Audion

The Audion is an electronic amplifying vacuum tube invented by Lee De Forest in 1906. It was the forerunner of the triode, in which the 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 larger current from the filament to the plate, allowing the Audion both to detect radio signals (that is, make them audible) and to provide amplification. However, De Forest's Audion is quite distinct from the true vacuum triode in that it is not capable of linear amplification.