Showing that a spark can pass through air
Practical Activity for 14-16
The Van de Graaff generator always produces excitement for students.
Apparatus and Materials
- Microammeter, light spot type, optional
Health & Safety and Technical Notes
The makers' instructions should be followed for the care and use of your Van de Graaff generator.
Read this comprehensive safety guide:
Sparks should be shown passing between the large sphere and the smaller sphere supplied with the generator.
- As an introduction to the demonstration, you could say: "Can gases carry currents? Does air carry currents? Suppose the air carried electric currents as easily as copper, what would happen to the electric circuits that you have been working with? Then air cannot carry a current as easily as copper or it would spoil all these experiments. What would happen to cells? Or to the wall terminals at home? It looks as if air must be a non-conductor, or a very good insulator like paper, glass, cotton, wood or things like that. Yet it is impossible to make gases to carry currents."
- When the charge on the insulated dome becomes high enough, a spark will pass between it and a second dome which has been connected to earth.
- To show that it is the same kind of electricity as found in electrical circuits, connect a microammeter into the earth connection lead. Each time a spark jumps across the gap, a sharp burst of current will be indicated on the meter.
- Once the Van de Graaff generator is set up, the kit that comes with it has lots of toys to demonstrate. Doing a series of experiments with these will delight students. Look at the collection
This experiment was safety-tested in July 2007
- A video showing how to use a Van de Graaff generator: