Ingredients:

• One exhaust temperature gauge [stolen] from a military jet
• One lemon
• 3" of copper pipe (any diameter)
• One galvanized nail
• Alligator clips & wires

Assembly:
Jam the copper down into the lemon. Jam the nail into the lemon, making sure it doesn't touch the copper. Connect the alligator clips to the "leads" and the terminals on the gauge. Done deal.

You can vary the current produced based on how far you push the nail and/or copper pipe into the lemon. Illustrated here are three different nail positions. First, barely inserted:

Next, inserted most of the way:

Last, inserted all the way (the gauge is pinned):

I actually set up three lemons in this fashion, with jumpers joining the cathode of each lemon to the anode of another. When run in series, I got the following measurements (yeah, I know, a digital multimeter would provide much more accuracy):

• 2.4 volts
• 1.1 milliamps

The insertion depth of the leads inside the lemons varies the current, but the voltage stays constant regardless.