The thermionic vacuum tube diode, originally called the Fleming valve, was invented by John Ambrose Fleming in 1904 as a detector for radio waves in radio receivers, and evolved into a general rectifier. It consisted of an evacuated glass bulb with a filament heated by a separate current, and a metal plate anode. The filament emitted electrons by thermionic emission (the Edison effect), discovered by Thomas Edison in 1884, and a positive voltage on the plate caused a current of electrons through the tube from filament to plate. Since only the filament produced electrons, the tube would only conduct current in one direction, allowing the tube to rectify an alternating current.
The thermionic vacuum tube diode originally called the Fleming valve was invented by John Ambrose Fleming in 1904 as a detector for radio waves in radio receivers and evolved into a general rectifier It consisted of an evacuated glass bulb with a filament heated by a separate current and a metal plate anode The filament emitted electrons by thermionic emission (the Edison effect) discovered by Thomas Edison in 1884 and a positive voltage on the plate caused a current of electrons through the tube from filament to plate Since only the filament produced electrons the tube would only conduct current in one direction allowing the tube to rectify an alternating current