located between the filament and plate, was patented January 29, 1907. Like the von Lieben vacuum tube, De Forest's Audions were incompletely evacuated and contained some gas at low pressure. von Lieben's vacuum tube did not see much development due to his death seven years after its invention, shortly before the outbreak of the First World War.

De Forest's Audion did not see much use until its ability to amplify was recognized around 1912 by several researchers, who used it to build the first