English: Early Fleming valves. Shows several different designs used by Fleming. The Fleming valve, invented in 1904 by John Ambrose Fleming, as a detector for radio waves in early radio receivers, was the first thermionic diode and the first vacuum tube. It consisted of an evacuated glass bulb containing two electrodes: a cathode in the form of a tantalum wire filament, and a anode (plate) consisting of a sheet metal cylinder surrounding the filament. A separate current through the filament heated it red-hot, releasing electrons into the vacuum of the tube. The AC voltage to be rectified was applied between anode and filament. When the anode is positive, the electrons from the filament are attracted to it and a current flows through the tube. When the anode is negative the electrons are not attracted so no current flows.