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Magnetic Field in a Flat Spiral



Original poster: "David Thomson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>

The magnetic field of a flat spiral coil has an elongated torus shape near
the surface of a flat spiral coil.

I've tried this experiment on single wound, bifilar wound, and wye coil
wound flat spiral secondaries, with the same results for all.  A 12VAC 3A
wall transformer was connected between the center tap and the outer lead.  A
NIB disk magnet is held in the hands.

With the AC power on, hover the NIB magnet in various positions around the
coil.  The magnet vibrates strongly, but has no magnetic pull in any
particular direction.  But as the magnet is turned through the various axes
in the hand it is found that at different locations over the coil the magnet
vibrates noticeably stronger along some axes than it does others.

In the center of the coil, the position with the least vibration is when the
N/S poles are perpendicular to the plane of the coil (x axis).  But as the
magnet is brought further out, the point of least vibration occurs as the
magnet becomes more perpendicular to the y axis (in a rotational coordinate
system.)  So as the magnet travels toward the outer winding, the N/S poles
are pointing toward the z axis (the z axis is up and down through the center
of the coil.)

The magnetic field turns 90 degrees in a very short space near the outer
windings and continues around to the bottom such that it is 180 degrees from
the upper magnetic field.  The magnetic fields above and below the coil are
parallel to each other but pointed in opposite directions.  The magnetic
field in the plane of the coil is pointing in the same direction as the
field in the center of the coil, but in opposite directions.

I also noticed that when I did this experiment near a large ferrous object,
the magnetic field around the space of the coil leaned in the direction of
the ferrous object.

There are many other interesting geometric features of the magnetic field.

Has anybody on this list done research into the magnetic fields around coils
in general?  I'd like to learn more about this.

Dave