Technology History | Dollis Hill — Colossus Was Built in a Post Office Research Station

The machine that read Hitler's own cipher traffic was built by a telephone engineer, in a Post Office research station, out of parts meant for telephone exchanges.

Tommy Flowers had joined the General Post Office at Dollis Hill in 1930 and spent a decade on switching equipment, which is how he came to know something nobody at Bletchley Park believed: that thermionic valves are reliable if you never switch them off.

Valves failed on the thermal shock of warming and cooling. Leave the machine powered and the temperature steady, and a design using more than a thousand of them becomes possible.

He proposed exactly that after a 1941 secondment to Bletchley, and was turned down. The codebreakers thought a machine with 1,500 valves would spend its life broken.

So he built it anyway, on his own initiative and partly with his own money, in eleven months from February 1943.

Colossus was demonstrated here in December 1943, taken apart, and delivered to Bletchley Park in January. Flowers wrote in his diary on Saturday 5 February 1944 that it had done its first job.

A Mark 2 with roughly 2,400 valves followed, running five days before the Normandy landings.

What it was first at needs saying precisely, because the field is crowded. Colossus was the first programmable electronic digital computer — special-purpose rather than general, programmed by switches and plugs rather than by a stored program, and secret for three decades, which is why older accounts of computing's origins skip it entirely.

Then it was erased. Churchill ordered the machines destroyed and the drawings burned, and the work stayed classified until the 1970s, so for most of his life Flowers could not explain what he had done or why he was owed for it.

The building is still here, and has been flats since the research station closed — Chartwell Court, a long brick block of 1933.

English Heritage put a blue plaque on the end wall on 6 September 2023, seventy-nine years after Colossus first ran and twenty-five after Flowers died.

It is best read from the street beside it, which is named Flowers Close.

A brass plaque at the entrance records the building's own history, and another marks something buried underneath: Paddock, the reserve Cabinet War Room built beneath this site in case Whitehall fell.

Sources: English Heritage blue plaque record and its 2023 announcement; The National Museum of Computing on the Colossus timeline; Flowers's diary as quoted in published accounts of the first run. Cover photo: Spudgun67, CC BY-SA 4.0, via Wikimedia Commons.