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Fleming, Sir John Ambrose

current electrical cathode anode

(1849–1945) British physicist and electrical engineer: inventor of the thermionic diode.

Fleming had a mixed education, studying at times at University College and at the Royal College of Chemistry, London, and at Cambridge under . After intermittent periods of teaching and study he was appointed professor of electrical technology at University College, London.

Although he worked on a number of electrical engineering problems, including radio telegraphy and telephony, Fleming’s outstanding contribution was the invention of the thermionic valve, in 1900. This was based on an effect noticed by (to whose company Fleming had been a consultant) and consisted of a vacuum tube containing a cathode heated to incandescence by an electric current, and an anode. When the anode was maintained at a positive potential with respect to the cathode, an electric current could flow from cathode to anode but not in the opposite direction. This electric ‘valve’, or diode, was to form an essential component in electronic devices such as radios, television sets and computers for half a century, until eventually superseded by the cheaper and more robust transistor. Fleming’s rules illustrate the fact that the force on a wire carrying a current is perpendicular both to the magnetic field and to the current. (Also for the trio of force, field and current, they give the direction of any one if the other two are known.)

Flemming, Walther [next] [back] Fleming, Sir Alexander

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