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    Re: RES: Recharging Compas Magnet
    From: George Huxtable
    Date: 2004 Jun 23, 20:51 +0100

    In response to a question about how can one remagnetise a current needle,
    Jose Almeida responded on 20 June-
    >You can recover the compass by applying a strong manetic field on it. You
    >can do that by winding a coil around the compass, and applying DC
    >(continuous current) on it. The coil plane should be perpendicular to the
    >plane of rotation of the compass. The compass "needle" should be left free
    >to rotate, so that when you apply dc to the coil, it will align itself to
    >the magnetic field generated.
    >Try, say about 50 turns of AWG 24 or 28 wire, and apply 12v DC from a car
    >battery. The contact should be brief, less then a second, as the current
    >will be very high, almost a short circuit. If you leave it connected it will
    >generate a lot of heat and burn.Just scratch the contact a few times, that
    >shoud do it.  Jos? Almeida.
    Something worries me about that proposal, though it's not based on any
    practical experience, because I have never tried to do the trick that Jose
    suggests. Alarm-bells ring when he says-
    >The compass "needle" should be left free
    >to rotate, so that when you apply dc to the coil, it will align itself to
    >the magnetic field generated.
    The problem that I can foresee is as follows. When you close the circuit
    through the battery and coil of wire, the current will increase to a high
    value very rapidly indeed: within a few microseconds, at a guess, because
    the inductance of the coil will be small, of the order of a few
    microhenries. There's no way that the needle is going to be able to rotate
    to align itself with the field of the coil in that short time.
    So the needle will become magnetised by the coil, in those few
    microseconds, in a direction that depends on the direction of the field of
    the coil, and the direction the needle happened to be pointing in at that
    time. It may magnetise the needle in the direction that you want, or, just
    as likely, in the opposite direction so that the compass will afterwards
    point South instead of North. Whichever direction it was, while the current
    flows the needle will then rotate to align itself appropriately, according
    to the applied field from the coil.
    All would not be lost, though, if the needle happened to end up pointing
    South. It should be possible to remagnetise it again, the other way, using
    another burst of current, but with the direction of the coil reversed with
    respect to the pointing of the needle.
    I have no idea whether the (theoretical) difficulty suggested above
    presents real problems in practice (Jose Almeida might know), but it might
    be worthwhile to bear it in mind.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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