Thank you for your very interesting question regarding how to make a safe science display to demonstrate step potential. It is our pleasure to help.
The first thought that comes to mind is to demonstrate a real-life scenario. Farmers have had to deal with step potentials for a very long time. Imagine an open field with cows milling about casually eating hay, a storm forms and lightning strikes the center of the field. All the cows that are facing the lightning strike die, and all those at a 90-degree angle to the strike are perfectly fine. Why?
When lightning strikes the earth, the electrical energy propagates across the surface of the earth, sort of like dropping a pebble into a calm body of water. The waves roll out across the surface getting smaller as they travel further away from the epicenter. If you were to take a volt-meter and measure the voltage on the surface of the earth as the electricity rolled across its surface during a lighting strike, you would see that the voltage drops with distance. You could imagine voltage rings sort of like a target symbol with the voltages decreasing with distance away from the center.
If the front legs of the cows are 100-ft away from the lightning strike which equals 1,000-volts, and the back legs are 94-ft away from the lighting strike which equals 750-volts, then the difference in voltage between the back and front legs is 250-volts, killing the cow.
Now, if the front and back legs are both at 100-ft from the lighting strike point, then both legs are at 1,000-volts, and the difference in voltage is Zero-volts, and the cow is safe (just like a bird landing on a high-voltage wire).
Perhaps you can build a simple farm landscape with a light bulb in the center to represent the lighting strike point. You could then just mark concentric circles branching out from the center showing the voltage gradients. Some colorfully decorated cows could then be used to differentiate the cows being electrocuted from the ones that are not.
Now, if you are trying to go very high-tech, you could easily use some simple low-voltage batteries. 1-volt on the display can represent 1,000-volts, and .25 volts can represent 250-volts, etc. You could have pins placed in key locations across the surface of the model so that the model cow can be plugged into the pins either facing the strike or perpendicular to the strike. When the cow is plugged into the pin, it either displays a hazardous voltage or it doesn’t. This would be an interactive display.
Best of luck on your model! Send us some pictures when you are done with it!
The Engineering Team at E&S Earthing Solutions
Photo credit: http://www.flickr.com/photos/bergfotograaf/3887128769/sizes/l/in/photostream/