English: An early De Forest audion tube, the first electron tube that could amplify. The first triode, or tube with three electrodes, a filament, grid, and plate, the audion was invented in 1906 by American engineer Lee De Forest. The top metal electrode is the plate. The zigzag metal wire partly visible under it is the grid. The filament was originally under that, attached to the wires at left, but has burned out. As many Audions did, this model had two filaments (indicated by the four filament supply wires) so that when one burned out the "spare" could be used. The filament wires were attached to the screw terminal at left, while the grid and plate terminals were brought out through the right end of the glass envelope to the two wires at right. Tube belongs to History of San Jose, Perham Collection of Early Electronics.