Lee De Forest invented the Audion, a forerunner of the triode, which is an amplifying vacuum tube. The grid controls the filament which sends the current from the filament. The Audion can detect radio signals by applying a small amount of power to the grid that can control a larger current from the filament plate. However, De Forest’s Audion is distinct from the true vacuum triode since it is not capable of linear amplification.

De Forest found that when gas was in a partial vacuum heated by a convectional lamp with a wire wrapped around the glass housing that it would act as a detector of radio signals. In his first design he had a small metal plate sealed into the lamp housing connected to a positive terminal of a 22 volt battery via a pair of headphones while the negative terminal was connected to one side of the lamp filament. Wireless signals applied to the wire caused a disturbance in the current and therefore produced sound in the headphones. He eventually applied an antenna to the circuit to increase sensitivity.

The Audion was the first system that did not draw power significant power from the antenna/tuned circuit. It is still argued whether De Forest invented the triode vacuum tube; however, it is apparent that he underestimated the potential of his original device. He did not see it had the potential as a telephone repeater amplifier.
Edwin Armstrong published an explanation of the Audion in 1914 and later during a dispute with De Forest he was able to prove that De Forest had no idea how the Audion worked. The Audion had problems due to the low-pressure gas being absorbed by the metal electrodes. Many tried to find ways to improve the reliability of the device, however, Dr. Irving Langmuir of General Electric tried to prevent the absorption of gas by starting with a higher vacuum. Although successful his device was completely different device from the Audion.

De Forest claimed that true vacuum triodes simply wouldn’t work if there was any gas in the envelope; however, this was a direct contradiction to his Audion which required gas in its operation. De Forest produced Audions for the US Navy up until the 1920s even though by most standards they were considered obsolete by then.

Later vacuum triodes allowed the signal to be amplified to any desired level by feeding the amplified output from the triode into the grid. Vacuum tubes were also used to make superior radio transmitters. These tube radios became common in Western households until the introduction of the transistor radio in the mid 1950s. In most cases solid state devices have largely replaced the vacuum tube.

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