Hidden away by the lifts on the 7th floor of the ugliest building at University College London are a couple of glass display cabinets. Almost no-one knows they are there, and most people pass by without noticing them. Which is a shame, because the contents are really rather remarkable. This small exhibition commemorates the invention in 1904 of the Thermionic Valve (or Radio Tube if you’re American) by John Ambrose Fleming.
Fleming was the first Professor of Electrical Engineering at UCL, and was also a consultant scientist to the Edison Electric Light Company, investigating the “Edison effect”. In early Edison lamps, carbon from the lamp filament became deposited on the inside of the glass dimming the light. In an attempt to reduce this, Edison added an extra electrode to attact the vapour, and discovered that a current can flow from the hot element to the extra electrode but not in reverse.
From 1899 Fleming became a consultant to the Marconi company, even designing the transmitter that made the first transatlantic transmission. But the big problem was producing a sensitive radio detector, and Fleming’s inspiration was to use one of his vacuum tubes to rectify the signal. Thus was born the thermionic diode valve, which led on to the whole field of electronics. Based on Fleming’s work, Lee deForest added a third electrode in 1908 to create the Triode which could amplify signals. The rest, as they say, is history.
My pictures below capture some of what is on display; click for larger pictures. The information comes mostly from the signs in the exhibition.
Experimental Thermionic Valves (c. 1904/5). These are the prototypes of valves used by Marconi for radio. The case also contains production versions, which are almost identical to the prototypes except more professionally produced.