Hi James, That's cool! But I'm not sure how the ferrite works in the
Earth's field. Does a ferrite 'take up' more magnetic field space,
than just it's physical volume? (If I stick a piece of ferrite next
to
my hypothetical gradiometer will I get a signal?)
You know a loopstick antenna? The length of the permeable (typically
ferrite) rod intercepts some fraction of the B-field of a propagating
radio wave. It's the magnetic dual of a mag-loop antenna, which is itself
the E-M dual of a dipole. Consider:
- A [short] dipole intercepts some fraction of the electric field of a
propagating wave. Any E-field pointing along the axis of the antenna
polarizes it, inducing a voltage.
- A [small] loop intercepts some fraction of the flux of a propagating
wave. Any B-field pointing through the center of the antenna induces a
current.
- We can enhance the amount of flux entering the loop by adding a
permeable material, which "sucks in" nearby fields (not strictly true, but
close enough for hand-waving).
- Ironically, the result is a dipole again, but a magnetic dipole: it's
sensitive to B-field along the axis of the core (assuming typical
geometry, like a cylindrical "loop stick").
Now, that's all well and good but it doesn't work for static fields,
because the stick isn't moving (intercepting different regions of B
field).
Suppose you stick a strong magnet onto a the protruding end of the ferrite
rod, enough to saturate it. The saturated part no longer functions as
ferrite, so the rod appears shorter. The influence from external fields
has changed. Now change the magnet to an electromagnet and turn it on and
off. The influence from external fields alternates with the saturable
section.
The only rub -- and many people forget this about saturable reactors --
is, once you saturate one part of the core in the assembly, the whole
thing now ceases to be balanced, and flux from the control winding couples
into the sense winding. The same will occur here, where a lot of MMF
(magnetomotive force, amp-turns) is dropped across the reluctance of the
saturated piece of ferrite (which now has very high reluctance). The
control section ceases to be contained in a core, so flux leaks through
the air and screws everything up. And obviously, the error is in phase
with the measurement, so you can't separate the measurement from the
error.
I'm not damning the approach -- you could, for example, attempt to shield
the control winding with extra pole pieces, or a good conductor. Maybe
the induction can be balanced with a winding not around the core (ooh,
that could be tricky since we're trying to measure small differences in
the first place!). Not sure how well that would work. I'll also addthat
I forget how fluxgates work. It could be they already do this (as
mentioned earlier), which would make the solution a whole lot easier to
imagine.
Tim