Colossus Mark 1

Editor’s note: This is a guest blog by Peter Zawistowski, who wanted us to know that this is a 70-year-old computer that is programmable and digital.

The National Museum of Computing, located at Bletchley Park, celebrated the 70th anniversary of Colossus, the first partially programmable electronic computer. Colossus went into service on February 1944, used for code breaking and the deciphering of Lorenz-encrypted communications used between Hitler and his generals.

(Source: The National Museum of Computing)

(Source: The National Museum of Computing)

It was designed by a single British telephone engineer, Tommy Flowers. So incredible are the statics of Colossus Mark 1. Its size, 7 feet high by 17 feet by 11 feet. It contained 1,500 vacuum tubes. The next version, known as Colossus Mark 2, had 2,500 tubes of which 501 were thyraton switches. The Mark 2 also contained nearly 4.5 miles of wiring, roughly 10,000 plus resistors, and so many other components. It weighed about 11,000 pounds and used 8 kW of power.

The Mark 1 began operation on February 5, 1944. The Mark 2 unit replaced it in June of 1944, just in time for D-Day invasion. One of the early deciphered messages allowed the Allies to know that Hitler had swallowed the bait for a false landing at Calais on D-Day, giving the Allies a decisive advantage. This Mark 2 version was easier to program and was five times faster than the Mark 1. By the end of the war, nine Colossus Mark 2 units had been built. The original Mark 1 was converted into a Mark 2, making ten Mark 2 units delivered and operating.

The American built ENIAC received the credit of being the first programmable digital computer. The National Security Agency had released some Freedom of Information Act (FOIA) information in the late 1970s that had shown that Colossus had beaten ENIAC by two years. Note that Colossus was a partially programmable computer. It was not a general purpose machine, but was designed for a specific cryptanalytic task. ENIAC was the first fully programmable digital electronic computer.

The Colossus computers, hardware, and documentation were classified from the inception and remained so long after the war ended. Winston Churchill specifically ordered the destruction of most of the Colossus machines into “pieces no larger than a man's hand.” Tommy Flowers, the designer of Colossus, was instructed to burn all the records, which he did. Some components were used for training purposes. In 1960, the remaining two Colossus machines were disassembled and stored. The need for these machines disappeared in the early 1960s as digital transmission and all-digital encryption became widespread.

Using eight photographs of Colossus and a few diagrams kept hidden or stored by engineers, the project of rebuilding a fully functioning Colossus began. Tony Sale, along with engineers who originally worked on the project, along with many volunteer technicians and engineers labored for years on the project. November 15, 2007, the rebuilt and fully functioning Colossus Mark 2 was unveiled to the public at the National Museum of Computing at Bletchley Park. A new and extended Colossus Gallery opened on March 6, 2012. The new gallery allows visitors to the museum to walk around the huge machine.

8 comments on “Colossus Mark 1

  1. eafpres
    April 1, 2014

    For me, the most astonishing part is the destruction of not only the machines, but the records.  Makes me wonder how much important information is not just delayed by government classification, but actually lost by the destructive paranoia of the governments?

  2. antedeluvian
    April 1, 2014

    My visit to Bletchley Park (which is nominally the same site as the Museum of Computing) was without doubt the best museum I have ever visited (and I have been to a quite a few). It should be the pilgramage of every electronic engineer.


  3. samicksha
    April 2, 2014

    I didn't knew much about colossus but yes it privilege to be there and learn more, i read about its destruction and found some lines by Tommy Flowers when it was ordered to destroy all documentation and burnt them in a furnace at Dollis Hill. He later said of that order: That was a terrible mistake. I was instructed to destroy all the records, which I did. I took all the drawings and the plans and all the information about Colossus on paper and put it in the boiler fire. And saw it burn

  4. eafpres
    April 2, 2014

    @samicksha–I imagine for such creative giants, the destruction was as if they were ordered to kill their own children.  Just horrible.  Thank you for sharing the comment.

  5. alan_#1
    April 4, 2014

    About 12 years ago I was at an EMC seminar at Bletchley Park and to be honest it was rather dull so rather than return after a coffee break I explored the site and was lucky enough to be “caught” by a security guard who gave me a tour.  

    I met (and had lunch with) some of the Colossus rebuild team, they were really dedicated volunteers. With my work hat on I supplied and fitted a PC oscilloscope to the paper tape reader so the signals could be viewed by the public – its still running today.  Seemed rather cool to fit the oldest computer with modern test equipment

    To echo the other comments – its well worth a viisit if you are in the area.



  6. Navelpluis
    April 4, 2014

    We had and still have much contact with a couple of the key persons @ Bletchley Park. In the beginning of the 2000's this museum -we find- was at its best: It told the complete story from interception to decoding messages. This was the Enigma story, not about Colossus yet because a lot still was classified. We know that Government(s) (not to mention them) had trouble with some of the rebuilld work done @ BP, since they want the lid on the can, never to be opened.

    In the last 3 years or so you see that some documents here and there are finally unclassified (!!!) showing how Colossus together with the Lorenz coding machine worked. An amazing archievement. The British -as of my humble opinion- were put on a 'held back' after the war as a result of this: They paid for the war until a couple of years ago, and they almost had no profit of these amazing developments. As an engineer I find that analyzing a bit of this history give you very interesting insights.

    About the Lorenz machine, see this link:

    Have fun on this website anyway…. It eats away days if you don't watch the clock 😉

  7. Sachin
    April 10, 2014

    Even in spite of its colossal size (and I believe that's why it was named Colossus in the first place) I still can't help but wonder about how much faster the process of computer evolution would have moved had these computers been left intact or the technical details relating directly to their construction been made available to the public or even to a few select engineers tasked specifically with their improvement. We would probably have started using iPhones and the like in the early nineties (or sooner).

  8. SunitaT
    April 29, 2014

    The colossus was a very good invention. The fact that it was used to maintain peace during wars was such a good factor. It also made work easier in that department since it could break codes and decipher unknown or unwanted codes. It was generally an asset to the government. Why they chose to shut down the project is only known to them. This is something of value we have clearly lost and we will never be able to recover.

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