audion

Audion is the name given to a vacuum-tube device introduced by Lee De Forest in 1906. It is commonly described as the first practical triode, though its operation and reliability were later refined. The Audion added a control grid between a heated cathode (filament) and an anode (plate) inside an evacuated bulb. By applying a voltage to the grid, a small change in plate current could produce a larger variation in output, enabling amplification of weak signals and detection of radio waves.

The device works through thermionic emission: electrons emitted by the heated cathode are directed toward the anode, with the grid modulating this flow. When properly biased, the Audion could amplify signals across audio and radio frequency ranges, improving receiver sensitivity and enabling various regenerative and feedback circuits that enhanced performance.

In practice, early Audions faced reliability issues, including leakage, noise, and unintended oscillations. The term "Audion" was used for the broader family of triode devices, though De Forest's original design was later refined and superseded by more stable triodes with improved performance.

Impact and legacy: The Audion played a crucial role in the rapid development of radio technology, long-distance communication, and audio amplification, and is considered a pivotal step in vacuum-tube history. It laid the groundwork for modern electronic amplification and, by extension, much of 20th-century electronics, even as later tube designs and, eventually, semiconductor devices replaced its predecessors.