Argon - Ar
The gas argon is a colorless gas, which is not interesting at all to display. One could equally well display a bottle full of air. As with helium and neon, argon also can be used in a discharge tube, but the visible light output for a given power input is much lower than for helium and especially neon. An important part of the light output is in the UV-range, and hence is not visible for us. The picture shows a tube, filled with argon at low pressure, of approximately 20 cm length, which is connected to a 3300 V AC source, in series with a 400 kΩ resistor in order to limit the current.
When the same tube is connected to a 5 MΩ resistor, as the helium and neon tubes, then the light output is barely visible, while for the helium and neon tubes the light output is very bright. This low light output is shown in the picture below.