This is a simple demonstration that requires a piece of copper and zinc and a sensitive electrical meter or other low current consumption electrical device. We use a precision motor called a low-inertia motor. Such a motor has almost no torque rating, but is instead designed to register motion using the smallest electrical current possible. Our low-inertia motor requires only around 15 microamps of current to spin.
To form the battery, the pieces of metal are connected to wires leading to the motor, and then the metals are submersed in cola, or any other fizzy acidic drink. The motor spins, showing that a chemical reaction is occurring between the metals and the acid in the drink. The acid forms an electrolyte - a liquid which takes an active chemical role in the process, and in doing do, allows an electrical circuit to be completed. In schools, the demonstration is usually performed with dilute sulphuric acid. When using cola, it is actually phosphoric acid that plays the role of the electrolyte. The diagram below shows what is happening. Oxidation at the zinc electrode destroys that electrode and liberates electrons in the metal. The zinc ions end up in the electrolyte. Copper and zinc have sufficiently different electrode potentials so as to produce a potential difference of over a volt between the electrodes (copper and zinc have electrode potentials of +0.34V and -0.76V respectively, measured with respect to a standard hydrogen electrode). The electrons liberated in the zinc thus flow to the copper and enter the electrolyte to combine with hydrogen ions from the phosphoric acid. The reduction associated with this reaction creates hydrogen gas.
The copper electrode should not decay as rapidly as the zinc, although there is a secondary reaction that can go on when the copper oxidises to produce copper ions, and electrons. This reaction, which is not shown in the diagram, is encouraged by single hydrogen ions H+ combining with water molecules H20 to produce oxonium ions H3O+. The oxonium ions take electrons from the copper, and thus the copper decays also. In addition, this undesirable reaction also serves to reduce the potential produced by the cell, since there is oxidation taking place at both electrodes.
The table shows the electrode potential chart, with the voltage values measured against a standard hydrogen electrode (which is taken to have the arbitrary value of zero). Any two metals sufficiently far apart in the table will form a battery, although some of the more reactive metals oxidise in air too readily and are so not clean enough to work properly when submersed in the cola.
Finally, instead of cola, try using a lemon, or even better, many lemons in parallel - then there is better accuracy in the term 'battery' applied to this demonstration. Vegetables work too, and the potato clock which can be bought in science toy stores, is a novel example.