Need something like an EMP, but not....

Not really possible to reverse such a large magnetic field in a short period of time.

I'd say some sort of exceedingly powerful electromagnet would be your best bet, maybe you could say it's a sort of Gauss Gun that shoots focused electromagnetic fields.

The US navy is working on a Gauss Gun for ship to ship warfare, it's essentially a bunch of electromagnets that fires a projectile, sometimes they're referred to as rail guns as well.

Rhoda Nightingale said:

Doing pre-research for my NaNo. I'm using crack science on this (i.e. making most of it up) since it's NaNo and all, but I'd like it to at least sound science-y. What I need is a device that will reverse the polarity of a largish magnetic field--like the size of a building. My protag is a space-age ghost buster ("pandimensional anamoly neutralizer") and one of his team's methods of attack is to get electromagnetic readings on whatever place is supposed to be haunted and then reverse the polarity to clear out the ghosts. Allegedly. So I need a machine that will do that, and something to call it that gets that idea across.

Thanks in advance!

Click to expand...

Once you start reversing the polarity of something, anything really, you might as well just do it by recalibrating the main deflector dish. Seriously. "Reversing polarity" is 20 carat technobabble, there's no way to make it sound plausible, just invent some name for that device.

Addendum: As for the basic polarity thing, if polarity is used in any context but chemistry (electron distribution in molecules) or trying to burn out a DC circuit, it's technobabble.
Reversing a magnetic field is also not something that can be done externally. You could turn the magnet around (as in, 180°) but there's no way to change the magnetic field in itself. At most, you could simply set up a vastly stronger magnet to drown out the field from the first one.

Drachen Jager said:

The US navy is working on a Gauss Gun for ship to ship warfare, it's essentially a bunch of electromagnets that fires a projectile, sometimes they're referred to as rail guns as well.

Click to expand...

Almost. Railguns are very different from Gauss Guns, although the two terms get confused from time to time. Coilgun is a (correct) different name for a Gauss Gun. Afaik, no-one's doing research on Gauss Guns yet, only Railguns, since it's not realistic to build a useful Gauss Gun without superconducting coils.

Gauss Gun and Coilgun do refer to the same thing. Functional models have been built but they're impractical with current technology.

Yeah I guess railguns are somewhat different. Same basic principle though, using electro-magnetics to propel a projectile. A Gauss Gun would be closer to the kind of device needed to provide a focused magnetic field.

Drachen Jager said:

Gauss Gun and Coilgun do refer to the same thing. Functional models have been built but they're impractical with current technology.

Click to expand...

Well, the principle is sound, and technology not that difficult. You might see one in highschool physics class. But to get projectile velocities that make it useful as a gun, you need to get magnetic fields from the coils that would blow any non-superconducting material, so it's not feasible with currently available materials.

Yeah I guess railguns are somewhat different. Same basic principle though, using electro-magnetics to propel a projectile.

Click to expand...

Well, if you frame "basic principle" in very general terms. For practical purposes, there is a huge difference between the two. A railgun accelerates a conductive projectile by passing a huge current through it and two rails, which will propel the projectile along the rails, while a coilgun accelerates a magnetic projectile by switching a series of electromagnets so the ones behind the projectile repel it, and the ones in front attract it.

A Gauss Gun would be closer to the kind of device needed to provide a focused magnetic field.

Click to expand...

The magnetic fields of a gauss gun are more controllable (well, a railgun doesn't contain any magnets, electro- or otherwise) but there isn't really such a thing as a focused magnetic field. Not that it matters much for the op, just nitpicking for the fun of it.

I'd invoke monopoles; since they're purely theoretical at this point, you can make them do whatever you want. Ish.

Don't "reverse the polarity". It's a trope name for "stupid technobabble". It was even made fun of in Stargate. Twice.
Find another name, unless this is a parody.

Rhoda Nightingale said:

I like the sound of "monopole," but I have no idea what that means.

Click to expand...

Sheng-Ji Yang said:

I maintain nonetheless that yin-yang dualism can be overcome. With sufficient enlightenment we can give substance to any distinction: mind without body, north without south, pleasure without pain. Remember, enlightenment is a function of willpower, not of physical strength.

Click to expand...

Emphasize added

Monopole Magnets have a north-pole but no south-pole or vice versa.