Audion triodes featured on a US postage stamp issued in 1973 to mark the centenary of Lee de Forest's birth.
The development of the Auction triode is detailed in the book 'Lee de Forest and the Fatherhood of Radio' by James A Hijiya (Associated Universities Presses Inc, 1992). To quote briefly:
Fleming was looking for a rectifier that would enable him to use a mirror galvanometer; when he discovered the valve, he had reached his goal and had little incentive to continuing developing the instrument. De Forest's two-electrode Audion (1904) was much less successful, never working well enough as a detector to be practical; only one receiving set using the de Forest diode was ever sold. Therefore, de Forest needed to continue improving his Audion, and he ultimately did so by adding a third electrode.
On 25 November 1906, de Forest ordered from manufacturer McCandless a new kind of tube, one with a third electrode interposed between the filament and the anode. To prevent the third electrode from blocking the passage of 'particles' between the filament and the anode, de Forest specified that it consist of a wire instead of a solid plate. John Grogan, an assistant to McCandless, suggested that to create a greater surface, drawing electrons from the filament, the wire be bent back and forth, and de Forest named this innovation the 'grid'. He then made this triode (Audion) into a detector by connecting the anode to an earphone and connecting the grid to an antenna for receiving wireless signals.
De Forest began testing the new tube on 31 December 1906, filed for a patent on 23 January 1907, and received it (No. 879,532) on 19 February 1908.
Although the triode Audion worked, de Forest did not accept the 'electronic' theory that Fleming was by then expounding, and continued to believe that some residual gas was necessary - that is as a 'soft valve' rather than with a hard vacuum. It was not until 1912 when engineers Harold Aitken and Irving Langmuir, at AT&T and General Electric (GE) respectively, realised the need to evacuate the tube thoroughly, that the Audion became suitable for practical use as an amplifier. The World War of 1914-18 gave enormous impetus to valve design and the manufacture of valves with consistency of characteristics. De Forest was destined to die virtually penniless on 30 June, 1961. Twelve years later, the hundredth anniversary of his birth in 1873, the important contribution made by the de Forest Audion was recognised by the issue of an 11c Airmail postage stamp.