linguistic:paraphrasing, linguistic:voice_change
"Grid" Audion detector

De Forest's most famous invention was the "grid Audion", which was the first successful three-element (triode) vacuum tube, and the first device which could amplify electrical signals. He traced its inspiration to 1900, when, experimenting with a spark-gap transmitter, he briefly thought that the flickering of a nearby gas flame might be in response to electromagnetic pulses. With further tests he soon determined that the cause of the flame fluctuations was due to air pressure changes produced by the loud sound of the spark. Still, he was intrigued by the idea that, properly configured, it might be possible to use a flame or something similar to detect radio signals.

After determining that an open flame was too susceptible to ambient air currents, de Forest investigated whether ionized gases, heated and enclosed in a partially evacuated glass tube, could be used instead. In 1905 to 1906 he developed various configurations of glass-tube devices, which he gave the general name of "Audions". The first Audions had only two electrodes, and on October 25, 1906, de Forest filed a patent for the diode vacuum tube detector, that was granted U.S. patent number 841387 on January 15, 1907. Subsequently, a third "control" electrode was added, originally as a surrounding metal cylinder or a wire coiled around the outside of the glass tube. None of these initial designs worked particularly well. De Forest gave a presentation of his work to date to the October 26, 1906, New York meeting of the American Institute of Electrical Engineers, which was reprinted in two parts in late 1907 in the Scientific American Supplement. He was insistent that a small amount of residual gas was necessary for the tubes to operate properly. However, he also admitted that "I have arrived as yet at no completely satisfactory theory as to the exact means by which the high-frequency oscillations affect so markedly the behavior of an ionized gas."

In late 1906, de Forest made a breakthrough when he reconfigured the control electrode, moving it from outside the tube envelope to a position inside the tube between the filament and the plate. He called the intermediate electrode a grid, reportedly due to its similarity to the "gridiron" lines on American football playing fields. Experiments conducted with his assistant, John V. L. Hogan, convinced him that he had discovered an important new radio detector. He quickly prepared a patent application which was filed on January 29, 1907, and received U.S. patent 879,532 on February 18, 1908. Because the grid-control Audion was the only configuration to become commercially valuable, the earlier versions were forgotten, and the term Audion later became synonymous with just the grid type. It later also became known as the triode.

The grid Audion was the first device to amplify, albeit only slightly, the strength of received radio signals. However, to many observers it appeared that de Forest had done nothing more than add the grid electrode to an existing detector configuration, the Fleming valve, which also consisted of a filament and plate enclosed in an evacuated glass tube. De Forest passionately denied the similarly of the two devices, claiming his invention was a relay that amplified currents, while the Fleming valve was merely a rectifier that converted alternating current to direct current. (For this reason, de Forest objected to his Audion being referred to as "a valve".) The U.S. courts were not convinced, and ruled that the grid Audion did in fact infringe on the Fleming valve patent, now held by Marconi. In contrast, Marconi admitted that the addition of the third electrode was a patentable improvement, and the two sides agreed to license each other so that both could manufacture three-electrode tubes in the United States. (De Forest's European patents had lapsed because he did not have the funds needed to renew them).

Because of its limited uses and the great variability in the quality of individual units, the grid Audion would be rarely used during the first half-decade after its invention. In 1908, John V. L. Hogan reported that "The Audion is capable of being developed into a really efficient detector, but in its present forms is quite unreliable and entirely too complex to be properly handled by the usual wireless operator."