effect of my glasses and my watch on the compass needle in the aiming circle.http://www.google.com/search?q=aiming+circle&hl=en&tbo=u&tbm=isch&source=univ&sa=X&ei=0ASmUIetA4jpigK4moHICw&sqi=2&ved=0CCsQsAQ&biw=1280&bih=635

    I'm guessing the problem with high voltage power lines is discharges around the insulators.


    --- On Fri, 11/16/12, Bill Morris <engineer@clear.net.nz> wrote:

    From: Bill Morris <engineer@clear.net.nz>
    Subject: [NavList] Re: Test your magnetic compass.
    To: NavList@fer3.com
    Date: Friday, November 16, 2012, 12:20 AM

    I am little puzzled by some of the remarks made in response to Byron's original post. Things which on the face of it can be expected to have no effect on a magnetic compass are said to have an effect, or at least, are to be avoided.

    Power lines almost always carry alternating current and so could be expected to have a magnetic field alternating at 50 or 60 Hz and I would not expect an effect on a magnetic compass.

    Power lines in the immediate vicinity of a magnetic compass are another matter if they are carrying direct current and are unpaired, but they nearly always are paired so the fields tend to cancel. I tried placing a marching compass and a P4A aircraft compass on top of a power cord carrying 10 amps AC and could detect no effect when I switched the current on or off.

    Underground steel or water pipes I can understand might have an effect on a magnetic compass (and no one has mentioned reinforcing steel in concrete pavements), but I do not understand one reference to moving water. Is the writer suggesting a magnetohydrodynamic effect and is the (usually fresh) water being proposed as the conductor? Is this effect something that has been observed in, say, plastic water lines or is it an untested proposition?

    Keys, belt buckles and knives are more of a risk than most modern watches, but I fail completely to understand how an aluminium beverage can could affect a magnetic compass unless there was included somewhere on it a significant mass of ferrous metal. Again metal spectacle frames are unlikely to be a hazard, as most are not made of iron, steel or nickel, and the mass of steel in the two screws that hold the "temporals" to the front is unlikely to have an observable effect. At least, I could not observe one.

    Like Byron, I believed that magnetic force between two magnets varies inversely as the square of the distance between them, but the Admiralty Manual of Navigation 1970 (B.R.45 (1) states that "...the field strength due to a short magnet varies inversely as the cube of the distance from the magnet..." Nevertheless, the manual recommends that in swinging a compass, other ships should be at least three cables distance away, presumably because a _large_ mass of ferro-magnetic material has a local effect on the earth's field.

    Most large veseels do not roll appreciably, though I have experienced rolling of over 12 degrees each way on a large container ship. However, yachts very commonly are heeled in one direction when on a tack and while this does not cause the same oscillation of the needle caused by rolling, the vertical component of the vessel's magnetism that may have had no effect when on an even keel has a horizontal component when the vessel is heeled, and appreciable errors may result.

    Bill Morris
    New Zealand
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