Counting matches with a Van de Graaff generator

Demonstration

This ‘match counter’ is a useful step towards understanding a Geiger-Müller tube.

Apparatus and Materials

• Microammeter, light spot type, optional

• Van de Graaff generator

Health & Safety and Technical Notes

Read this comprehensive safety guide...

Van de Graaff generator safety note

Use the same apparatus and set up as for the demonstration:

Showing that a spark can pass through air

Procedure

1. Set up the Van de Graaff generator and switch it on. Bring the small sphere up to the dome of the Van de Graaff so that sparks are jumping between them.

2. Move the spheres apart until they just stop sparking. Keep the generator running.
3. Light a match and hold it under the gap between the spheres. This should produce some ions which will set off a cascade of ions – i.e. a spark.
4. Each lit match you put under the gap between the spheres should set off a spark.

Teaching Notes

• Explain that the apparatus is behaving like a match counter - although this is a roundabout way of counting matches. It is doing so because of the invisible ionisation that is happening in the gap, i.e. it is detecting something that can’t be seen.
• You could ask your students to turn away from the apparatus and see if they can hear when you put a match under the gap. They can’t hear the match flame but they can hear the effect it produces – a spark in the gap. This is a useful step towards building a radiation detector: the match is inaudible, but the effect it produces can be made audible. Similarly, although ionising radiations are invisible, the effect they produce can be made visible.
• You can do this demonstration in conjunction with

  Counting matches with an EHT supply

  It is useful to start with the Van de Graaff generator (as described here} because students will have seen it sparking before.

This experiment was safety-tested in February 2006

• A video showing how to use a Van de Graaff generator: