(15 minutes or less)
Mount both metal plates on a piece of wood or simply clamp them to a nonmetallic surface. (If you prefer, you don't even have to mount the two plates. You can attach the wires as described below and then simply hold one plate in each hand. This has the benefit of allowing you to substitute other metals easily.)
Using the clip leads, connect one plate to one of the meter's terminals and connect the other plate to the other terminal. At this point it doesn't matter which plate attaches to which terminal.
(15 minutes or more)
Place one hand on each plate. You should notice a reading on the meter. If the meter doesn't show an electrical current, simply reverse your connections, attaching the copper plate to the terminal that the aluminum was connected to and vice versa. If there is still no current, check the connections and the wiring. If that doesn't produce current, try cleaning the plates with a pencil eraser or steel wool to remove oxidation.
Experiment with different metals to find out what combination produces the most current. Try pressing harder on the plates. Get your hands wet and try again.
Repeat the above experiments with the plates wired to a voltmeter. For accurate readings you will need an FET input voltmeter.
Have one person put a hand on the copper plate and another person put a hand on the aluminum plate, and then have them join their free hands.
When you touch the two metal plates, the thin film of sweat on your hands acts like the acid in a battery, reacting with the copper plate and with the aluminum plate. In one of these reactions, your hand takes negatively charged electrons away from the copper plate, leaving positive charges behind. In the other reaction, your hand gives electrons to the aluminum plate, causing it to become negatively charged.
This difference in charge between the two plates creates a flow of electrical charge, or electrical current. Since electrons can move freely through metals, the excess electrons on the aluminum plate flow through the meter on their way to the copper plate. (In metals, positive charges cannot move.) In your body, both positive and negative ions move. Negative electrons move through your body from the hand touching the copper to the hand touching aluminum. At the same time, positive ions move in the opposite direction. As long as the reactions continue, the charges will continue to flow and the meter will show a small current.
Your body resists the flow of current. Most of this resistance is in your skin. By wetting your skin you can decrease your resistance and increase the current through the meter. Since two people holding hands have more resistance than one person, the flow of current will be less.
If you disconnect one of the wires to the current meter, the aluminum becomes negatively charged: Electrons pile up on the aluminum side because they cannot cross the gap in the wire. The copper becomes positively charged as your hand removes electrons from the metal. These piles of charge create a voltage, which is measured when the microammeter is replaced by a voltmeter.
Most batteries use two different metals and an electrolyte solution to create piles of charge and thus a voltage. (In this exhibit, your sweat acts as an electrolyte solution.) When the terminals of the battery are connected with a wire, this voltage produces a current.
You can use other pairs of different metals in a circuit to produce a current. The success you have using various metals will depend on a metal's electric potential, that is, its ability to gain or lose charges. Try various metals to see which produces the highest current reading. An electromotive series table (found in every chemistry textbook) shows the electric potentials of metals and allows you to predict which metals will work well in making a hand battery.
You can sometimes get a small current even between two plates made of the same metal. Each plate has a slightly different coating of oxides, salts, and oils on its surface. These coatings create slight differences in the surfaces of the metals, and these differences can produce an electrical current.
The slightly painful sensation of a fork tine touching a metal filling, the process of plating metals, sacrificial anodes used to preserve ship hulls and iron bridges, potato clocks, and dielectric unions to prevent deterioration of copper and iron plumbing are all everyday examples of metals transferring charges.