Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Showing that a spark can pass through air


The Van de Graaff generator always produces excitement for students.

Apparatus and materials

Van de Graaff generator

Microammeter, light spot type, optional

Health & Safety and Technical notes

Read this comprehensive Van de Graaff generator safety.

Read our standard health & safety guidance

The makers' instructions should be followed for the care and use of the Van de Graaff generator. 

video demonstration of the Van de Graaff generator is available at the National STEM Centre eLibrary.



Van de Graaff generator

Sparks should be shown passing between the large sphere and the smaller sphere supplied with the generator.

Teaching notes

1 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. 
2 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. 
3 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. 
4 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 Van de Graaff generator
This experiment was safety-tested in July 2007


Related guidance

Electric charge and current - a short history

Van de Graaff generator - the basics


Related experiments

Experiments with a Van de Graaff generator


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