A Geissler tube is an early gas discharge tube used to demonstrate the principles of electrical glow discharge, similar to modern neon lighting. 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 fluids; 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 dissociates electrons from the gas molecules, creating ions, and when the electrons recombine with the ions, the gas emits light by fluorescence. The color of light emitted is characteristic of the material within the tube, and many different colors and lighting effects can be achieved. The first gas-discharge lamps, Geissler tubes were novelty items, made in many artistic shapes and colors to demonstrate the new science of electricity. In the early 20th century, the technology was commercialized and evolved into neon lighting. Application Geissler tubes were mass-produced from the 1880s as novelty and entertainment devices, with various spherical chambers and decorative serpentine paths formed into the glass tube. Some tubes were very elaborate and complex in shape and would contain chambers within an outer casing. A novel effect could be obtained by spinning a glowing tube at high speed with a motor; a disk of color was seen due to persistence of vision. When an operating tube was touched by the hand, the shape of the glowing discharge inside often changed due to the capacitance of the body. Simple straight Geissler tubes were used in early-20th-century scientific research as high voltage indicators. When a Geissler tube was brought near a source of high voltage, alternating current, such as a Tesla coil or Ruhmkorff coil, it would light up even without contact with the circuit. They were used to tune the tank circuits of radio transmitters to resonance. Another example of their use was to find nodes of standing waves on transmission lines, such as Lecher lines used to measure the frequency of early radio transmitters. Another use around 1900 was as the light source in Pulfrich refractometers. Geissler tubes are sometimes still used in physics education to demonstrate the principles of gas discharge tubes. Influence Geissler tubes were the first gas discharge tubes,