Ions produced by a flame carry a current
Practical Activity for 14-16
A match flame will ionise the air. Use this to show ionisation and how it can be detected by charging an electroscope.
Apparatus and Materials
- Power supply, EHT, 0–5 kV (with internal safety resistor)
- Metal plates with insulating handles, 2
- Hook for electroscope
- Retort stands and bosses, 2
- Flexicam or webcam linked to a projector (optional)
Health & Safety and Technical Notes
See guidance note on Radioactive sources (UK guidelines) in
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.
A web cam or ‘flexicam’ could be used to project an image of the gold leaf onto a screen. Alternatively, use a bright lamp to cast a shadow of the leaf on a screen or wall. The lamp should be on the same side as the observers and below the line of sight. Remove the glass plate from the back of the electroscope.
The advantage of using an EHT supply is that it looks like an electric circuit – albeit a strange one. The electroscope plays the part of very sensitive meter and the air between the plates is a component whose resistance changes.
Take care when using the electroscope not to make it all seem like a sleight of hand – especially when moving leads around.
- Fix the two metal plates so that they are parallel to one another and about 5 cm apart.
- Connect one of the plates to the positive terminal of the EHT supply through the safety resistor.
- Place the hook in the electroscope and connect the other plate to the leaf of the electroscope through the hook.
- Connect the case of the electroscope to the negative terminal of the EHT supply.
- Connect the negative terminal of the EHT supply to the earth terminal.
Getting a current to flow and charge the electroscope
- Set the EHT supply to about 3 kV and switch it on. The leaf of the electroscope will rise due to induced charges; reset it by momentarily connecting the leaf to earth.
- Hold a match flame beneath the gap between the plates and watch what happens. The air in the gap is ionised and allows a charge to flow across the gap and charge the leaf of the electroscope.
Discharging the electroscope
- You can also discharge the electroscope by ionising the air around it.
- Disconnect the electroscope from the supply but keep the base earthed. Place the disc in the top of the electroscope and charge it using a flying lead from the positive terminal of the power supply via the safety resistor.
- Light a match over the top of the disc. This will ionise the air and allow the electroscope to discharge.
- Charge the electroscope again and light a match to the side of the disc. To discharge the electroscope, you will need to blow gently over the flame so that the ions pass over the top of the disc.
- The electroscope should discharge quickly to earth when the air above its disc is ionised. If it is discharging too slowly, bring an earthed metal plate up close to the disc of the electroscope.
- You could start the demonstration by asking students what they know about atoms and ions. The demonstration reveals something about the structure of atoms: that electrons are removed with relative ease. The temperature of the flame makes the air particles move so fast that, in collisions, they knock electrons off each other.
- Discuss what happens when the air is ionised.
Airatoms are losing electrons leaving behind positive ions. The electrons then move to the positive plate and ions move to the negative plate. You can demonstrate this with the experiment
- The demonstration is also a step towards measuring ionising radiations. The apparatus is, in effect, an ionisation measurer. It is a circuit that allows a small current to flow when there are ions between the plates. The current is too small to measure with a normal ammeter (there are extremely expensive ammeters that will do it). The best way to measure it is using an electroscope. This idea is developed in
- With more advanced students, you can discuss the capacitance of the circuit (a combination of the parallel plates and the electroscope). When you first switch on the EHT, there is a separation of charge induced on the plates (the positive plate induces a negative charge on the facing plate). In turn this draws some negative charge away from the electroscope, which is left charged. In effect, the electroscope measures the charge that has been displaced by the very small and very short-lived charging current of the capacitive circuit.
This experiment was safety-tested in May 2006.