Re: Why won't my Potato Battery work?

OK, there are several things going on at once.  Let's simplify things:
1.  Set your voltmeter for millivolts, *NOT* milliamps;  that will just 
short out the battery you're trying to construct. Your text or teacher 
should have told you that that's what you're trying to do.
2.  Study your textbook for the difference in voltage and current; that 
will help you a lot in understanding what you're doing and what I'm going 
to tell you. If your text isn't clear, go to the library and investigate 
the physics texts that make sense to you.
3.  For your initial experiment, use only one potato.  To make things 
easier, use 20 or 24 gauge wire.  Stick the penny and the nickel into the 
potato (each potato will end up with two wires coming out of it), attach 
the alligator clips, and measure the VOLTAGE coming out of the potato.  
What you've really made here is a battery and again, studying your text or 
spending some time in your library with physics books should help make that 
clearer.  OK, you've measured the voltage coming out of the potato battery. 
  If you try to measure the CURRENT coming out of the potato battery, the 
behaviour that you're seeing is exactly what I'd expect, and almost 
certainly due to local depletion of the ions.  You can prove this by 
measuring the current, watching it deplete, and then waiting 10--15 
minutes.  You should see the same behaviour as in the initial test.
  Your text should have told you that using multiple potatoes will boost 
the VOLTAGE, not the CURRENT coming out of the rig.  To use multiple 
potatoes, connect the nickel from the first potato to the penny of the 
second potato, and so on; use as many potatoes as you like.  Each potato 
should have TWO wires coming out of it.  The final setup is connect the 
voltmeter from the penny of the 1st potato and the nickel of the last 
potato, so you should have something very similar to the one-potato 
startup.  A question for you:  if the one-potato battery puts out 10 mV, 
what will a five-potato battery put out?
I hope that this helps get you started on the right track.  GOOD LUCK!