> maximum amp-turns and shaping the field density

I plan, unless advised otherwise, to make the core long and slender,
probably 6mm soft iron rod. That way I can get the turns up, 26 or
28 gauge, so the distance from the core should be minimised, and
will help keep the current down

> Practical aspects quickly push you into a particular solution.  You
> probably have some maximum volume you can occupy and some
> power source with a maximum current available

The initial energising will be a DC dump via a FET and reservoir cap.
After that I can let the PSU take over. There's no real restriction on
parameters but obviously the coil and PSU don't need to be unnecessarily
big

..  Wire has volume and resistance.
> Smaller wire gives more turns in the available volume but higher
> resistance means you need a higher voltage to get the current.  And so on=
..

> In the solenoid core, the flux departs immediately at the ends of the cor=
e
> and bends back around to the other end.  The effect is that the flux=20
> density
> quickly reduces

But it would be strongest and most repulsive at the point it leaves the cor=
e=20
?

> The problem you are solving determines the shape that can be used so
> it's up to you

If what I just asked is correct, then a straight bar would be best to repel
a magnet as it's pole against pole, with the attracting pole at the other=20
end
of the EM core

> As for permanent magnets, Brendan mentioned the fact that the fields
> add together.  This is up until the core is saturated in which case no
> further increase of field strength is had even if the current is increase=
d

I think I picked up on that on my travels. My question was whether a
permanent magnet can improve the efficiency or response of the EM
by "wetting" the domains before current is applied

> For a quick and horrible demonstration of this, bring a high power
> rare earth magnet up to the inductor in a switching power supply
> under maximum current load - you can easily saturate the inductor and
> then destroy the switching circuit due to high current.

Much as I'd like to see that (on someone else's of course), I'll take
your word for it ;-))

Joe

--=20
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