Fleming, Sir John Ambrose

Fleming, Sir John Ambrose (1849-1945), English physicist and engineer, the inventor of the first electronic tube used to detect radio waves. Fleming was born in Lancaster, Lancashire, on November 29, 1849, and was educated at University College, London, and the Royal College of Chemistry, London. He then studied at the University of Cambridge under James Clerk Maxwell.

Fleming worked in London as a consultant with the Edison Electric Light Company and as an adviser to the Marconi Wireless Telegraph Company. In 1885 he was appointed professor of electrical engineering at University College, London; he held the post until 1926. Fleming focused on experimenting with the practical applications of electricity rather than exploring its theoretical aspects. His experiments resulted in improvements to the electric generator, the incandescent lamp (see Electric Lighting), and electric meters.

In 1904 Fleming devised his most important invention, the diode (two-electrode) radio rectifier, also called the Fleming valve or thermionic tube. It was the first vacuum-tube radio detector. The invention led to the development of other multi-electrode vacuum tubes, which made possible the early radio and gramophone electronics industry.

Fleming’s work had involved using a metal cylinder, which served as an anode, surrounding a filament, which served as a cathode, and a high vacuum. He connected the cylinder plate and the filament through a second current circuit with a battery to increase the current of electrons. The diode that he created allowed electrical current to flow in only one direction. Fleming used this property to detect high-frequency radio waves with the tube. Through an antenna circuit, he applied changing voltages created by radio signals to the filament and plate. These changes altered the strength of the plate current, which in turn could be used to reproduce the radio signal in a receiving apparatus.

Fleming received patents worldwide for the functional radio-wave detectors, which became important to radio telegraphy. He was knighted in 1929. His 20 books include Principles of Electric Wave Telegraphy (1906), Fifty Years of Electricity (1921), and Memories of a Scientific Life (1934). He also wrote more than 100 scientific papers.