Diode valve, early 20th century.
© Science Museum / Science & Society Picture Library
Early experimental Fleming diodes such as this one were the forerunners of other types of vacuum tubes. Diode valves control electrical currents and were, for instance, used to amplify radio signals. Sir John Ambrose Fleming (1849-1945) invented the thermionic valve and pioneered the use of electricity for light and heat on a large scale.