(So it turns out I can pack boxes faster than expected! Unfortunately that means I’m now just killing time until tomorrow when I load everything into the truck and head off for my new job in Waterloo, Iowa. But here’s another article, just for you! I know, I know, it’s not a project or a circuit, but I left my electronics stuff in St. Louis…)
You know what? The Discovery Channel is right, the world is just awesome.
The other day I was surfing Wikipedia and happened across an article on telluric currents. Apparently, changes in our planet’s magnetic field induce fairly substantial currents into the surface of the earth (both across lands and oceans). Now, I’ve heard of the earth’s magnetic field, and I’m familiar with grounding rods acting as current paths. But telluric currents? Well, like 99.9% of all Wikipedia articles, it’s new to me.
So what’s the deal with these mysterious earth currents? Well believe it or not, this is a phenomenon which was first observed way back in the mid-1800s. In fact, it used to wreak havoc with telegraph and, later, telephone lines. You see, electrical currents tend to follow the path of least resistance. So if there happens to be a wire connected between two points on the earth’s surface (e.g. a communications ground line), any current that might normally have flowed through the earth itself will instead flow along the lower-resistance wire. For example, according to The Earth’s Electrical Environment (Pg. 244), between August 28th and september 2nd, 1859, an enormous geomagnetic storm induced 800V on a 600km wire in France. Much later, on March 24, 1940, a similar event damaged two communications sites in Tromso, Norway:
“Sparks and permanent arcs were formed in the coupling racks and watch had to be kept during the night to prevent fire breaking out… One line was connected to earth through a 2mm thick copper wire, which at once got red hot, corresponding to a current more than 10amps.”
Now telluric currents aren’t all bad. In fact, they’ve recently been used to map and explore underground structure. By taking measurements of voltage and current along an array of points at the earth’s surface, scientists can characterize the conductivity of different areas of the ground. This method can even be used to identify mineral or petroleum deposits. For more details on this, see this article on Magnetotellurics.
Well as you may have guessed, naturally-occuring telluric currents can even be harnessed to provide electrical power. Of course, this requires a wire of substantial length. And this point, combined with the fact that there’s not much energy to be drawn from most telluric currents anyway, makes this an impractical power source. However, there is one related invention which at least solves the length issue: the earth battery. Basically, the earth battery works just like any other chemical battery – you insert two electrodes made of different metals into the ground, and the earth acts as your electrolyte. The ground needs to be slightly wet for this to really work properly. But with such close spacing, you’re not really deriving energy from the earth’s magnetic field, you’re just making a simple chemical battery (like that potato battery you made in elementary school). However, earth batteries did work well enough to power some early telegraph stations. If you’re curious, here’s the patent for an improved earth battery, issued in 1874.
By the way, in researching for this article, I ran across a (seemingly) amazing “patent” for a device that claims to be able to produce 3000W of electrical power from a 500W input. It says this can be accomplished through a simple high-frequency oscillator and a half-mile antenna which derives energy through resonance with telluric energy. Now, I’ll let you come to your own conclusions, but I think this is bunk. For one thing, US Patent #253,765 is for a portable fence, not an electrical power accumulator (and I couldn’t find this “patent” via term searches). But secondly, how could telluric currents possibly resonate at 500kHz? Everything I’ve read describes naturally-occuring telluric currents as having periods on the order of, at shortest, minutes. Which means we’re talking about frequencies in the millihertz, not kilohertz. In fact, most telluric current oscillations are diurnal, meaning they follow a daily, 24-hour cycle. Oh and third, the rest of the website hosting that “patent” is unbelievably sketchy…
Anyway, if you’re curious, take a read through this chapter, available for free online, and tell me what you think. I’d absolutely love to try this out sometime. Anyone have any suggestions for how to do it? I’m thinking of just buying the cheapest, longest length of wire I can get from Home Depot, along with a couple of pieces of re-bar. Then I’ll just go find a field somewhere, set my two electrodes pointing north and south (as that seems to be the predominant direction of telluric current flow in the US), then check it with a voltmeter. Perhaps nobody would mind if I tried this at a park someplace…