Sir John Ambrose Fleming

1849-1945

British Physicist and Electrical Engineer

Fleming invented the thermionic valve or vacuum tube diode, making possible the rectification or conversion of alternating current into pulsating direct current. The diode was the first reliable rectifier and permitted a vast expansion of radio technology. Fleming remained active in research in electronic technology for 65 years.

Fleming, the son of a minister, attended University College, London, graduating in 1870. He taught science for a time, and then in 1877 entered Cambridge University to work under the eminent British physicist James Clerk Maxwell (1831-1879), with whom he had begun a correspondence. In 1881, Fleming joined the faculty of the newly established University of Nottingham, but left after a year to return to London, where he worked as a consultant to the London Telegraph Company and the Swan Lamp Factory. In 1885 he became a professor of electrical engineering at University College London, a position he held for the next 41 years.

In 1884 Fleming visited American inventor Thomas Edison (1847-1931) in the United States. There he learned about the Edison effect, in which a current would flow from the heated filament in an evacuated light bulb to a separate electrode sealed in the bulb if the electrode was connected to the positive terminal of the battery providing power to the bulb. No current would flow if it were connected to the negative terminal. When Joseph John Thomson (1856-1940) discovered the electron in 1897, the explanation of the effect became apparent. Electrons could escape from the heated filament into the vacuum, a process dubbed "thermionic emission" and were attracted to the positive charge on the electrode.

The first demonstration of radio waves occurred in 1887 when Heinrich Hertz (1857-1894) induced a spark in a coil of wire a short distance from his transmitter. The main obstacle to using radio waves for communication was their frequency. The signal voltage reversed so many times per second that no mechanical device could respond. In 1889 Fleming became a collaborator with Italian inventor Gugliemo Marconi (1874-1937) and began exploring the use of the Edison effect to turn the alternating radio signal to one that consisted of a series of pulses in only one direction. In 1904 Fleming was awarded a patent for a device, the vacuum tube diode or "valve," which would rectify, that is allow current to flow in only one direction. The diode was exactly what was needed by the new radio industry since it could convert a radio signal, which changed directions thousands of times a second, to a series of pulses all in the same direction, which could then register on a meter or other physical device. In 1906, American physicist Lee De Forest (1873-1961) added a third electrode, initially to improve the performance as a rectifier. In the right type of circuit, De Forest's new triode allowed for the amplification of signals, allowing both the production of stronger signals and the detection of much weaker ones.

Although Fleming did not receive the public acclaim that would go to De Forest, he enjoyed a very long and productive career in physics, and in the new field of electronic technology. In 1874 he gave the first scientific paper ever presented to the newly formed London Physical Society. In 1939, 13 years after his official "retirement," he gave his last paper to the society at the age of 90. He was knighted in 1929, and received many awards from scientific societies.

DONALD R. FRANCESCHETTI