While trying to make a microbial fuel cell apparently I stumbled upon how to make an Earth battery, which is something whose existence I really had no clue about until I made one. While the current it generates is pretty much non-existent I've found that several of these in series are capable of dimly lighting a 3 volt LED for weeks, which I still think is pretty good for what amounts to a jar of mud.

To make this "mud battery"  you'll need:

Aluminum Foil
An Empty Container 
Insulated Copper Wire
At Least 5 cm of Graphite (taken from a pencil)
A Digital Multimeter
A Hand Shovel
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Step 1: Obtain and Insert the Dirt

For this step, just go outside and find a spot where you can dig up some dirt. Though I've yet to experiment with how various types of dirt affect the mud battery's function, I've found while making the some dozen cells I've tried that the blackish-brown sort of topsoil pictured works well. If I had to guess, I'd say the dirt I use is towards the "loamy" side of things because it retains water well without holding it up so much that it prevents the diffusion of ions between the electrodes

Besides finding the right kind of soil to use, this step really is easy as just filling up your container with dirt.

Step 2: Prepare the Anode

To prepare the anode strip a piece of wire such that one end has a little bit of copper exposed (~2 cm) while the other has significantly more copper exposed (~4 cm). Now take a piece of aluminum foil and wrap it around the long end of the wire so as to form a sort of "pin" of aluminum foil. Once finished stick the anode into the dirt.
TheBarton3 months ago

What about using it to charge a deep cycle battery over a period of time. Anyone try that? I'm new to researching this stuff so I don't know what would keep you from being able to move the electricity from the dirt to the storage battery.

alpual1 year ago
So I'm not entirely sure about this, but I think that what you've made here is pretty similar to a potato battery (or lemon or salt water...). The soil is probably full of electrolytes that facilitate the flow of electrons from your cathode to your anode. This is distinct from the microbial fuel cells which get electricity from bacteria's anaerobic metabolism. Does this sound right?
20StoryLincoln (author)  alpual1 year ago
That sounds exactly right. Shortly after I posted this however long ago another user pointed out the same thing. After finding out I was wrong about what was going on I re-wrote a lot of the instructable to reflect how it really most likely works. Unfortunately I have no idea how to get the url for the page to change from what the original title was, so its kind of still stuck like that. Sorry!
Thank you so much.I was searching for a working model for science fair
Alex Mercer2 years ago
Yucky Good! Thanks! I tricked my lil' sister that those mud are powerful by my powers so that it lights!
20StoryLincoln (author)  Alex Mercer2 years ago
Nicely done, good sir, nicely done.
HarveyH443 years ago
I always enjoy alternative methods of generating electricity. Getting a voltage is great, but you need current for it to do any work. You can get get around 1.0 volts many different ways. This link might help you, from an electrical point of view...

Basically, if you stick two electrodes, of different materials, into most anything moist, you will read a voltage. Unfortunately, there is hardly any current to do much of anything with. Hope this helps some.
20StoryLincoln (author)  HarveyH443 years ago
Yes, this does help. And after finding that cooking my mud for 30 minutes made the danged thing work better, I'm going to have concede that this is probably just a contained earth battery. The current is also pretty much non existent (~0.1mA), but it's apparently enough for several batteries in series to keep a 3 volt LED lit in my room like a ghostly little firefly. 

Though I'm a bit bummed that I completely missed the mark here, I'd still like to thank both of you for pointing it out and for providing sources for further research into this and for explaining how this thing is probably working. 
I'm more into electronics, only a few biology classes, and very little about bacteria. Don't let it completely discourage you, you still might be onto something, just need to look into it a little more. Lots of cool stuff has been discovered, and refined by people tinkering around in their spare time. You saw something interesting, and you investigated. Maybe you are onto something, just haven't isolated it yet. Wasn't looking to put you off on this, just figured from my point of view (electronics), and experience, seemed like dissimilar metals could account for the voltage.

Now, knowing that, you can factor that first volt in to you investigations. If you can double it, you know you are really onto something spectacular. Like I said, I don't know much about bacteria, but some can do fascinating things, like glow in the dark. They are some amazing chemical factories, you should continue your research.
20StoryLincoln (author)  HarveyH443 years ago
Some more testing I did suggests that the explanation you (and Kiteman) put forth for my observed voltage is closer to what's actually going on then my "it's microbes" idea (neither boiling water nor cooking a battery had any negative affect on the voltage generation, though I still plan on trying to cook it at the proper temperature for complete sterilization just to put it entirely to rest). But even though I'll be a bit bummed that I'm most likely completely wrong I do still intend to keep tinkering around with this to see if I can't get whatever it is I have here to generate enough of a current/voltage to make at least a cool little lamp or something.

I don't know how long it takes bacteria to multiply, or to sufficiently produce their magic. I remember that most bacteria don't play well with other species. Some are resistant, where others fail. There are still many things to look at, and I don't know a whole lot about the biology. Maybe take a look under a microscope, and figure out if you have a dense enough population, of the right species.

