Flemings original thermionic valve, 1889.

UNITED KINGDOM - JUNE 12: The original thermionic valve made by Sir 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 Guglielmo Marconi's (1874-1937) 1901 transatlantic transmissions. Fleming later developed these valves as sensitive detectors of radio signals. (Photo by SSPL/Getty Images)