Mike’s third valve machine is an eight-bit Harvard computer built entirely out of NOR gates — 920 thermionic triodes across five backplanes, on a 190-by-130-centimetre panel bolted to a brick wall. Sixteen single-step instructions, and a carry-chaining ADDC that will do 128-bit arithmetic if you have time for a cup of tea. It needs ten minutes to come up to temperature, and: “The Tube Computer has a pleasant homely aroma from the hundreds of double triodes, all quietly burning off their dust.”