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    Re: Test your magnetic compass.
    From: Lu Abel
    Date: 2012 Nov 16, 19:33 -0800
    Sorry, wish I could put comments in-line, but Yahoo mail doesn't let me.

    1.  Magnetic deviation when bringing a compass near a light switch may have nothing to do with electrical flow.  Instead, depending on the age of your house and the building codes in effect, it may be due to either metallic sheathing around the wires (BX cable) and/or metal electrical boxes holding the switch.

    2.  Most stud finders work on the basis of ultrasound.  Sound will go through plasterboard but is reflected if there's a stud behind it.   Most dramatically demonstrated if you bring a studfinder near a pre World War II wall.   These were plaster on wooden lath and a studfinder goes nuts -- thinks the whole wall has wood behind the plaster (which it does).

    From: Alexandre E Eremenko <eremenko@math.purdue.edu>
    To: NavList@fer3.com
    Sent: Friday, November 16, 2012 3:20 PM
    Subject: [NavList] Re: Test your magnetic compass.

    I don't really understand, why alternating current produces a
    constant magnetic field.
    Ken confirms this but does not explain.
    But this is a phenomenon which I noticed long ago and can even measure
    with my compass. It is about 10 degrees, close to a wall electric switch.

    There are also constant magnetic fields produced by nails in the studs
    of an American house. (And by iron rods inside the concrete of Soviet
    And there is even a simple magnetic devise based on this,
    it is called the stud finder.

    But the field produced by AC wiring is stronger.


    On Fri, 16 Nov 2012, Ken Muldrew wrote:

    > On 2012-11-16, at 2:55 PM, Bill Morris wrote:
    >> Can some physicist or power engineer explain how an alternating current can produce a constant magnetic field? Could corona discharge really account for the observation or must we postulate rusty joints in transmission towers acting as rectifiers?
    > AC power lines produce AC magnetic fields, but you have to be within about a meter of the line for the strength of the B field to equal that of the earth. DC power lines are becoming more popular due to advances in power engineering. Currently there are only about one or two hundred DC high voltage lines worldwide but this number will increase soon (at least power companies seem to be marketing the safety of DC lines pretty aggressively, and they do make sense for technologies such as wind power).
    > Ken Muldrew.
    > View and reply to this message: http://fer3.com/arc/m2.aspx?i=121152

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