The positive terminal was to be made of copper and placed at the top of the Tower. The negative terminal was iron, and placed in the dirt. In chemical reactions, copper is positive because it may receive electrons. In chemical reactions, iron is negative because may give off electrons. The reaction of copper and iron is sometimes referred to as a ox/redox reaction.
Just as we could use the electrodes of a battery to power a lightbulb, so can we use the current generated from the completion of copper and zinc electrodes embedded in a lemon to register on a voltmeter. Yes, that is it! Use a lemon to power a light bulb! Make sure the penny and the nail are actually part way into the lemon so that they are in contact with the juice. You should be able to find video instructions that are plain on www.youtube.com (search for lemon battery). You may try lighting up a Christmas tree light bulb with it by connect it.