Editing Audion (section)

== History ==
[[Image:Audion receiver.jpg|thumb|An Audion [[radio receiver]] by [[Lee de Forest|de Forest]]. The Audion tubes were mounted upside down to prevent the delicate filaments from sagging and touching the grids. This receiver provided the ability to choose operation of either one of the two provided detector tubes.]]

It had been known since the middle of the 19th century that gas flames were [[Electrical conduction|electrically conductive]], and early wireless experimenters had noticed that this conductivity was affected by the presence of [[Radio frequency|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 were 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 which produced sounds in the headphones.

This was a significant development as existing commercial wireless systems were heavily protected by [[patent]]s; 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 space current path greatly improved the sensitivity; in his earliest versions, this was simply a piece of wire bent into the shape of a [[Gridiron (cooking)|gridiron]] (hence ''grid'').

The Audion provided power gain; with other detectors, all of the power to operate the headphones had to come from the antenna circuit itself. Consequently, weak transmitters could be heard at greater distances.

=== Patents and disputes ===
De Forest and everybody else at the time greatly underestimated the potential of his grid Audion, imagining it to be limited to mostly military applications. It is significant that de Forest apparently did not see its potential as a [[long distance calling|telephone repeater amplifier]] at the time he filed the patent claiming it, even though he had previously patented amplification devices and crude electromechanical ''note magnifiers'' had been the bane of the telephone industry for at least two decades. (Ironically, in the years of patent disputes leading up to World War I, it was only this "loophole" that allowed vacuum triodes to be manufactured at all since de Forest's grid Audion patent did not mention this application).

{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = First internal grid Audion tube.jpg
| width1   = 100
| image2   = Audion tube.jpg
| width2   = 162
| footer   = ''(left)'' The first prototype Audion with the grid (zigzag wires) between the filament and plate.<ref name="RadioNews">{{cite journal | last = de Forest | first = Lee  | title = Evolution of the Vacuum Tube | journal = Radio News | volume = 9 | issue = 11 | pages = 990 | publisher = Experimenter Publications |date=May 1930 | url = http://www.americanradiohistory.com/Archive-Radio-News/30s/Radio-News-1930-05-R.pdf | access-date = August 3, 2014}}</ref> ''(right)'' Later design of an audion tube. The grid and plate are in two parts on either side of the central filament. In both these tubes the filament is burned out.
}}

De Forest was granted a patent for his early two-electrode [[diode]] version of the Audion on November 13, 1906 ({{US patent|841386}}), and the "triode" (three-electrode) version was patented in 1908 ({{US patent|879532}}). De Forest continued to claim that he developed the Audion independently from [[John Ambrose Fleming]]'s earlier research on the [[thermionic valve]] (for which Fleming received Great Britain patent 24850 and the American [[Fleming valve]] patent {{US patent|803684}}), and de Forest became embroiled in many radio-related patent disputes. De Forest was famous for saying that he "didn't know why it worked, it just did".{{citation needed|date=July 2017}}

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. It is true that after the invention of the true vacuum triode in 1913 (see below), de Forest continued to manufacture various types of radio transmitting and receiving apparatus, (examples of which are illustrated on this page). However, although he routinely described these devices as using "Audions", they actually used high-vacuum triodes, using circuitry very similar to that developed by other experimenters.

