Flemings original thermionic valve, 1889.

UNITED KINGDOM - OCTOBER 22: Fleming's original thermionic valve, 1889. Made by John Ambrose Fleming (1849-1945) for experiments on an effect seen in early electric lamps. The source of current is a heated negative electrode, the cathode. Current flows through the vacuum to the positive electrode, the anode. In 1899 Fleming became a technical adviser to the Marconi Company, helping design the Poldhu transmitter in Cornwall, used in Marconi's 1901 transatlantic transmissions. Fleming later developed these valves as sensitive detectors of radio signals. (Photo by SSPL/Getty Images)