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:
An Empty Container
Insulated Copper Wire
At Least 5 cm of Graphite (taken from a pencil)
A Digital Multimeter
A Hand Shovel
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.
Step 3: Prepare the Cathode
Preparing the cathode is simple: cut second piece of wire, strip about 2 cm of insulation from one side, strip about 4 cm of insulation from the other, and then wrap the exposed wire from the 4 cm end around one of the ends of the piece of graphite. Once this is all done, just stick the end of the graphite that doesn't have copper wire wrapped around it into the dirt within your mud battery.
Step 4: Turn the Dirt Into Mud and Then Wait
At this point I usually test the battery to see if an electric potential exists within it using my digital multimeter. Though the voltage usually wavers a bit, all of the mud batteries that I've made with this method at this point have displayed a voltage that hovered around a point between 0.3-0.6 volts +/-0.09.
Now feel free to correct me if I'm wrong on this, but I've found that even though the battery does (or at least should) generate an electric potential, wiring several of these devices together in series in an attempt to get at least a little function from them provides a far lower voltage than would be expected due to (what I'm assuming is) the high resistance posed by each battery in the series.
Though I knew from I've found, however, that moistening the dirt until it becomes mud greatly ameliorates this situation, as mud cells (in comparison to dirt cells) not only boast higher voltages (0.8-1.0 volts) but also seem to fair far better when wired into a series.
This step is pretty much as easy as it sounds: just add water (tap water works) to your mud battery until the soil within it is completely saturated with water.
The benefits of mud over dirt in this battery aren't immediately presented; to see the increased voltage one must allow the battery to incubate overnight. While I don't really know exactly why it has to incubate over night to work (I initially thought that it was because the voltage was generated by microbial activity and that the increase in voltage following incubation was due to microbial colonization of the anode), if I had to guess I'd say that the incubation is likely necessary because it takes time for the water to dissolve some of the salts within the soil into solution.
People to Thank
In closing I'd kind of like to thank egbertfitzwilly because I gleaned a lot of what I know of homemade electrodes from reading his instructables and HarveyH44 and Kiteman because they pointed out why what I thought I made (a microbial fuel cell) wasn't what I actually made and showed me what it was I actually concocted (an Earth battery).