In reading about Tesla, he made many references to the enormous, free energy source ion the earth’s magnetic field. However, this field is very weak-indeed, Michael Faraday tried to detect the currents induced by the field (in the River Thames), and could not do so. Does anybody know what this mysterious source of energy was, that tesla proposed exploiting?
People’s wallets.
WAG: Electromagenitc current produced by the Earth’s magnetosphere?
I don’t know about the magnetic field, but there’s abundant electricity flowing in the atmosphere; and using it seems to be a legit, albeit tricky, concept. See “atmospheric electricity” and “radiant energy”.
Atmospheric electricity - Wikipedia
Extraction of vibrations from the aether. Most people (like, those that aren’t Tesla) are incapable of comprehending this. 
I wish I could find a good technical critique of his work. He pretty much invented today’s alternating current power system but then went off the deep end with radiated power.
An imaginary one.
In many ways, he was trying to make antennas that would take advantage of the various differences in energy potential between earth ground and what is available in the air. An antenna will absorb energy that is radiated through the air. Usually tiny amounts, in somewhat narrow bands of energy. He wanted to capture more. He succeeded in many ways. It is a possibility. But gathering such a wide array of energy is difficult.
I believe they tried towing a conductive cable from a space shuttle once. To test if current would flow. Enough current flowed to melt the cable.
Yeah, nobody’s denying the electrical potential exists. It’s just that we have so many other potential and current sources of electricity which are so much more practical we just don’t really need that one. It would probably be more practical to capture the motion of the tides and use that to power turbines than to run massive wires up into the high atmosphere all over the place.
The best critique of broadcast power as Tesla imagined it is the inverse square law: At a distance r from the transmitter, the amount of power you get is proportional to r[sup]-2[/sup]. That is, your power gain is a quarter as much at two units of distance, a twenty-fifth as much at five units, and so on. This is intimately related to the fact we live in a three-dimensional universe, so it isn’t something we can engineer around while keeping the basic design.
Practical wireless charging mats are, therefore, mats: They work by not broadcasting power, but rather by getting the device being charged involved in a small electrical circuit which involves a relatively tiny air gap. More practical broadcast power designs aren’t broadcasting much at all, either, so much as narrowcasting, in that they presume they’ll have a practical way to aim beams of power to things. This is an interesting technical problem, but not nearly what Tesla imagined.
Tesla was an amazing man for the latter 19th Century. He had a tragic personal life. A large part of that tragedy wasn’t due so much to Edison as the insanity which rendered him a crackpot in the early 20th Century. Even if he had remained entirely sharp and on top of things throughout his life, he still died decades ago, and science has moved on since then. The best way to honor the spirit of Tesla is to keep up with it.
You mis-spelled “Electromagical” there?
While experiments have been done with power tethers on satellites, I have no idea why, because they’re a very impractical way to power satellites. That’s not free energy you’re harvesting; you’re just converting orbital energy to electrical energy. Which means that if you want your satellite to stay up for any reasonable amount of time, you’re going to need rockets (well, more rockets) to continually boost the orbit. And we all know how inefficient rockets are.
If anything, you’d want to do the reverse: Use a conventional solar panel for power, and use that to force a current through the wire to increase the satellite’s orbital energy, as an alternative way to replenish energy lost to orbital decay.
And the use of electrodynamic tethers for propulsion isn’t some kind of theoretical notion; several companies, and most notably Tethers Unlimited are developing electrodynamic tether systems for satellite applications which not only allow stationkeeping but orbital adjustment or end-of-life retirement for spacecraft in Low Earth Orbit, and potentially even on-orbit servicing operations so a repair spacecraft or tug could service or reposition other satellites. This could be used for spacecraft of any size, albeit the low thrust levels limits the rate at which changes can be made, and is probably most practical for nanosats or picosats. Trying to extract this energy in a terrestrial, non-moving generator, however, is not very practical.
There is a lot of energy stored in the geomagnetic field, primarily as charged particles emitted by the Sun, along with some particles produced by cosmic ray scattering and, in one instance, by the high altitude detonation of a nuclear weapon. However, extracting this energy is logistically difficult, and it isn’t at all clear what the effects would be of draining the Van Allen belts, as we only have a very incomplete understanding of how the belts interact with the upper atmosphere. Given that we can get solar energy in orbit at an average of 1366 W/m[SUP]2[/SUP], there would seem to be little utility in dragging large tethers through the belts to extract energy in exchange for the loss of orbital energy due to drag unless there was some limitation on the size of solar arrays a satellite could deploy. It probably makes more sense to used beamed power in that case with a base satellite beaming high frequency microwaves to a compact receiver on the host client satellite versus deploying a long tether system for power generation.
Stranger
No – Tesla apparently believed in his notions. He wasn’t a con man.
If you want someone who was trying to tap people’s wallets under the guise of tapping an inexhaustible electromagnetic power source, you want John Ernst Worrell Keely, con man extraordinaire. There are still people around who believe that he was onto something, despite revelations of his fraudulent setup:
“If you want someone who was trying to tap people’s wallets under the guise of tapping an inexhaustible electromagnetic power source”… in the past you had to look no further than the Straight Dope website.
For the longest time one of these Free Energy companies was running ads that showed up here in SD.
As I recall because of the way web advertising is done it wasn’t the SD’s fault and there was no way they could shut it off.
I haven’t seen it for a while now so maybe they either figured out how to get rid of it, or the advertiser just went away.
And don’t forget Joe Newman.
There is no aether. Michelson and Morley took care of that.
No, M&M disproved the ether. The æther is something else entirely, as any proper mad scientist could tell you.
Ah. 
Eh?
I guess I need to wait for DrFidelius to come explain the joke for everyone.
(yes, it was my joke, but I’m sure he’d explain it better than I could)
It’s not an æther/or proposition.