Conductivity
Electricity and Magnetism

Conduction in a gas

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

The spark through the air from a discharging Van de Graaff is bluish in colour, just like a more powerful lightning flash.

Apparatus and Materials

  • Van de Graaff generator


  • Metal plates with insulating handles, 2
  • Miniature neon lamp
  • Power supply, EHT, 0–5 kV
  • Power supply, HT, 0–250 V
  • Resistor, 220 k ohms, 1 W
  • Fluorescent tube

Health & Safety and Technical Notes

Read our standard health & safety guidance


A miniature neon lamp is often supplied as an accessory with a Van de Graaff generator.

It is unwise to operate a computer close to a running Van de Graaff generator, particularly a laptop which is not earthed.

Procedure

  1. Hold the neon lamp near to the large sphere of the Van de Graaff generator and observe the glow. The illustration shows a convenient arrangement. (Alternatively, put the neon tube in a holder with leads attached. Plug one lead from the holder into the Van de Graaff generator while the lead from the other end of the holder dangles. Bring up an earthed body such as a finger near the dangling end, and the lamp glows.}
  2. Show that an ‘electric current’ supply does the same thing. Set up the two metal plates with insulated handles parallel to each other and, say, 15 cm apart. Connect the positive terminal of the EHT supply to one of the metal plates; connect the earthed negative terminal of the supply to the other metal plate. Hold the neon lamp between the plates. It will glow.
  3. Show that the neon lamp can be lit using the HT power supply. Connect a safety resistance (220 kΩ, 1 watt) in series with the lamp. Connect to the neon lamp using 4 mm leads and crocodile clips. Set the HT supply to give a d.c. voltage of, say, 200 V. The neon lamp will glow.
  4. Hold (in the hand or clamp stand) the fluorescent tube near to the dome and it will glow.

Teaching Notes

  • These demonstrations show that neon gas glows red when a potential difference from either an electrostatic generator or a power supply (a.c. or d.c.) is used as the source of a high voltage.

This experiment was safety-tested in January 2007

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

Conductivity
appears in the relation σ=1/ρ G=σA/L
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