A homemade Audion radio receiver constructed by an amateur radio hobbyist 1914. The Audion (triode), the first amplifying vacuum tube, was invented by Lee De Forest in 1907 but was not widely used until its amplifying ability was recognized around 1913, so this is one of the earliest vacuum tube radios. From left, the apparatus consists of an automobile storage battery to provide the filament current, and a tuning coil. Audions were always mounted hanging upside down, with the delicate tungsten loop filament hanging down, to prevent it from sagging and touching the grid.