Selection of valves, 1889-1906.

UNITED KINGDOM - MAY 10: Six valves, including the original thermionic valve of 1889 (back right), 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. Two of the other valves are: the audion valve of 1906 invented by Lee De Forest (front second from left); a large transmitting triode (back left). (Photo by SSPL/Getty Images)