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    Re: Learn the stars, by phone
    From: George Huxtable
    Date: 2009 May 16, 00:39 +0100

    Brad had mentioned the presence of a mumetal shield around the batteries of
    the Skyscout instrument.
    
    I had cast a bit of doubt on that, pointing out that the mumetal screen, in
    itself, would locally distort the Earth's magnetic field, even as it
    shielded any magnetic effects from the batteries.
    
    I've now had a chance to see the manual that went out with the instrument,
    and it does indeed refer to the use of mumetal to screen the batteries.
    Presumably, then, the evil effect of the magnetism of the batteries is such
    that any such perturbation of the ambient field is a price worth paying to
    eliminate it. I imagine that the magnetic sensors will have been placed as
    far as possible from the battery assembly, so as to take advantage of the
    inverse-cube law, which applies in this case.
    
    Does anyone in the list know about the construction of such alkaline
    batteries? It's presumably the steel case that is the origin of the
    magnetism. Is that a fundamental part of the chemistry, or could they be
    encased in another material, non-magnetic, just as well, if at greater
    expense? Has any specialist battery maker explored such a niche market, I
    wonder (if so, they would be expensive). I'm interested in such effects on
    other types of battery too, having substituted LED illumination for the
    faded Tritium lighting of a Mini-compass, using a 3 volt coin cell (CR
    2032). This has to be spaced several inches outside the hockey-puck profile
    of the compass, to preserve magnetic accuracy. Perhaps use of a mumetal
    shield, for that little battery, would allow it to be placed against the
    compass body. It seems worth investigating. Does anyone have experience? Can
    anyone offer a bit of mumetal? I used to use masses of the stuff, in my
    working days, for magnetic shielding of photomultiplier tubes, but that was
    long ago.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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