That first volt will most likely be found anywhere, but you might find a way to get more. A red LED, needs about 2 volts to light, the brightness is determined by the current. White or blue take over 3 volts. Even if you can't get a higher voltage, maybe you can get a boost in current.

Being a biologist, I'm sure you have your own microscope, but you might be interest in one the hooks up to your USB port, around $30 surplus. Have one in a box some place, a little more magnification than I expect from a cheap piece of equipment. Work surprisingly well, but not sure if it's good enough for bacteria. Figure if you can see individual blood cells, probably close though.
20StoryLincoln (author)  HarveyH443 years ago
While I'd still be at a loss for why adding boiling water and cooking a cell didn't seem to alter it's voltage generation at all much beyond the notion that maybe I just didn't heat it up enough (which is a distinct possibility), it hadn't occurred to me that maybe the voltage increase following the "incubation" could be attributed to microbial action at the anode.

I don't mean for this to sound like I'm trying to shoot down your suggestion, but unfortunately my own personal microscope is a bit old: not only am I not sure if it works, but I also don't know that it'd be able to differentiate much between types of bacteria even with a good one. But from what I can gather from the link you gave me, an earth battery's voltage is due to differences in electric potential between the metals used for the electrodes, and from how it seems they used to be used I'm guessing that (beyond the tapping into the telluric currents) it doesn't matter where in a "unit" of soil they're placed so long as they're placed within the same "unit". Now this differs from how the electrodes in the type of soil-based microbial fuel cell I'm going for need to be placed, where (since the voltage is due to the fact that redox potential decreases as one gets deeper in the soil due to the activity of certain bacteria) the anode needs to be placed at least several inches in the soil beneath the cathode for it to work.

Now if I'm correct in this difference, I was wondering if I'd be right to assume the following: I presently have set up a "mud battery" series with the electrodes set up in such a way that they only should work to generate voltage as an earth battery does, so if the increase in voltage following incubation is due to microbial activity then it'd be expected that letting the whole thing sit out overnight shouldn't alter the voltage any because it isn't set up to take advantage of the microbes. But if the voltage does go up then it's likely due to some inorganic process, right?

Also, you can get USB microscopes for $30 surplus? Do you know if you can take pictures with the one you have? I mean I guess one could always just do a print screen of whatever one was looking at, but being able to take microscope pictures would be pretty neat. A female lab tech I know took a bunch of pictures of molds with a microscope in the lab she works in for a research project and they all turned out really pretty.
Kiteman3 years ago
Nice project, but are you sure it's microbes producing the electricity? It could just be "ordinary" electrochemical activity involving the aluminium, carbon and dissolved salts from the soil, which would also be dependent upon moisture content and temperature.

It would be interesting to try this with a sample of soil, bake it to kill off the microbes, then re-hydrate and compare the voltage.

20StoryLincoln (author)  Kiteman3 years ago
I was actually wondering if microbes were actually at "fault" here too while I was experimenting with this. Before you made me aware that temp changes could similarly modify non-biological electrochemical activity, one of the ways I chalked up the voltage to microbes was indeed the fact that a shift in temperature altered the voltage read out. Besides this indicator, however, I was also hoping that the fact that the cell had to be incubated over night before it really started working well also supported the idea that microbes were the active agent here. In addition to the "incubation time", I thought that the fact that the voltage my cell produced mirror that which (I think) would have been expected by figure 2 in this (links to a paper download) paper provided some support to my microbial claims. Finally, though it probably lends weak support, when I was experimenting with it I found that using a graphite anode also generated a voltage, which I hoped would at least partially suggest that it wasn't due to something with salts in the soil.

But like I said, my experience lies more on the "microbial" bits of things than the "fuel cell" bits. Since you pointed out that the temperature thing doesn't really lend any support that it is microbes, however, I'm going to go stick one of these cells I have contained in a glass jar into the oven for 35 minutes at 350 degrees to see if it alters anything. And well, even if it proves this isn't an MCF I'd have to say that generating a bit of voltage with a bit of mud is still pretty cool.
Oh, I'm not putting your project down, just picking at the details.

It would be cool if you could charge batteries by hammering a couple of electrodes into damp ground. Maybe a set of specially-designed tent-pegs could keep your mobile topped up...

20StoryLincoln (author)  Kiteman3 years ago
Oh no, I understand; I wasn't trying to come off as angry or upset, I was just going over the things about it that made me think it was microbes and not salt or something. It really was something I was worried about and while wandering about during the week I tinkered with it I did ponder baking up some dirt to see it it stopped the cell. I just kind of didn't have the impetus to actually do it until you brought it up.

But yeah, the people who made the MudWatt actually intended their experimental version for something like that; the soil-based MCF they designed was meant to be used as a kind of thing to get power for sensors and transmitters out in places where it'd be awkward to have batteries or normal generators.