Counting matches with an EHT supply
Practical Activity for 14-16
An EHT supply can be used to produce sparks, which can be increased if you add a capacitor in parallel with the air gap. You can use the sparks to count matches. Use this to develop the ideas behind the spark counter and, eventually, the Geiger-Müller tube.
Apparatus and Materials
- Power supply, EHT, 0–5 kV (with internal safety resistor)
- Conducting spheres, 2
- Retort stands and bosses, 2
- EHT capacitor, 0.1 μF
- Connecting leads
Health & Safety and Technical Notes
See guidance note on
A school EHT supply is limited to a maximum current of 5 mA. which is regarded as safe. For use with a spark counter, the 50 MΩ. safety resistor can be left in circuit so reducing the maximum shock current to less than 0.1 mA.
Although the school EHT supply is safe, shocks can make the demonstrator jump. It is therefore wise to see that there are no bare high voltage conductors; use female 4 mm connectors where required.
The EHT capacitor is a special component. It has to be able to withstand a voltage of 5,000 V. Do not use an ordinary capacitor in the circuit and certainly not a low voltage electrolytic.
can advise on a source for a suitable EHT capacitor.
This demonstration works in conjunction with the demonstration
It shows that the spark from an EHT supply is the same as that from a Van de Graaff generator.
To get a decent spark, you need to connect a high voltage capacitor in parallel with the air gap. This is because, for safety reasons, the EHT supply has a big internal resistance and cannot deliver a large current. On its own, the EHT will produce a small spark across a gap of a couple of millimetres. But it will not let enough charge flow quickly enough to produce a bigger spark. The capacitor solves this by storing some charge. It lets the charge go when the air breaks down. It is the capacitor’s charge that allows the breakdown to produce a decent spark.
Although the Van de Graaff produces sparks without needing a capacitor (because it has capacitance and stores its own charge), the EHT supply offers more control. You can change the voltage rather than the separation of the spheres to set the field strength so that the air is just on the verge of breaking down. This will be more useful in the long run when making a spark counter and Geiger-Müller tube.
- Set up two conducting spheres about 1 cm apart. Put their insulating handles into bosses on retort stands.
- Connect the capacitor in parallel with the spheres.
- Set the voltage on the EHT to about 5,000 volts. Move the spheres together until they start sparking.
- Reduce the voltage until the sparking just stops. Carrying out...
- Hold a match under the gap between the spheres. The ions in the flame will set off a cascade of ions between the spheres – i.e. a spark.
- Each lit match you put under the gap between the spheres should set off a spark. Optional...
- With advanced students you can show that, without the capacitor, the EHT will produce only very weak sparks across a small gap.
- 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 them 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. Although the radiation is invisible, the effect it produces can be made visible.
- The high voltage produces a strong electric field between the two spheres as positive and negative charges build up on the spheres and across the plates of the capacitor so that there are charges waiting to be driven across the gap. A few ions in the air in the gap will start a spark.
- Point out that the sparks are the same as those produced by the Van de Graaff generator. Explain that the EHT supply gives more control than the Van de Graaff generator when setting the field strength. And that control will be useful later.
- With advanced students, discuss the need for the capacitor (see technical note above). When it is charged to 4,500 V, a 0.1 μF capacitor stores about 1 J. This is about the same as the energy stored elastically in the spring of a mousetrap. The snap of the mousetrap is about the same as the snap of the spark from the charged capacitor.
This experiment was safety-tested in March 2006