Portrait of Sir John Fleming (1849-1945), British physicist and inventor of the thermionic valve. Fleming had a mixed education, studying variously at London and Cambridge. After intermittent teaching, he finally became professor of electrical technology at University College, London. Fleming's outstanding contribution to electrical engineering was his invention in 1900 of the thermionic valve. This one-way electrical valve, or diode, formed the basis of all sorts of electrical devices such as radios, televisions and early computers, and was not replaced until the advent of transistor technology.