A Geissler tube[1] is an early gas discharge tube used to demonstrate the principles of electrical glow discharge. The tube was invented by the German physicist and glassblower Heinrich Geissler in 1857. It consists of a sealed, partially evacuated glass cylinder of various shapes with a metal electrode at each end, containing rarefied gasses such as neon, argon, or air; mercury vapor or other conductive liquids; or ionizable minerals or metals, such as sodium. When a high voltage is applied between the electrodes, an electrical current flows through the tube. The current disassociates electrons from the gas molecules, creating ions, and when the electrons recombine with the ions, the gas emits light. The light emitted is characteristic of the material within the tube, and is composed of one or more narrow spectral lines. Many different colors and lighting effects can be achieved. In the early 20th century, Geissler tube technology was commercialized and evolved into neon lighting.
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Geissler tubes were mass produced from the 1880s as entertainment devices, with various spherical chambers and decorative serpentine paths formed into the glass tube. When the tube was handled (the terminals were insulated) the shape of the plasma changed. Some tubes were very elaborate and complex in shape and would contain chambers within an outer casing. If these were spun at high speed a visual disk of color was seen due to persistence of vision. (Somewhat similar devices in the form of stationary globes are now produced and sold for personal amusement.) As an educational tool they are also used to demonstrate the movement of electrons and the principles of a vacuum.
It was observed that under some conditions the glass envelope would itself glow at the positive (anode) end. This glow was attributed to the transmission of a ray from the negative cathode at the opposite end of the device, and so were named cathode rays. William Crookes developed a modification of the Geissler tube into what is known as the Crookes tube to demonstrate and study these rays, later determined to be a stream of electrons. This device was further developed into the cathode ray tube with applications in electronics development and diagnosis, and in radar and television displays.
Geissler tubes have had a large impact on the development of many instruments and devices all of which use related vacuum and discharge principles.