Audion – 1906

Two years after Englishman John Ambrose Fleming invented a two-electrode vacuum tube, American inventor Lee De Forest one-upped him by developing a tube with three electrodes.

audion triodeAudion Triode | Larger sizeDe Forest, who claimed to have been unfamiliar with Fleming’s work, dubbed his device the Audion, a term he formed by contracting the words “audio” and “ion”. The name De Forest chose for the invention hints at the fact that the Audion detected radio signals, served as a rectifier (a device that converts alternating current to direct current), and was the first solely electronic component to make amplification of electromagnetic signals possible. Its ability to amplify set the Audion apart from the Fleming valve. With his new device, De Forest became a radio pioneer, wirelessly broadcasting music from the Metropolitan Opera House in New York.

Both the Fleming valve and the Audion evolved from the light bulb. As can be seen in the illustration here, like a light bulb, the Audion contained a metal plate (the anode) and a filament (the cathode) inside of glass housing. In addition, between these two components, the Audion featured a piece of bent wire called the control grid (because it was shaped like a gridiron). Applying a small electric current to the grid made it possible to modulate and amplify the signal between the other two electrodes.

De Forest made the filaments in his earliest Audions out of tantalum, but later switched to tungsten, which proved more stable. De Forest, and later other engineers, fine-tuned the Audion’s design in other important ways. According to De Forest’s initial patent, the first Audion was not a true vacuum tube. In fact De Forest, who did not understand how or why the device worked very well, at first mistakenly claimed that a small amount of gas must be present in the tube for it to function properly. This caused some difficulties with his patent rights, and eventually others recognized that the tube worked best when evacuated as much as possible.

By the time three-electrode tubes were adopted into the technologies of Western Electric and other corporations, they were commonly called triodes rather than Audions. Triodes revolutionized the electronics industry. Before the invention of the transistor, they could be found in almost every household in America. Whenever someone listened to the radio or watched television, they were benefitting from De Forest’s discovery.

Last modified on 10 December 2014