A volunteer is asked to hold a pair of metal electrodes. These are connected to a voltmeter and it can be shown that a voltage has been produced.
- One zinc electrode (a galvanised bolt is fine for this).
- One copper electrode (a short section of copper pipe works well).
- Wires and connectors to make the circuit.
- High impedance voltmeter (e.g. multimeter) that can be seen by the audience.
This requires preparation of some electrical connections. Sufficiently long wires with crocodile clips at the electrode ends and other suitable connections at the voltmeter end will work well.
Zinc and copper are the basis of the first battery made by Volta. The human being is acting as a solution of ions between the two dissimilar metals. Had the electrodes been the same metal no voltage would be produced. The difference between these two metals in the reactivity series ensures that a voltage is produced. Other metals would produce different voltages. It would be straightforward to test say, aluminium (be aware of the stable oxide layer that forms on the surface) and iron in addition to copper and zinc.
In each of these cases the battery works on the principle of reduction and oxidation. One of the metals is being oxidised to form a stable ion in solution and ions of the other metal (which will have dissolved slightly in the acid it has been exposed to) are being reduced back to the metal. Exactly which metal is doing which of these processes depends on their relative position in the reactivity series. The further they are apart in that series the larger the voltage that will be produced.
Wet hands work best, so if the human wire is a little nervous it will help.
Copper pipe makes a good copper electrode and galvanised steel is sufficient for the zinc electrode. It does not have to be solid zinc.
Humans do not have to be used in this experiment, potatoes, oranges, lemons, vinegar or even salty water can be used instead.
This can be used to investigate the reactivity series. Compare copper, aluminium (foil), zinc (galvanised nails), iron (iron nails) and other metals. The larger the voltage the further they are apart on the reactivity series.
The current flowing is very small and unlikely to be a hazard. The major hazard in this demonstration is the trip hazard of the electrical wires that may be used. Hands should be washed well after handling the electrodes.