Fleming valve

The thermionic diode was later widely used as a rectifier — a device that converts alternating current (AC) into direct current (DC) — in the power supplies of a wide range of electronic devices, until beginning to be replaced by the selenium rectifier in the early 1930s and almost completely replaced by the semiconductor diode in the 1960s.

In operation, a separate current flows through the cathode "filament", heating it so that some of the electrons in the metal gain sufficient energy to escape their parent atoms into the vacuum of the tube, a process called thermionic emission.

In 1901 Fleming designed the transmitter used by Guglielmo Marconi in the first transmission of radio waves across the Atlantic from Poldhu, England, to Signal Hill, St. John's, Newfoundland, Canada.

Although the contact, reported December 12, 1901, was widely heralded as a great scientific advance at the time, there is also some skepticism about the claim, because the received signal, the three dots of the Morse code letter "S", was so weak the primitive receiver had difficulty distinguishing it from atmospheric radio noise caused by static discharges, leading later critics to suggest it may have been random noise.

After reading Fleming's 1905 paper on his oscillation valve, American engineer Lee de Forest in 1906 created a three-element vacuum tube, the Audion, by adding a wire grid between cathode and anode.

De Forest quickly refined his device into the triode, which became the basis of long-distance telephone and radio communications, radars, and early digital computers for 50 years, until the advent of the transistor in the 1960s.

Around 1914 Irving Langmuir at General Electric developed a high voltage version called the Kenotron, which was used to power x-ray tubes.