English: A replica of one of the bulbs with which Thomas Alva Edison discovered the Edison Effect (thermionic emission) in the early 1880s. Made by Clayton H. Sharp in 1921 as an experiment to see if Edison's bulb could be used like a Fleming valve as a detector in a radio receiver to rectify radio waves. It worked.

It consists of an Edison incandescent light bulb, an evacuated glass bulb with a hairpin shaped bamboo carbon filament, with an additional platinum plate (visible between the arms of the filament) attached to wires emerging from the base of the bulb. A current through the filament heated it white-hot. Edison discovered that the hot filament emitted negatively-charged particles (later discovered to be electrons) an effect that was called the Edison effect. He demonstrated this by applying a separate voltage between the filament and plate. When the plate had a positive voltage, the electrons were attracted to it and a current flowed through the tube from filament to plate. When the plate had a negative voltage, the electrons were repelled so no current flowed through the tube. Edison found no practical use for this effect, but in 1904 John Ambrose Fleming invented a similar tube called the Fleming valve which was used to rectify radio signals in the first radio receivers, which evolved into the diode vacuum tube.

Alterations to image: partially removed aliasing artifacts (crosshatched lines in background) due to scanning of halftone photo using FFT filter in GIMP.