Well, it is.. on geological timescales. And with sudden flips at that. The record is in the orientation of suitable partices at the divergent plate boundaries ('frozen' like that when the lava cooled), which also is an indicator for ocean floor spreading and plate tectonics. It's pretty awesome, actually. But as the sibling comment says, in addition to being slow it's far too weak as well.

edit: Here's an image, because it's so awesome: http://en.wikipedia.org/wiki/Vine%E2%80%93Matthews%E2%80%93M...

It could exploit the earth's motion through the interplanetary magnetic field[0] to charge your batteries... reeeeeeaaaally... sloooooowly.

[0] http://en.wikipedia.org/wiki/Interplanetary_magnetic_field

That was a joke example of battery I made up that sounds believable enough to generate traffic from sites like this...