Fleming's original thermionic valve, 1889.
© Science Museum / Science & Society Picture Library
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.