Sir John Ambrose Fleming's invention of the thermionic valve (tube) jumpstarted modern electronics. He also made many other contributions to the field of electrical machinery.
He was born in 1849, the eldest of seven children to a Congregational minister. Although born in Lancaster, his family soon moved to London.
After studying at University College, London, and at Cambridge University, Fleming was a consultant for the Edison Electric Light Company in London. He later became an adviser to the Marconi Wireless Telegraph Company and a popular teacher at University College (UCL) from 1885 to 1926, where he was the first to hold the title of professor of electrical engineering.
Early in his career Fleming investigated photometry, worked with high-voltage alternating currents and designed some of the first electric lighting for ships. In addition, Fleming designed the transmitter that made Marconi's first transatlantic transmission in 1901 possible.
Fleming recognized that the major problem preventing vast improvements being made was that of detecting the signals themselves. Fleming wondered about using the Edison effect to rectify the radio waves and thereby act as a detector. Having previously performed some experiments using these bulbs, he gave the idea to his assistant who implemented the experiment and found it to work.
The two-electrode radio rectifier, which he called the thermionic valve; also known as the vacuum diode, kenotron, thermionic tube and Fleming valve was patented in 1904. Fleming's invention was the ancestor of the triode and other multielectrode vacuum tubes.
The vacuum-tube diode contains two electrodes: the cathode, which is either a heated filament or a small, heated, metal tube that emits electrons through thermionic emission; and the anode, or plate, which is the electron-collecting element). Vacuum tubes have been almost entirely replaced by transistors, which are cheaper, smaller and more reliable.
The invention of the diode was a revolutionary idea, but it had little impact at first. "Valves" were expensive to make, and in less than two years, the "cat's whisker" was produced, a crude form of semiconductor rectifier that consisted of a thin wire positioned on a lump of suitable material (even coal) to produce a point contact rectifier. This was more convenient than Fleming's diode and it soon caught on.
Around 1906 the de Forest Company in the U.S. introduced a device called an Audion. It used the same basic thermionic technology as Fleming's diode, but a third electrode had been added. This was called a grid because of the nature of its construction. Initially the Audion was only used for detection of signals. It took another four years before it was used as an amplifier. Fleming lost a patent infringement case regarding the thermionic technology in the courts.
Fleming authored more than 100 scientific papers and books, including The Principles of Electric Wave Telegraphy (1906) and The Propagation of Electric Currents in Telephone and Telegraph Conductors (1911).
He retired in 1926 and was knighted in 1929 for the advances he had made to electrical and electronic engineering. For 15 years he was president of the Television Society. He died in 1945.