A late 19th-century Geissler tube used for teaching purposes Credit: The Cathode Ray Tube site.
Different forms of Geissler tube.
The Geissler tube was a forerunner of the modern electron tube, invented in 1857 by Heinrich Geissler (1814–1879). It is a glass tube containing a gas at low pressure which glows with a characteristic color when a high voltage is applied to the metal electrodes at the ends of the tube. Originally, they were used mainly for amusement – essentially toys for Victorian adults. Modified forms of the Geissler tube are used as spectroscopic light sources and in neon or argon signs.