Valves are electronic switches, first invented in the early 1900s. They were used in radios, and later radar and television, up until the 1960s when transistors took over.  Colossus was built in 1943 under such secret conditions that very few people knew about and it did not become public knowledge until the 1970s. It was designed to help decode secret messages sent by the Germans from their headquarters in Berlin during World War II. There is now a working replica at the National Museum of Computing at Bletchley Park.

 

There is an obvious similarity in appearance to the old fashioned incandescent light bulb and they do indeed glow and get warm. But this heat then allows a stream of electrons to pass from a cathode to an anode. Other electrodes control the flow of electrons similar to the way turning a tap controls the flow of water.

 

 There were about 2,400 valves in Colossus,compared to the 15 billion transistors in an iPhone. There are still a few uses for valves today however: the magnetron in your microwave oven is a valve, and they are loved by hifi enthusiasts and guitarists for their amplifiers.