Mercury discharge tube.
A discharge tube contains a gas at low pressure through which electricity is passed [A]. Electrons (negative particles) and ions (positive particles) move towards the electrodes [B]. Ions strike electrodes to produce more electrons [C]. Light is produced as electrons collide with gas atoms [D].
A discharge tube is a glass envelope containing gas at low pressure, which gives light due to the collision of electrons with gas molecules. Discharge lamps, filled with one or more of various gases, are used to give colored light: mercury vapor gives purple, sodium vapor gives yellow, and neon gives red. Early discharge tubes, called Geissler tubes, were used as toys. A fluorescent lamp is a discharge tube coated internally with a fluorescent substance.