One photograph of three technicians working on a thermonic valve, titled 'Largest Radio Valve'. Caption on back reads: 'Technicians at work on the largest radio valve in the world -- a 1 1/2-ton monster -- at a laboratory in Harlow New town, Essex. The giant thermonic valve is being developed by a research group of the Admiralty for space exploration. When completed it will be installed in a lead-lined pit at Great Baddow, Essex'.