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From: José Otavio O. de Almeida (no email)
Date: Wed Jun 23 2004 - 18:20:11 EDT
A good point, but if the needle still has some residual magnetisation, the
possibility of remanetising in the opposite direction is small, it requires
more energy than otherwise. Being free to rotate, the needle tends to align
itself in the position of minimum energy. But in order to guarantee the
process, make sure the initial position of the neddle is paralel to the
plane of the coil: this way it will align itself perpendicular to the coil,
in the correct sense, as sonn as current is aplied.
-----Mensagem original-----
De: Navigation Mailing List
[mailto:]Em nome de George Huxtable
Enviada em: quarta-feira, 23 de junho de 2004 16:52
Para:
Assunto: Re: RES: Recharging Compas Magnet
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. José 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.
George.
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