The original machine was designed during 1943-4 by a team led by Tommy Flowers at the Post Office Research Station at Dollis Hill - the machine's Post Office roots are plainly evident in the Strowger-type relays, uniselectors and equipment racks that were then much in evidence in the UK's automatic telephone exchanges.

Colossus machines (eventually there were ten in all) were the world's first programmable, digital, electronic, computing devices. They used thermionic valves (vacuum tubes), the fastest switching devices then available, to perform calculations aimed at deciphering German wireless traffic that was encrypted using the Lorenz SZ40/42 machine. In the absence of magnetic disc or semi-conductor technology, encrypted messages were read by Colossus at high speed using punched paper tape for storage and an optical reader. Following the end of WWII., most of the machines were taken apart and their components recycled, but two survived at GCHQ Cheltenham where they were used for various purposes until 1960.

The fully-functional replica of a Colossus Mark 2, shown here and now on display at the National Museum of Computing (in H Block) Bletchley Park, was reconstructed by a team under the direction of Tony Sale, and completed in November 2007. For more detailed information on Colossus, see . . . . http://en.wikipedia.org/wiki/Colossus_computer

A general view of the machine from the front. To the left is a teleprinter-type keyboard, behind which is an equipment rack housing banks of thermionic valves (shielded by the red covers) and a panel of rotary selectors. The central rack houses banks of relays (behind the metal covers) and a couple of motor "uniselectors" (a high-speed electro-mechanical rotary selector). Then comes rack housing yet more electro-mechanical switches, manual switches, plug-boards and indicator lamps. And furthest away are yet more banks of thermionic valves. The Mark 2 Colossus was equipped with over 2,400 valves. For other views of Colossus, see:

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