Tag Archives: magnetism

Measuring Telluric Currents – First Trial

Way back in November of last year (2010), I wrote a short little article on telluric currents, their history, and related applications.  Now, in case you’re unfamiliar with this topic (as I was prior to November of last year), here’s the executive summary: telluric, or earth currents, are electrical currents which travel through ground or water, primarily near the surface of the earth.  They may be naturally occurring (due to changes in the earth’s magnetic field via solar wind), or man-made (e.g, from mineral exploration).

Well towards the end of my previous post, I expressed a desire to try and measure these currents.  Unfortunately at that time, it was winter, and I was in the process of relocating to Iowa.  But now that I’ve settled in here, and the ground has finally thawed, I’ve gone out and performed a quick first measurement.  Here’s the procedure I followed:

  1. Obtain two 36″ lengths of standard rebar and 100′ of insulated 14 AWG copper wire (solid core works, but I used stranded for better contact with the rebar).
  2. Sand/file any rust from the surface of the rebar (to reduce contact resistance).
  3. Strip about two inches of insulation from the ends of the wire, then fray these ends and wrap them tightly around one end of each piece of rebar.  Cover these attachment areas with electrical tape.
  4. Cut the wire, which is now linking the two pieces of rebar, at any point (this is where the multimeter will be inserted).
  5. Space the two lengths of rebar as far apart horizontally as possible, then drive them into the ground as deeply as possible (in my case, this was about 20″).  For my first test, I configured the two so that they would point north to south based on the map shown here and my location in Iowa).  In other words, if I were to stand at the southern-most length of rebar, facing the other rod, I would be facing north.  They were separated by the 100′ of wire.
  6. Measure both current (short circuit) and voltage (open circuit).
  7. Finally, if anyone should question what the @#$% you’re doing pounding rebar into the ground, simple employ this catch-all excuse: “solar flare protection.

So, without further ado, I give you my results (in low-quality cellphone pic format):

Telluric Current - Well, it is measurable...

If you can’t quite make out that reading, my apologies.  The meter indicates 0.55mA (DC).  Yea, not too incredible, I know.  I also measured the voltage between the two rebar rods, but at just 105mV, it’s not terribly impressive either.  So, at best, we’ve got 14.5uW of power to play with – barely enough to run a digital watch (please see this excellent page on Thevenin equivalent circuits and the maximum power theorem for details on how that number was calculated).

Overall, these results are a little disappointing, both in quality and in quantity.  I had hoped to reconfigure my rods a couple of times so as to measure the current’s heading as well.  Unfortunately, for this test I picked a slightly wooded area that also happened to be teeming with mosquitoes.  I’ll do a lot of things for the sake of science, but serving as a meal for blood-sucking insects isn’t one of those things.

In the future, I’d like to leave the rebar in place for a while longer – say, 24 hours – and record data continuously during that time.  I’ve read in a number of sources that telluric currents tend to vary over the course of a day.  So, when I did my test this morning (8:30AM CST), I may have been measuring things as a low point.  The only trouble with capturing data for such an extended length of time is that I’ll need to find a more controllable location, and I’ll need to figure out how to log the data automatically.  I’ve got a few development boards I can probably re-purpose for that though…

So, in summary, for round two of testing I shall make the following changes:

  • Take measurements with the rods configured along different compass headings.
  • Log data for a consecutive period of at least 24 hours.

If anyone has other suggestions, please leave a comment.  Stay tuned for more.  Thanks!

Telluric Currents and the Earth Battery

(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.

Global Map of Telluric Currents, Created 1936 - This is likely no longer very accurate by today's standards.  Understandably, collecting such data wasn't so easy in 1936, so a lot of this map came from interpolation.

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.

An example of data produced using the methods of 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… 🙂

Magnetostriction (aka: Why Transformers Hum)

Have you ever wondered why transformers hum?  I have.  And no, it’s not because they don’t know the words.  But seriously, at first thought, it makes no sense.  They’ve got no moving parts, and how can something produce sound without moving a little air?  Well as it turns out, with transformers, there’s more than meets the eye.

Yes, admittedly, I did just make two terrible jokes within the same paragraph.  Tough.

According to Swiss scientists, the low-frequency hum produced by transformers is often due to the phenomenon of magnetostriction.  Basically, when a ferromagnetic material is exposed to a magnetic field, it can actually change shape, albeit very slightly (think microns).  You see, at the microscopic level, such materials consist of individual magnetic domains.  Think of these domains as tiny bar magnets.  Whenever a magnetic field is applied to the material, each domain actually rotates.  This rotation can cause the material to either expand or contract depending on the orientation of the magnetic field.

Fun fact: if you were to “turn off” earth’s own magnetic field, its diameter would expand by 10cm.  That doesn’t sound like much, but it would result in ten square kilometers of new land area.  Can you say earthquake?

So, when you’ve got a transformer connected to a 50/60Hz AC line, it’s dealing with a magnetic field that oscillates at 50/60Hz.  However, the transformer’s core actually undergoes magnetostriction twice during each electrical cycle (see this animation), so the whole thing vibrates at 100/120Hz, thus producing sound.

Now I should mention that there are a few less-sophisticated effects which may also cause transformers to hum.  Quite often, it’s because their windings or laminations (layers of iron sandwiched together to form a transformer’s core) are not held tightly together.  Thus, forces resulting from the oscillation of the transformer’s magnetic field can cause these parts to vibrate (as opposed to magnetostriction, which causes parts to change shape).  According to one article, this effect can be amplified by any DC offset in your AC signal.  This DC offset results in an asymmetrical magnetic field which causes increased vibration, not unlike the vibrations produced by an asymmetrical rotating weight.

Of course, most of the time, the hum or buzz of a transformer is so quiet that it doesn’t much matter.  However, I once worked on a transformer-isolated DC-DC converter that absolutely screamed.  You see, this converter was to be built as part of a class project.  We were given planar E-cores and copper foil with which to create our transformer.  Normally such cores are used with PCB traces, like so:

A Planar E-Core Transformer

However, we were told that we could just cut loops of copper foil and stack them together between the two halves of the core.  Well, it turns out our twenty-one foil windings didn’t quite fit.  And in the process of trying to force the core together, we cracked it.  Badly.  But not so badly that we couldn’t put it back together with superglue and electrical tape.  We then proceeded to wind the core with standard insulated wire.

Amazingly, our cracked transformer worked wonderfully.  The converter even yielded an efficiency of nearly 90% at 100W.  However, at one point we needed to perform a small-signal analysis on the system in order to improve our mathematical models.  Doing this meant introducing a small variation into the duty cycle of our main 50kHz PWM signal.  This small, 1% variation ranged in frequency from 1Hz all the way up to 10kHz.  And I tell you, when we started to hit 1-2kHz, you could’ve heard our circuit through a pair of cinder block walls.  It was uncomfortably loud.  And I’m willing to bet this was because of our cracked transformer core.  Perhaps we hit some sort of mechanical resonance as well.  Regardless, it was a pretty exciting/frightening experience.