Measuring Internal Resistance of Batteries
Contributors:
Shawn Hymel
Shawn Hymel 
Build a Lemon Battery
We get to build a battery! This is a crude fruit-based battery that will not last very long nor drive much current. However, it is useful for measuring internal resistance.
IMPORTANT: Please don't eat the fruit after you have made the battery! Zinc ions that are floating in the lemon juice certainly can't be healthy for you.
Insert the zinc screw or nail into one side of the lemon.
Use the hobby knife to cut a slit in the other end of the lemon.
Insert a penny into the slit so that half of the penny is still outside the lemon's skin.
And that's it! We successfully created a battery. Make sure that the penny and screw cannot touch each other, or you will create a short circuit between the electrodes. While the short circuit will not start any fires, it will not produce a voltage for our experiments.
NOTE: If you would like the battery to last longer than a few hours, remove the screw. Even without a circuit connected, the screw will continue to oxidize in the presence of the lemon juice.