The Human Circuit: Batteries in Nature
A simple battery is made with two metal plates and an electrolyte, which is a substance that is electrically conductive. The metal plates must be made from two different metals. One metal plate serves as the positive (cathode) side of the battery and the other plate serves as the negative (anode) side. Together the cathode and anode are called the electrodes. The chemical reactions that produce the charge occur at the electrodes. The anode is involved in an oxidation reaction, which releases electrons. At the same time the cathode is involved with a reduction reaction, which attracts the anode’s electrons. The electrolyte provides a path for the flow of electrons from the anode to the cathode and a “current” is generated. Did you know that we can make a battery out of anything with these properties? Let’s try a couple!!
Experiment #1: The Potato Battery
- 1 potato (the electrolyte)
- 1 piece of copper wire about 4 in. long (the cathode)
- 1 paper clip (the anode)
- A voltmeter capable of measuring millivolts or one light emitting diode (LED)
- Wire cutters (optional)
- Rubber gloves (optional)
Straighten the paper clip. Place about 1 inch of the straightened paper clip and the copper wire into the potato as close together as possible without touching. Connect an LED or one end of the voltmeter to one wire and the other end to the other. This will complete a circuit. What does the voltmeter register, or how bright does the LED glow? What happens if we use a different length of wire (e.g., cut 1 inch off each wire and check the voltage/LED brilliance)?
What happens if we use a different electrolyte? Let’s do a similar experiment, only this time we will use ourselves as the electrolyte (Don’t worry you will not get shocked!!)
Experiment #2: The Human Battery
- 1 copper bottom pan
- 1 aluminum bottom pan
- Two 1-ft pieces of copper, or other, wire
Place both pans upside down on a wooden table or other non-conducting surface. Make sure the pans do not touch. If insolated, strip about 1in. of insulation off of each end of both wires. Use the tape to secure one wire to the bottom of each pan. Connect the voltmeter to each end of the wires. What does the voltmeter register? It should read zero, because we have not completed the circuit yet. Now place one palm on the copper bottom and your other palm on the aluminum bottom. What does the voltmeter register?
For the potato battery, the steel paper clip sends a current through the water in the potato, which easily conducts the electrical current, to the copper wire.
For the human battery, you noticed the voltmeter only registered a potential when both hands were placed on the pans. This is because the oils and sweat on your hands serve as the electrolyte for the metal ions from the aluminum pan to flow through your body to the copper pan. Try the human battery experiment twice more, using wet palms and then using rubber gloves? What happens?
The very first batteries were discovered near Baghdad, Iraq, and date to about 200 B.C.