[ChuckMcM]

It isn't a stupid question, it is a good one. The answer would depend on how the field is generated in the first place.

Given a field generated by asymmetric rotation of the molten core at the center of the Earth, 'shorting it' (apply a load) would presumably affect the core's rotation. In terms of relative energy however, the poor coupling at the surface would suggest that this would be a very challenging way to divert any meaningful amount of power from the core itself. It would however have to deal with points in time where the core reverses its magnetic field. The papers on core reversals are fun to read.

I think more usefully, the presence of the voltage, might be an interesting way to localize one's location and orientation.

I remember brainstorming "off the wall" power generation ideas and one that has yet to be realized would be to inject dust ahead of a wind turbine with a collector in the back. Then using the Van DeGraf effect to generate power instead of lightning as it currently does.

[fpoling]

If one needs location, then the magnetic field can be measured directly. It is already considered as a potential alternative for GPS, https://www.electronicdesign.com/markets/automation/article/...

The main problem is that locally measured Earth magnetic field varies on a daily basis and is strongly influenced by solar storms.

A better alternative is to use variations in Earth gravity to improve inertial navigation. That vastly more stable.