Hydrogen - H2
Hydrogen is a colorless gas, which is very flammable. Displaying the gas itself is not really interesting, but an electrical discharge into the gas is quite interesting to display. The gas emits a nice pink light. The picture above shows a tube, filled with hydrogen 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.
Deuterium - D2
Hydrogen exists in two isotopic forms, which have very close properties, both chemically and physically. Deuterium is an isotope with one extra neutron in the nucleus, its symbol is 2H, but for this isotope a special symbol D is used frequently. Below, two tubes are shown, one is filled with hydrogen, and the other is filled with deuterium. Both are connected in series with a 6000 V AC source, and an 800 kΩ series resistor. Both tubes, hence, carry the same current. Their spectra are very close, and this is to be expected, due to the same structure of the molecules H2 and D2. The top tube contains deuterium, the lower tube contains hydrogen.