actually i just wiki'ed it.
I thought the earths magnetic pole could be used for making a small current, it turns out I was slightly wrong and found telluric current. I can't find too much out on how many volts though or standard current, but I have found that it was used to help telegraph poles in US so I was thinking it would have to be at least 1volts, then that little device you showed us from linear technologies might be useful in upping it to a decent voltage
I thought the earths magnetic pole could be used for making a small current, it turns out I was slightly wrong and found telluric current. I can't find too much out on how many volts though or standard current, but I have found that it was used to help telegraph poles in US so I was thinking it would have to be at least 1volts, then that little device you showed us from linear technologies might be useful in upping it to a decent voltage