Vacuum tube computers: The WWII 1943 Colossus Mark I

I have always been interested in electronics engineering ingenuity used to solve “impossible” problems. One such problem was the task of deciphering the German Army's most secret transmissions during WWII via a German teletype machine named Tunny (that's what the British called tuna). The SZ40 was one such cipher machine used as a teleprinter attachment.

The German SZ40 cipher machine (Image courtesy of Stanford University, Reference 2)

The German SZ40 cipher machine (Image courtesy of Stanford University, Reference 2)

Due to a key mistake made by a German operator on an SZ40 in 1941, the details of which the British have never revealed, the cryptographers were able to determine that the German machine had wheels with variable pin settings, plus that there were 12 of these wheels, the sizes of the wheels (which were in essence the number of variable pin settings on each) were 23, 26, 29, 31, and a continued progression. They also were able to determine that the rotors moved in relation to one another as a message was enciphered. This was the first of two things that helped change the course of the war.

The second one was that in order to decipher the SZ40, they needed a very fast rate of operation in a British cryptanalysis machine which was not presently available in the mechanical devices used in order to determine the patterns around the circumferences of all 12 wheels in the SZ40 at a particular point in time since the Germans were regularly changing the pin-setting patterns. After that, they would need to figure out the settings of the wheels at the beginning of the message. Maxwell Newman, a 45 year-old mathematician from the University of Cambridge, was chosen to head up the effort to break the German teletype traffic.

That same year, 1943, work was in progress on Colossus, a high-speed programmable machine. Thomas H. Flowers was the chief engineer on the Colossus project, who had been looking into how he could use electronics in the electro-mechanical telephone switching systems which were being used before WWII. This effort proved critical to his work on Colossus.

Flowers suggested that one of Heath Robinson’s tapes, which had the unchanging data on the internal mechanism actions of the SZ40 machines, be replaced by Colossus (The reliability of mechanical machines was poor in reading at high speeds). There was only one tape that needed to be read at high speed and Colossus would perform that task with great reliability.

The problem here was that tube failures were common in electronics and Colossus had 1,500 vacuum tubes. Flowers said that the Colossus system would work reliably if it was not turned off and on, which was the common cause of vacuum tube failures at that time. They ran 24 hours per day. Later, in 1944, the Mark II was introduced with 2,400 vacuum tubes assisted by 800 electromagnetic relays.

The Mark II machines were able to work in parallel on five successive characters simultaneously and could read 5,000 characters per second to process characters at an astounding rate for that time of 25,000 per second.

In 1943, the British government Code and Cypher School, operated by the Women's Royal Naval Service, began using the Colossus I, a high speed programmable vacuum tube-based computer, to help solve an 'impossible' to break cypher code from the German teletype cipher machine called Tunny. (Image courtesy of Reference 1)

In 1943, the British government Code and Cypher School, operated by the Women’s Royal Naval Service, began using the Colossus I, a high speed programmable vacuum tube-based computer, to help solve an “impossible” to break cypher code from the German teletype cipher machine called Tunny. (Image courtesy of Reference 1)

In a speech in London in 1986, Hinsley told an IEEE audience, how deciphered Tunny messages gave British intelligence in the campaigns in North Africa, Italy, southern France, and eastern Europe. But the biggest contribution of Tunny decryptions was right before the Normandy invasion.


1 Electrotechnology in World War II, Breaking the enemy's code, Glenn Zorpette, Associate Editor, IEEE Spectrum, 1987.

2 The Lorenz Schluesselzusatz SZ40/42, Stanford University.

6 comments on “Vacuum tube computers: The WWII 1943 Colossus Mark I

  1. raj_at_anasim_dot_com
    October 2, 2016

    Fascinating, Steve!


  2. Thomas7
    October 2, 2016

    Steve this is awesome!

  3. antedeluvian
    October 3, 2016

    Steve If you're ever in England, you should make a point of visiting Bletchley Park. It is actually two museums in one. Obviously the first is the site where Turing and many others did their work in cracking the Enigma machines. They have working Enigmas as well as a reconstructed “bombe” which was used to work through the possible combinations of coding wheels used on the Enigma. It was a glorious coaster and whirring of electromechanical parts when they fire it up. The second museum includes a reconstructed Colossus, also operational albeit an awful lot quieter the the “bombe”. Hallowed ground for electrical engineers- if you only visit one place in England this should be it.

  4. Steve Taranovich
    October 3, 2016

    @antedeluvian—thanks—I have been to the London Museum, but never knew about Bletchley Park—I definitely must see the next time I am in the UK

  5. Navelpluis
    October 5, 2016

    Talking about Bletchley Park…

    In the early years (2001..2008) it really really was a beautiful museum for non-insiders. You had the 'Crypto Trail', the complete picture from the Germans creating their codes, morse radio, intercept by the Y-stations, the role of Bletchley Park, and at the end the most important messages ended up under the eyes of Churchill. Tony and Margaret Sale were the 'father and mother' of the museum.

    Today, Bletchley Park (BP) is a collection of 'crypto snippets'. The complete story is gone. Pity. More worse is that David White's collection is not on display anymore. He has a beautiful collection of spy radio & crypto gear, really really nice. His cottage was full of it, great atmosphere. I remember an elder lady (in her 80s! ) walking in, listening to morse on an old HRO receiver… She said “It is an Italian, I can hear it in emotion”. Lots of partial collections are gone, also the Churchill collection has gone. “Not part of the park” said the management, well, the room above the BP entrance is called “Churchill Room”, so certainly the man had involvement there. certainly in the preliminary stage of the park at the start of the war. The BP management arranged to shoo off their best volunteers, people that were active from the start of the BP excistance, a shame I find this.

    Why this happens? I think because the big money boat came in. Money can make people behave ugly, pity…

    But more worse is the stupidity of placing a barrier to divide two sites of the park. There is a legal issue between management and the Colossus computer team. BP management decided to place this barrier. For visitors (most older people) this is an utter misery, leading to frustrations.

    Think about this: If this barrier was there during the war, the war may have been lost !!!

    So, BP, give the Colossus team their fair share, remove this stupid barrier and behave like grownups, please !!


  6. jimfordbroadcom
    October 5, 2016

    as I was just pondering engineering tradeoffs this morning and how when the tradeoffs tip too far to one direction, the other direction disappears.  That made me think of how nobody uses vacuum tubes for computing anymore.  And then I come in to work to find this blog!  Neat stuff, and yes, navelpuis, it sounds like a damn shame that the BP museum(s) has gone to pot.

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