Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Conduction in a gas


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

This comprehensive safety note comes courtesy of the Scottish Schools Equipment Research Centre.

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

A video demonstration of the Van de Graaff generator is available at the National STEM Centre eLibrary.

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


Apparatus set-upa 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.) 

b 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.

c Show that the neon lamp can be lit using the HT power supply. Connect a safety resistance (220 kW, 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.

d 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 

Related guidance

Van de Graaff generator - the basics

Electric charge and current - a short history


Related experiments

Experiments with a Van de Graaff generator