In 1914, [[Columbia University]] student [[Edwin Howard Armstrong]] worked with professor [[John Harold Morecroft]] to document the electrical principles of the Audion. Armstrong published his explanation of the Audion in ''[[Electrical World]]'' in December 1914, complete with circuit diagrams and [[oscilloscope]] graphs.<ref name="Armstrong-audion">{{cite journal
|last=Armstrong
|first=E. H.
|author-link=Edwin Howard Armstrong
|title=Operating Features of the Audion
|journal=Electrical World
|volume=64
|number=24
|url=https://books.google.com/books?id=6WgfAQAAMAAJ&pg=PA1149
|pages=1149–1152
|date=December 12, 1914 }}</ref> In March and April 1915, Armstrong spoke to the [[Institute of Radio Engineers]] in New York and Boston, respectively, presenting his paper "Some Recent Developments in the Audion Receiver", which was published in September.<ref name="Armstrong1915" /> A combination of the two papers was reprinted in other journals such as the ''Annals of the New York Academy of Sciences''.<ref name="Armstrong-audion" /> When Armstrong and de Forest later faced each other in a dispute over the [[Regenerative circuit|regeneration]] patent, Armstrong was able to demonstrate conclusively that de Forest still had no idea how it worked.<ref name="Lee" /><ref>{{cite book |url=https://books.google.com/books?id=3iIOAAAAQAAJ&pg=PA178 |pages=178–184 |last=McNicol |first=Donald Monroe |title=Radio's Conquest of Space the Experimental Rise in Radio Communication |publisher=Taylor & Francis |year=1946}}</ref>

The problem was that (possibly to distance his invention from the Fleming valve) 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 adsorbed by the metal electrodes. The Audions sometimes worked extremely well; at other times they would barely work at all.

As well as de Forest himself, numerous researchers had tried to find ways to improve the reliability of the device by stabilizing the partial vacuum. Much of the research that led to the development of true vacuum tubes was carried out by [[Irving Langmuir]] in the [[General Electric]] (GE) research laboratories.

===Kenotron and Pliotron===
[[Image:Early triode vacuum tubes.jpg|thumb|upright=1.4|Audions and early triodes developed from them, 1918
<ul>
<li>''Bottom row (D):'' De Forest Audions and oscillaudions</li>
<li>''Third row (C):'' Pliotrons, developed at [[General Electric]] by Langmuir</li>
<li>''Second row (B):'' triodes developed at [[Western Electric]] which bought the rights from de Forest in 1913.</li>
<li>''Top row (A):'' French triodes. The French government gained the right to manufacture Audions in 1912 when de Forest failed to renew his French patents for lack of $125.</li>
</ul>]]

Langmuir had long suspected that certain assumed  limitations on the performance of various low-pressure and vacuum electrical devices, might not be fundamental physical limitations at all, but simply due to contamination and impurities in the manufacturing process.   His first success was in demonstrating that, contrary to what Edison and others had long asserted, incandescent lamps could function more efficiently and with longer life if the glass envelope was filled with low-pressure inert gas rather than a complete vacuum. However, this only worked if the gas used was meticulously 'scrubbed" of all traces of oxygen and water vapor.
He then applied the same approach to producing a rectifier for the newly developed "Coolidge" X-ray tubes. Again contrary to what had been widely believed to be possible, by virtue of meticulous cleanliness and attention to detail, he was able to produce versions of the Fleming Diode that could rectify hundreds of thousands of volts. His rectifiers were called "Kenotrons" from the Greek ''keno'' (empty, contains nothing, as in a vacuum) and ''tron'' (device, instrument).

He then turned his attention to the Audion tube, again suspecting that its notoriously unpredictable behaviour might be tamed with more care in the manufacturing process.

However he took a somewhat unorthodox approach. Instead of trying to stabilize the partial vacuum, he wondered if it was possible to make the Audion function with the total vacuum of a Kenotron, since that was somewhat easier to stabilize.

He soon realized that his "vacuum" Audion had markedly different characteristics from the de Forest version, and was really a quite different device, capable of linear amplification and at much higher frequencies.  To distinguish his device from the Audion he named it the "Pliotron", from the Greek ''plio'' (more or extra, in this sense meaning [[gain (electronics)|gain]], more signal coming out than went in).

Essentially, he referred to all his vacuum tube designs as Kenotrons, the Pliotron basically being a specialized type of Kenotron. However, because Pliotron and Kenotron were registered trademarks, technical writers tended to use the more generic term "vacuum tube". By the mid-1920s, the term "Kenotron" had come to exclusively refer to vacuum tube rectifiers, while the term "Pliotron" had fallen into disuse. Ironically, in popular usage, the sound-alike brands "Radiotron" and "Ken-Rad" outlasted the original names.