From the earth’s magnetic field?
Because we aren’t moving relative to it, as I understand it. It is possible to extract energy from it if you are moving; this has been suggested for use in space.
Are you suggesting magnetic induction? You’d need to run conductors way the heck up into the magnetosphere, which is constantly being compressed and distorted by the solar wind. It’s anywhere between 50,000 and 90,000 km up.
Could this be possibly be scaled down? The magnetic field is There, shouldn’t we be at least working on it?
The magnetic field provides no energy for extraction - the movement of a conductor in this field does.
And there aren’t any obvious cases where we can move a conductor without paying to do so (and if we have to drive the thing, it’s obviously a losing equation, thanks to the 2LoT)
My compass runs on energy from the earth’s magnetic field. However, you can move the needle just by shaking it, so you know there isn’t a whole lot of power holding it in place.
Conceptually, think of the Earth’s magnetic field as being similar to its gravity field.
You cannot directly turn gravity into useful energy. You can lift something up, let it fall down again, and extract energy from the movement as it falls down, but the energy you get out of it will always be less than what you need to invest into lifting it up first. Bummer. Now, there are exceptions, such as if you have a high waterfall, then you can extract energy from the falling water. It is still the case that the energy needed to get the water to the top of the waterfall in the first place, will be more than what you can get out of it by converting it to electricity, but that lifting energy is being provided by the Sun (via evaporation + rainfall).
Unfortunately I can’t think of something similar for the Earth’s magnetic field, where we can extract energy by moving metal particles through the magnetic field from a high-potential-energy to a low-potential-energy location, and then have Mother Nature move them back to us “for free”.
It’s not possible, at this time, to justify the expense. We currently have much, much cheaper ways to produce energy. Even far in the future when petroleum and coal become dear, solar and hydro will probably continue to be much, much cheaper than stringing cables into the magnetosphere.
There is energy stored in a static magnetic field. You extract that energy by reducing the strength of the field. If you have any ideas for how to implement this for the Earth’s field, I’m all ears.
Oh, and the electrodynamic tethers Der Trihs mentions do not extract energy from the Earth’s magnetic field: They instead extract it from the kinetic energy of the spacecraft the tether is attached to. But this kinetic energy is derived from chemical energy in the rocket fuel, and it’s much more efficient to skip the middleman and just convert chemical energy directly to electrical energy.
“The Van Allen Energy Corporation wishes it to be known that the failure of Earth’s magnetic field is in no way related to our efforts to extract energy from it by reducing the strength of said field, and no responsible scientists would disagree. The catastrophic failure of the field was an entirely natural event, and no one could have foreseen that weakening it would lead to failure. We do, however, apologize to anyone who who was offended by being mutated.”
I wish Richard Feynman was still alive. 
I’d consult him!
HA! That is great!!
David Brin has a nice story about an orbiting space station using tethers for enery transfer (it’s available on his website (David Brin's Official Web Site: "Tank Farm Dynamo" (short story))), though there’s a twist in how this is done.
The Hoover Dam disagrees with you.
Really? You’d think that the Hoover Dam would know better than that how it itself operates. Or maybe not; after all, it’s just a bunch of concrete and steel, and mineral construction materials are not noted for their cognitive abilities.
In any event, Hoover Dam does not directly turn gravity into useful energy. It uses gravity, along with a variety of other phenomena, to turn sunlight into useful energy.
You should maybe read the entire post before disagreeing with it.
There are variations in the Earth’s magnetic field caused by coronal mass ejections from the sun and you can get reasonable swings in the field. There have been currents in the hundreds of amperes measured in long power transmission lines due to big storms. So there is a bit of power available, but given the random nature of the storms and the relatively minor amount of power, it isn’t going to be useful to try to harvest it, except for perhaps very odd applications. I wonder if enough field variation exists to power scientific instruments in the Arctic and Antarctic winters if a big enough coil could be laid out over the ice. Again the power source is ultimately the sun.
The earth’s rotational energy can be harvested. Just need to anchor a flywheel at the equator.