File:First internal grid Audion tube.jpg

English: The first prototype Audion tube, invented by Lee De Forest in 1906. The Audion, the first triode vacuum tube and first amplifying electronic device, founded the field of electronics and made possible its benefits such as radio broadcasting, long distance telephone lines, audio recording, and talking motion pictures. It consists of an evacuated glass envelope containing three electrodes, which are visible in order from foreground to background: a U shaped wire filament (which in this tube has burned out), a grid consisting of zigzag wires, and a square metal plate. The three elements are located very close together. De Forest invented it by adding a third electrode to the Fleming Valve invented in 1904 by John Ambrose Fleming, which just contained a filament and a plate.

De Forest tried many locations for a third electrode in the tube. This was the first tube he built with the grid electrode located in its modern position, between the filament and plate. This position allowed a voltage on the grid to control the current of electrons flowing from filament to plate, thus enabling the tube to amplify. It took a few years for De Forest and others to understand what was happening in the tube, and the Audion languished in obscurity until 1912, when its ability to amplify was recognized and it was widely applied to radio and telephone circuits.

De Forest hired a small maker of electric light bulbs named McCandless to build his Audion designs, and it can be seen that the tube is just an early light bulb with the extra electrodes added. The filament is attached to the screw contacts in the base, and the grid and plate contacts are just brought out through the side of the glass bulb.

Caption: "First vacuum tube with the control element in the form of a grid"