sci.physics.electromag

On Apr 4, 6:45=EF=BF=BDpm, j...@specsol.spam.sux.com wrote:
> Paul wrote:
> > On Apr 4, 4:25???pm, j...@specsol.spam.sux.com wrote:
> > > Paul wrote:
> > > > On Apr 4, 10:14?am, Benj wrote:
> > > > > On Apr 3, 9:25 pm, j...@specsol.spam.sux.com wrote:
>
> > > > > > A non-magnetic cage around a compass will have no effect, conduc=
tivity
> > > > > > isn't the issue for static magnetic fields.
>
> > > > > What Jim means it a conductive enclosure (provided it isn't TOO
> > > > > conductive) won't effect the static readings of the compass. (a
> > > > > superconducting enclosure would block magnetic fields) On the othe=
r
> > > > > hand the conductive enclosure could affect the speed of response o=
f
> > > > > the compass due to eddy current effects as the magnetic field is
> > > > > changed or the device moved in that field. ?How much it affects th=
e
> > > > > the response depends on the details of the construction, of course=
.
> > > > ??? ??? ??? ??? ???Are you saying that the magnetic field will not
> > > > be distorted due to a non-magnetic, but conducting cage?
> > > > ??? ??? ??? ??? ???Both the compass and the cage will move within th=
e
> > > > field.
>
> > > Moving a conductor in a magnetic field will induce current, called
> > > eddy currents, in the conductor, which in turn cause magnetic
> > > fields.
>
> > > The magnitude of those induced fields will be proportional to the
> > > magnitude of the external field and the velocity of the conductor.
>
> > > Unless the external magnetic field is a lot greater than the Earth's
> > > and the conductor is moving really fast and is really close to the
> > > compass, you are unlikely to see any effect.
>
> > > However, accelerating or turning a compass will cause compass errors
> > > all by itself. I'm assuming an ordinary mechanical compass here.
>
> > > --
> > > Jim Pennino
>
> > > Remove .spam.sux to reply.- Hide quoted text -
>
> > > - Show quoted text -
> > =EF=BF=BD =EF=BF=BD =EF=BF=BD This will be a digital compass, with MEMS
> > accelerometers.
> > =EF=BF=BD =EF=BF=BD =EF=BF=BD Are you saying that the magnetic field wil=
l
> > not be distorted due to a non-magnetic, but
> > conducting cage? =EF=BF=BDAs if the cage wasn't there?
>
> If the cage is not moving relative to the field, no distortion.
>
> If otherwise, see above.
>

Unfortunately, the cage is this case will
be a conducting chassis, which will enclose the
digital compass. This will be a portable device,
which the user will walk around with.

It won't be a super-conducting shell,
so the eddy currents will dissipate as heat. Hopefully
the effect will be somewhat transient, but the fact
that the compass itself is sensitive enough to
measure the earth's magnetic field, means that
it just may be sensitive enough to pick up distortions
to the field due to the motions of the user, even
if those motions are relatively slow.