The Colossus Computers: British Secret Computer

The Colossus machines were the most useful machines for the British in the World War II in order to observe and read the communication of the Germans in the form of coded messages for their secret strategies during the war. Colossus computers are considered to be the first digital, electronic computing devices of the world that worked with the usage of the vacuum tubes for the processing of data and other programs such as calculations.

Colossus computers were the outcome of several ideas by a number of people including the designer of this machine, Tommy Flowers with the help of Bill Chandler, Max Newman and Allen Coombs. The series of Colossus machines includes Colossus Mark 1 which was ready by February 1944 for its operational purpose and Colossus Mark 2 that worked in the June 1944. By the end of the war Ten Colossi were used for the military purpose (Lavington 1980).

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The Colossus computers were highly secret machines for a specific purpose of cracking the messages which were encrypted with the help of Lorenz sZ40/42 machine. An interesting point to note here is that British code breakers used to call these encrypted messages as Fish. The process of reading these messages included three steps as follows:

  1. These coded messages were read from a paper tape at a higher rate of speed.
  2. In the second step an electronic simulation of Lorenz machine that is a stream is processed.
  3. The third step undergoes if the match count of the settings is higher than their constant of the threshold, it would be sent to the electronic typewriter device as the output of the process.

It took eleven months for Tommy Flowers to design and build the colossus computers in the research station in London. After the procedure of the testing of the machine, this machine was sent to Bletchley Park and processed the first message for the first time on 18th January 1944. After the Colossus Mark 1 there were further Colossi which were used by the end of the Second World War. The evident difference between the Mark 1 and Mark 2 was that there were 1,500 electronic valves in the Mark 1 and 2,500 electronic valves in the Mark 2. The greater amount of the electronic valves in the Mark 2 was the reason behind the fast speed of the Mark 2 (Reed 2007).

Colossus computers were efficient because these machines were the first to use shift registers and counters. These computers were built and designed in the most efficient way that they could easily perform the Boolean calculations. These machines were made programmable for the military use by switches and plug panels. Colossus computers could store the Lorenz setting and perform calculations to crack the codes. The best feature of the colossus computers was that these computers were capable to perform the calculations at higher rate. A paper tape was read by the tape system in these computers as these were synchronized with these tapes. The purposes of these are to limit the speed of these machines. These tape readers were capable to give high speed performance and could read 9700 characters per second. Just for the tentative measures, these readers were limited to read only 5,000 characters only which were quite better for the usage.

These computers were successful in meeting the requirement as these computers successfully cracked a great number of encrypted messages of the Germans. After the war these computers were destroyed for the security reasons except two of these computers. These two computers were sent from the Bletchley Park because right after the war Bletchley Park was closed. Colossus computer was kept as a secret from the world and that is why not many people knew about it and it was not given the credits because of the quality of being the first digital computer. Colossus computers were not even added to the history of the computing hardware because of the reason of its secrecy. Later on the secret was revealed by the authorities and the designers and engineers of the colossus computer were given the credits of their flawless work of that time (Knowledge Crush 2003).

Colossal computers were not the true computers because they were built and designed specifically for the military purpose. They were only able to perform calculations and read the characters on the tape to crack the codes of the messages sent for the communication purpose. The colossus computers were not being Turing-complete that mean we cannot assign a well-defined algorithm to this machine. It‘s performance and features were limited to certain points that why we can conclude this machine as not a true general-purpose computer (Copeland 2006).

Colossus was redesigned by the Tommy Flowers with the help of some visionaries it took a long time to rebuild the colossus computers. It has been a legend since 50 years and a lot of debates were conducted on the colossus computers regarding its design and speed. The rebuilt structure can be seen in the Bletchley Park where the ninth colossus computer stood in the World War II.


  1. Copeland, J. (2006). Colossus. Oxford: Oxford University Press.
  2. Knowledge Crush. (2003). Colossus computer . [Online], Available from Knowledge Crush: <> [Accessed on June 24, 2009]
  3. Lavington, S. (1980). Early British Computers: The Story of Vintage Computers and the People who Built Them. Manchester: Manchester University Press.
  4. Reed, D. (2007). A Balanced Introduction to Computer Science . New Jersey: Prentice Hall.


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