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    Re: Automatic deviation calculation by electronic compasses
    From: Antoine Cou�tte
    Date: 2009 Dec 11, 08:11 -0800

    Dear Joseph,
    Thanks for your nice comments which have been reviewed from (well) Above and 
    have been subsequently approved and most appreciated !
    Since you are quite familiar with Mathematics as applied to Physics, and since 
    we (supposedly) keep instructing ourselves about FLUX GATES and/or related 
    items, I will offer at last one more development here since we are starting 
    going off track.
    Sorry for the (simple) math hereafter (last time !! promised again !) But I 
    could not resist it for so many days !
    It concerns FLUX GATES as mounted on Aircraft (and probably on Ships too). 
    THESE ARE EXTREMELY CLEVER DEVICES, which puzzled me when first learning 
    about them for my ATPL exams.
    Simply stated, they work as follows.
    An Aircraft pure iron sensor is maintained horizontal and always aligned with 
    the Aircraft (Ship ?) Centerline.
    With :
           - "Mu" being the magnetic permeability of the iron rod, and
           - "H" (oriented vector) the Horizontal Component of the Earth Magnetic Field, and
           - "Mh" (oriented vector) Magnetic Heading of the iron rod direction
           - "B" (oriented vector also) the Earth induced Magnetic Induction in 
    the Rod (i.e. the "flow rate" of H through the rod)
    We have the following simple equation :
                          B = Mu H cos(Mh)
    B itself is not detectable per se with just such a device.
    However the first derivative of B with respect to time, i.e. "d/dt B", is 
    easily detectable since it immediately translates into a electric voltage 
    "E", as per the well known formula :
                      E = k d/dt B (with k being a constant)
    The NICE TRICK here is that the FLUX GATES inventor has not forgotten that B 
    is a vector. Accordingly its derivative - and accordingly E - is the result 
    of both any change in its direction (which is the Magnetic Heading of B), or 
    any (possibly achievable ??? ) change in its intensity.
    Can we possibly change anything regarding B, without any undesirable or even unsurmountable effect(s) ?
    WE CANNOT AND MUST NOT CHANGE THE DIRECTION OF B, since we are looking for its 
    direction itself which is (closely related to) the A/C Magnetic Heading.
    Change what then ???
           - H cannot be changed, obviously ...
           - only would remain a (technically and adequately feasible ?) change in Mu.
    The clever physical implementation of this pure mathematical idea is that the 
    system constantly modifies the Magnetic Permeability "Mu" of the iron rod 
    through alternative currents around it arranged in such a way that they are 
    "magnetically undetectable" outside the Flux Gate in order to not perturb all 
    other Aircraft Avionic systems.
    Just vary the Magnetic permeability of an iron rod, and you can get an 
    accurate reading of its magnetic heading ... Well done and congratulations to 
    whoever had both this great idea and could accomplish its very delicate 
    electrical implementation !!!
    That's all Folks !
    Antoine M. Couette
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