See message 1207, "Results from a new A-Theory".
An e-mailer inquired if flux-coupling to the main core
might influence the results I reported about the magnetic
field from the output coil being greater on the outside of
the core leg than in the interior of the core space.
Based on the measured inductance change with the ferrite
sense coils in air and then in place at the core leg,
there is some coupling (about a 40% change in inductance ),
but this is not enough to skew the results I measured. To
further eliminate any flux coupling, I built a fourth
set-up using a Honeywell AMCC-100 core. This is a small
core, and the size difference provides further variation.
Instead of ferrite sense coils, I used Allegro/Sprague
UGN-3503U Hall-Effect sensors. These devices have a
linear response to about 23 kHz and a sensitivity of
about 1.3 mV output per gauss. Prior to installing them
near the core leg, I measured their sensitivity and chose
two devices whose output was within five per-cent when
exposed to the same field from a solenoid coil resonating
at 10 kHz. I used balsa wood to provide a spacer and
support so that the devices were about one-tenth inch
away from the core surface, about one-tenth inch from the
first layer of the output coil, and held in the center of
the core leg. These supports were made as closely
similar as was practical. Unfortunately, I neglected to
add an electrostatic shield, so the measurements are
sensitive to frequency. Therefore, I made four
measurements at four separate frequencies to determine a
trend of frequency response. The results at the lowest
frequency I tried, 1.23 kHz, were output-side sensor:
49 mV, interior sensor: 11 mV for a ratio of 4.5 to 1.
At the highest frequency I made measurements, 3.71 kHz,
the output-side sensor: 78 mV, the interior sensor: 38 mV,
for a ratio of 2.1 to 1. These devices are packaged in
plastic and have an internal operational-amplifier which
is clearly sensitive to external electric fields. A more
precise set-up should use a thin brass layer to provide an
electrostatic shield between the output coil and the
sensors.

David J.