The Rebuilding of Colossus at Bletchley Park.

This article presents information on the construction of a replica of Colossus Mk2 computer. In 1943, the Bletchley Park code breakers had difficulties breaking the German High Command teleprinter traffic that was enciphered using the Lorenz SZ42 cipher machine. They thought that vacuum-tube electronics might be the answer to the decryption difficulties, and after experimenting with a prototype called Heath Robinson, the Bletchley Park group approached the Post Office Research Laboratories at Dollis Hill in North London, England. Colossus also generated patterns internally, using vacuum tube ring circuits that were stepped around synchronously with the paper tape. It was the comparison of these patterns with the cipher characters read from the tape that eventually revealed the Lorenz machine settings, which allowed the intercepted cipher text to be deciphered. Flowers knew that gas-filled vacuum tubes, called thyratrons, could store bit of information per tube, and he chose to use these in the original ring circuits.