Fleming, (John) Ambrose (1849–1945)

English electrical physicist and engineer who invented the thermionic valve in 1904 and devised Fleming's rules.

Fleming was born in Lancaster, Lancashire, and educated at University College, London, and at Cambridge, where he worked in the Cavendish Laboratory and studied under Scottish physicist James Clerk Maxwell. In 1881–83, Fleming was professor at Nottingham, and from 1885 at University College, London. He was a consultant at various times to the Edison, Swan, and Ferranti electric-lighting companies and the Marconi Wireless Telegraph Company, for which he designed many parts of their early radio apparatus.

In 1904 he produced experimental proof that the known rectifying property of a thermionic valve was still operative at radio frequencies, and this discovery led to the invention and production of what was first known as the ‘Fleming valve’. He called it a valve because it allowed electrical currents to pass in only one direction. It worked by allowing one of the electrodes – the cathode – to be kept hot so that electrons could evaporate from it into the vacuum. The other electrode – the anode – was left cool enough to prevent any appreciable evaporation of electrons from it. It revolutionized the early science of radio. He was knighted in 1929.