Sean Breheny wrote (and in response to Mike Keitz' answer):

> I don't understand why two windings, in phase and wired in parallel would
> not increase the flux.

  Because you can visualise them as winding of one piece of wire of
double
the thickness.  Nice to pass MORE current through if you can afford the
drive current.  Usually however, you want less current but can afford a
higher voltage as we are talking in the few ohms range, if that.

> In fact, I doubt that I want to wire them in series because they are
> wound starting and ending at the same location on the toroid and both go
> around the whole circumference of the toroid.

  So they should.  Your aim is as many TURNS as possible is it not?
Series
connection = more turns.

>>The purpose of the primary winding is to periodically saturate the core.
>>If you aren't applying enough drive to do this then there won't be any
>>nonlinear effect to detect DC magnetic fields with the other coils.

  .. etc.  Hint: if you want to saturate the core, use as narrow
(minimal
cross section) as possible.  If you want to expose it to as much
external
field as possible, it should be as large as possible.  Think of a ladies
wedding ring in shape.  Now the sense coils:  They should be wound not
around the ring of the toroid, but flat over it, and there are two wound
at right angles.  A coil with its axis coincident with the toroidal axis
will sense virtually nothing.  (It might just be possible to use three
sense coils with a really weird shaped toroid!)

>>Monitor the primary current and make sure it sharply increases at some
>>point during the drive cycle.  If it is a nice linear triangle wave (with
>>square wave drive) it isn't enough.

  And if it's mostly flat-topped, the frequency is too low for the
winding
inductance.

>>Obviously the choice of core material is important.
> The color code on the toroid is green.

  It's arguable whether it should be a ferrite.  Probably should be
ferrous
laminations, strap wound or even coiled wire itself.

> (I don't have specs on the toroidal core, so I don't know permeability of
> core material ), but shouldn't an amp of current produce enough flux to
> saturate it?

  Without specs, including material, size and cross-section, it is quite
impossible to surmise.

> At one point, when I had abt 2.5 amps going thru it, It was producing an
> audible sound at the excitation frequency and was getting hot

  You mean the WINDING was getting hot?

> (since the winding has a resistance of only abt .1 ohm, I assume that
> this was hysteresis heating, not resistive heating).

  I^2*R = 2.5 x 2.5 x .1 = 0.625W is a perceptible amount of heat.

  If the winding resistance was only .1 ohm, I fancy you were using wire
thicker than necessary and should have opted for more turns, thinner
wire.

  All of this equally "rule of thumb" of course.  This is NOT my
speciality!

  Cheers,
        Paul B.