cpo wrote:
would it be possible to solve the problem for the need of a huge
magnet by using an electromagnet instead of a permanent one.
obviously this would use up more power, but the strenght of your
magnet would be variable, and it would be relatively inexspensive

No, for any number of reasons.

1. No matter how you energize the structure, you will
always be limited to the saturation magnetization
of the ferromagnetic magnet structure. 10-11 kG is
about the limit of affordable materials. Put a bazillion
amps through your electromagnet coil, and you won't
get another microgauss out of your magnet once
you're past the saturation magnetization.

2. Hard magnetic materials such as barium ferrite,
strontium ferrtie, even materials such as alnico or
some of the more exotic magnetic alloys are FAR
cheaper than a copper coil and the power supply
needed to energize it.

3. And what do you plan on doing with the rather large
amount of heat that WILL be dissipated by the ohmic
losses in your voice coil. There's enough of a problem
dissispating heat from the voice coil: in current magnet
structures, at least it's most of the time cooler than the
voice coil, the heat has someplace to go. Now you're
talking about rasing the temprature of the magnet
tructure all the time.

Basically, electromagnets are a bad idea, from a design,
efficiency, cost, size, thernal dissipatyion, performance
viewpoint. That's why the idea was abondoned quite
permanently 70 years ago.