gloeikathodes

A glow cathode, or gloeikathode in its plural form, is a thermionic electron-emitting cathode used in early vacuum tubes. It consists of a filament that serves as the electron-emitting surface and is heated directly by electric current until it glows red hot, initiating thermionic emission of electrons into the tube’s vacuum.

Construction and materials typically involve a tungsten filament or a tungsten alloy. Some versions used thicker filaments or specially treated tungsten to improve longevity, but emission relies on reaching high temperatures rather than employing an emissive coating. The filament’s red heat is visible as a glow within the tube, indicating active electron emission.

Operation and characteristics: when current flows through the filament, the surface temperature rises and electrons are thermally emitted. These electrons are drawn toward the positively charged anode, enabling current flow and amplification in devices such as diodes and triodes. Glow cathodes can handle relatively high current densities but require higher operating temperatures compared with coated or indirectly heated cathodes. They are more sensitive to vacuum quality and exhibit shorter lifespans due to evaporation and thinning of the filament material over time, which degrades emission.

Historical context: glow cathodes were common in the early development of vacuum tubes in the first decades of the 20th century. They were gradually supplanted by oxide-coated and indirectly heated cathodes that operate at lower temperatures and offer longer lifespans and greater reliability. Today, glow-type emission is primarily of historical interest, with modern tubes relying on more efficient cathode designs.

See also: thermionic emission, hot cathode, oxide-coated cathode, vacuum tube.