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The Audion is an electronic amplifier device invented by Lee De Forest in 1906 and ancestor of the vacuum tube. The Audion is a three-element tube, the forerunner of what is generally known as a triode today, in which the flow of current from the filament to the plate was controlled by a third element, the grid. A small amount of power applied to the grid could control a large current flowing from the filament to the plate, allowing the triode to amplify radio signals by placing the weak signal from the radio antenna on the grid, with a larger current from a battery between the filament and plate.
History
It had been known since the middle of the 19th century that gas flames were electrically conductive, and early wireless experimenters had noticed that this conductivity was affected by the presence of radio waves. De Forest found that gas in a partial vacuum heated by a conventional lamp filament behaved much the same way, and that if a wire was wrapped around the glass housing, the device could serve as a detector of radio signals. In his original design, a small metal plate was sealed into the lamp housing, and this was connected to the positive terminal of a 22 volt battery via a pair of headphones, the negative terminal being connected to one side of the lamp filament. When wireless signals were applied to the wire wrapped around the outside of the glass they caused disturbances in the current flow which produced sounds in the headphones.
This was a significant development as existing commercial wireless systems were heavily protected by patents; a new type of detector would allow De Forest to market his own system. He eventually discovered that connecting the antenna circuit to a third electrode placed directly in the current path greatly improved the sensitivity; in his earliest versions, this was simply a piece of wire bent into the shape of a grid-iron (hence "grid").
Compared to all competing devices, the Audion was unique in that it did not draw significant power from antenna/tuned circuit, which allowed the tuning circuitry to operate with maximum selectivity. With virtually all other systems, all of the power to operate the headphones had to come from the antenna circuit itself, which tended to "damp" the tuned circuits, limiting their ability to separate stations.
Patents and Disputes
Arguments still continue about whether De Forest really invented the vacuum tube. It's fairly obvious that he (and everybody else) greatly underestimated the potential of his original device, imagining that it have mostly limited military applications. It is significant that he apparently never saw its potential as a telephone repeater amplifier, even though crude electromechanical "note magnifiers" had been the bane of the telephone industry for at least two decades. (In fact for several years it was only this "loophole" that allowed vacuum triodes to be manufactured at all, since none of the original patents specifically mentioned this application.)De Forest was granted a patent for the early version Audion on November 13, 1906 (), but the "triode" (three electrode) version was patented in 1908 (). De Forest continued to claim that he developed the Audion independently from John Ambrose Fleming's earlier research of the thermionic valve (for which he received Great Britain patent 24850 and the American Fleming Valve patent ()), and became a fixture at many radio-related patent disputes. De Forest was famous for saying that he "didn't know why it worked, it just did". He always referred to the vacuum triodes developed by other researchers as "Oscillaudions" although there is no evidence that he had any significant input to their development.
In 1914 Edwin Armstrong published an explanation of the Audion, and when the two later faced each other in a dispute over the regeneration patent, Armstrong was able to conclusively demonstrate De Forest still had no idea how it worked.
The problem was that De Forest's original patents specified that low-pressure gas inside the Audion was essential to its operation (Audion being a contraction of "Audio-Ion"), and in fact early Audions had severe reliability problems due to this gas being absorbed by the metal electrodes. The Audions sometimes worked extremely well; other times they would barely work at all.
Apart from De Forest himself, numerous researchers had tried to find ways to improve the reliability of the device by stabilizing the partial vacuum. One of these, Dr Irving Langmuir of General Electric, took a somewhat unorthodox approach: Instead of trying to prevent the absorption of the gas, he deliberately started with a higher vacuum and looked for ways of making the Audion work under those conditions. He succeded, but quickly realized that although superficially similar to the Audion, his "vacuum" tube was really a completely different device, capable of linear amplification and at much higher frequencies. (A significant weakness of De Forest's claims is that none of his Audion schematics denoted the provision for any sort of "grid bias", an essential feature of any true vacuum triode operation).
Unlike the Audion, the vacuum triode could not de-modulate radio signals directly, (although Langmuir and other researchers found ways to do this) but it was capable of linear (ie undistorted) amplification, which turned out to be a vastly more useful feature. It is ironic that many "faulty" Audions which had lost their ability to demodulate radio signals due to gas absorption, had actually turned into crude linear amplifiers (which was why they lost their demodulating ability) but nobody realized this at the time!
Applications and use
The Audion (more correctly, the vacuum triode) made practical radio broadcasts a reality. Prior to its introduction radio receivers had used a variety of detectors including coherers, barretters, and crystal detectors The popular crystal detector consisted of a small piece of galena crystal probed by a fine wire or "cat's whisker". They were sometimes unreliable, requiring frequent adjustment of the cat's whisker and offered no amplification. Such systems required the user to listen to the signal though headphones at low volume. The Audion was a considerable improvement on this, but the original devices could not provide any subsequent amplification to what was produced in the signal detection process. The later vacuum triodes allowed the signal to be amplified to any desired level, typically by placing the output of one triode into the grid of the next, eventually providing more than enough power to drive a full-sized speaker. Apart from this, they were able to amplify the incoming radio signals prior to the detection process, making it work much more efficiently. By the late 1920s such "tube radios" began to become a fixture of most Western world households, and remained so until the introduction of transistor radios in the late 1950s.
In modern electronics, the Audion has been superseded by the solid state transistor, invented in 1948 and implemented in integrated circuits in 1959.
"Audion tube"