Voltage/current relationship for a gas

Demonstration

Gases conduct electricity, but they have to be ionised first.

Apparatus and Materials

• Neon indicator lamp (230 V, 10 mm MES type with integrated resistor)
• Power supply, HT, 0-300 V, (current limited to less than 5 mA.)
• Voltmeter, 300 V, DC
• Ammeter, 5 mA., DC
• Leads, 4 mm, 5

Health & Safety and Technical Notes

This must be a teacher demonstration because an HT supply is being used. The lamp holder must be in an insulating box.

Read our standard health & safety guidance

To prevent excessive currents, the neon lamp should be provided with a ballast resistor (of about 2,000 Ω.) in the base.

Leads with shrouded plugs (sprung or fixed) are advantageous but not essential here.

Procedure

1. Set up a series circuit consisting of the power supply, the neon lamp, and the 5 mA. meter. Connect the voltmeter across the lamp and 100 mA. meter.
2. Apply increasing voltages from 0 to 200 V, recording both the current and the voltage at each stage.

Teaching Notes

• Take a set of potential difference and current measurements, and draw the characteristic graph (p.d. against current). The graph is a straight line once the current gets going, but it takes a potential difference of more than 60 V to get it started.
• The conduction of gases is complicated, but on a simple level the gas has to be ionised and so energy is need to do this. Once the ions are formed, they can drift to the electrodes in the same way as in liquids. As the potential difference is raised, a glow will start abruptly and then continue as the potential difference increases.
• The striking potential, for a neon lamp is about 60 V. The glow will be extinguished when the voltage applied is reduced to about 50 V.

This experiment was safety-tested in January 2007