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    Re: Northern Limits of Magnetic Compass Usefulness
    From: Bill Lionheart
    Date: 2019 Apr 10, 15:50 +0100
    Remember that the magnetic field "H-field" and the magnetic flux density "B-field" are different things. A bit like electric field E and current flux density J in a conductor are related by  J = sigma E where sigma is the conductivity, we have B = mu H where mu is the permeability of the medium. In the old CGS system the unit for H was oersted while B was in gauss.  In SI  we now use A/m for H (compare with Volt/m for E) and Tesla for B.  

    An oersted is 1000/(4 pi) A/m    

    In SI units vacuum mu is 4 pi 10^-7        so if you have an H of one oersted   it is a B of  10^-4 T =  0.1 mT = 10^5 nT

    (I had to look up CGS, I was born in 1962 so by the time I studied physics we were using SI units, but my teachers had not really switched yet, so we would tease them about using foot ounces per fortnight)

    On Wed, 10 Apr 2019 at 01:33, Bob Goethe <NoReply_Goethe@fer3.com> wrote:
    Now, the publication he cited was 50 years old, and we know that the magnetic north pole has migrated considerably in that time.  The other thing is that it seems that the use of "oersted" as a unit of measurement has been superceeded by nT (nano Tesla).  I have been unable to find any resources that relate oersted to nT.
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