@Theodosius: I googled "black holes magnetism", wasn't satisfied. And I know the answer to your imaginary question (I didn't ask wether black holes have magnetic fields) is yes (well, I found no proof that there's always magnetic fields around black holes, although that's an assumption you can quickly make, because, but definitely around some), but your "because" doesn't explain why they have magnetic fields, it only explains why we know they have magnetic fields.
I'm definitely looking some more on google, but I'd say this thread still has a purpose. Google isn't the answer to everything, sadly.

They have magnetic fields for the same reason the earth has one. They rotate. What more explanation do you need?

As I understand it, the most accepted theory about why the earth has a magnetic field is because lots of fluid metals within the earth flow around (fluid because of the heat), with the metals ionizing (easier because of the heat). You need the earth to rotate too, and then you get magnetic poles and a magnetic field.

Just rotating isn't enough as far as I know. So what happens in a black hole to allow for magnetic fields?

Better clarification of the questions. Well, pressure results in heat and heat liquifiea ferrous material. Black holes have the ultimate gravitational pressure so naturally would have liquid metal acting like the Earth's but much more enhanced.

I didn't ask specific questions, just wanted to talk about the phenomenon :)

Thanks for answering.
Is it easy enough for ionized molecules to become opposite poles though? How does a molecule get from one end of the dark hole to the other without following the flow, since you wouldn't get opposite poles if all molecules flow along the others?

Also, does the formula for magnetic fields work for black holes? Will be more specific on that later probably.

For the record, I have my thoughts for both questions. Will probably get to them tomorrow, or later.

http://www.dailygalaxy.com/my_weblog/2013/07/enormous-magnetic-fields-of-embryonic-black-holes-trigger-the-brightest-explosions-in-the-universe.html

This article, found on google, explains the theory, although the focus is on gamma-ray bursts.

The liquid metal that Draugnar refers to is a type of hot gas called plasma, like what is found in the sun or in lightning. It's conductive. Friction between the different layers of plasma spiraling in causes a magnetic field, vaguely like static buildup or the electric charge buildup that causes lightning in clouds, except organized because it is a spiral, and not just from random wind gusts.

Anytime there is matter falling into a black hole, something like that will happen since it will spiral in. What is questionable is how much of a magnetic field there is from gas farther out from the black hole that is not yet hot enough to be called plasma.

What is even more interesting, since black holes form from matter that spirals in, is the possibility that the black hole itself is rotating, forming a ring instead of a point. I haven't looked into this for a few years, so I don't know latest theories.

Thanks for the link and theory.

For now, I just want to react on the last paragraph:

Assuming the black hole itself rotates (not saying it does, not saying it doesn't, but for the sake of argument, assume for a second it does), how do you draw the conclusion that a black hole must be a ring because of that rotation?

I know this isn't a very strong argument, but to explain why I think there's at least a step missing before such a conclusion can be drawn:

The earth rotates too. It's not a ring though.
So my rather simple conclusion: not everything that rotates is a ring.

Why would a rotating black hole have to be a ring?

google "hawking black hole torus"

e.g.

http://www.theorie.physik.uni-muenchen.de/activities/schools/archiv/sfp10/lectures/dabholkar_lectures.pdf

http://www.newton.ac.uk/preprints/NI05053.pdf

...amongst many others.

"Is it easy enough for ionized molecules to become opposite poles though? How does a molecule get from one end of the dark hole to the other without following the flow, since you wouldn't get opposite poles if all molecules flow along the others?"

I think you have a misunderstanding of what a magnetic field is. Perhaps you are confusing it with an electric field. Electric fields are formed by poles of oppositely ionized molecules (generally), not magnetic fields.

Magnetic fields, on the other hand, can be formed by electrons rotating in circles. Consider the common electromagnet, the electrons are flowing in circles, generating a magnetic field that goes through the inside and back the outside of the wire tube. It is theorized to be similar in the Earth,

http://upload.wikimedia.org/wikipedia/commons/b/b4/Outer_core_convection_rolls.jpg

with swirling currents of molten metal (carrying electrons) inducing our magnetic field.

The same is theorized to be with black holes; black holes spin so there are theorized to be spin currents inside the black holes, producing the observed magnetic fields.

Hmm, this reads more confusingly than I anticipated...

Perhaps these images will make it clearer:

This is a one-loop electromagnet:
I is current, red lines are magnetic lines:
http://upload.wikimedia.org/wikipedia/commons/e/ea/Magnetic_field_of_wire_loop.svg

This is a simple electric field caused by two oppositely charged particles:
http://upload.wikimedia.org/wikipedia/commons/e/ed/VFPt_charges_plus_minus_thumb.svg

Thanks for the clarification, PSM.

A magnetic field forms from an electron moving. By having the electrons move in a circular path you can control the direction of the magnetic field. By having several loops around an iron core you can superimpose the magnetic fields of each loop to increase the strength.

I don't have the answer Steephie only quesions.

1/. I didn't think all black holes spin, already the majority may.
2/. Perhaps we shouldn't assume that because the earth spins and some black holes spin that the they both have magnetic fields in the same way. When the earth spins the molten metal may spin with it, but in a black hole the molten metal may be continually sunked into the middle. This force may overcome the spinning force so that no magetic field is produced.

Every time I see the title, I think of "Airplanes and aerodynamics" - Diplofools doctorate's thesis. :-)

Here's another short article to read:

http://phys.org/news/2013-08-strong-magnetic-field-milky-black.html