Audion tube
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The Audion is a vacuum tube device invented by Lee De Forest in 1906. The Audion is a three-element tube, 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 much larger current flowing from the filament to the plate, allowing the Audion 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.
The Audion made practical radio broadcasts a reality. Prior to its introduction radios had typically used semiconductor rectifiers (or detectors) which consisted of a small piece of galena crystal probed by a fine wire or "cat's whisker". They were unreliable; required frequent adjustment of the cat's whisker and offered no amplification. Such systems required the user to listen to the signal though tiny headphones with almost no volume. The Audion allowed the signal to be amplified to any desired level, typically by placing the output of one Audion into the grid of the next, eventually providing more than enough current to drive a full-sized speaker. By the 1920s such "tube radios" were a fixture of most western households, and remained so until the introduction of the transistor in the 1950s.
De Forest was granted a patent for the Audion on November 13, 1906. He continued to claim that he developed the Audion independently from John Ambrose Fleming's earlier research, 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". 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, Armstong was able to conclusively demonstrate De Forest still had no idea how it worked.