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Colossus
Birth of the digital computer

Colossus was an electronic digital computer, built during WWII from over 1700 valves (tubes). It was used to break the codes of the German Lorenz SZ-40 cipher machine that was used by the German High Command. Colossus is sometimes referred to as the world's first fixed program, digital, electronic, computer. It was developed and built before the American ENIAC computer. 1

Colossus was designed by Tommy Flowers, an electronics engineer of the Post Office Research Station (part of the General Post Office, (GPO) at Dollis Hill (UK), with input from Harry Fensom, Allen Coombs, Sid Broadhurst and Bill Chandler. It was used to solve a problem posed by Max Newman, a codebreaker at Bletchley Park.

The image on the right shows one of just eight photographs of an original Colossus Mark II in action that were taken during WWII. The images were used in 1993 by Tony Sale and his team of volunteers, to start the Colossus Rebuild Project.
  

One of the most prominent parts of Colossus is the input device on the right, nicknamed 'the bedstead'. It is an optical reader for punched paper tapes, than can read data at the phenomenal speed of 5000 characters per second! A complex system of supporting wheels and pulleys is necessary to regulate the tape tension and prevent the tape from ripping apart at this speed.

Contrary to popular believe, Colossus was not used to break the German Enigma cipher. The Enigma was broken by means of an electro-mechanical machine, known as the Bombe, designed by Alan Turing and Gordon Welchman.

Colossus on the other hand, was used to break the Lorenz SZ-40/42. A highly sophisticated teleprinter attachment with no less than twelve complex cipher wheels. It was used by the Ger­man High Command (OKW) for messages at the highest level, for example between Adolf Hitler and Field Marshal Rommel in North Africa.
  

The first Colossus was delivered at Bletchley Park on 18 January 1944 and broke its first message on 5 February of that year. It was succeeded by Colossus Mark II, which consisted of no less than 2400 valves. The first Colossus Mark II was delivered on 1 June 1944 — just five days before the D-Day landings on the Normandy coast — and immediately produced good intelligence. In total 10 Colossi were installed before the end of the war. They were decisive for the Allied victory.

After the war, eight of the ten Collosi were dis­mantled and their parts were reused. The other two remained in use at GCHQ for solving crypt­analytic problems. The image on the right was taken at GCHQ in 1963. It shows Colossus from the front left. At the right, in front of Colos­sus, is some kind of curved console that might be a post-war addition. This rare image was released by GCHQ on 18 January 2024 to commemmorate the 80th birthday of Colossus [5]. Further rare photo­graphs in this series are available below.   

In the image above, an additional device on wheels is visible at the far right. This might be an alternative high-speed tape reader that replaced the original 'bedstead' to achieve faster pro­cessing. Eventually, in the mid or late 1960, the remaining two Colossi were also dismantled.

 More about the Lorenz SZ-40/42 cipher machine

  1. It is often thought that ENIAC was the first electronic digital computer, but after the disclosure of COLOSSUS in the mid-1970s, this part of history had to be corrected.  Wikipedia

Colossus in 1963, seen from the front left. Note the large console at the front and the new tape reader at the far right. Image kindly provided by GCHQ [5]. Crown copyright.
The console in front of Colossus in 1963. Image kindly provided by GCHQ [5]. Crown copyright.
Colossus in 1963, seen from the rear. Image kindly provided by GCHQ [5]. Crown copyright.
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Colossus in 1963, seen from the front left. Note the large console at the front and the new tape reader at the far right. Image kindly provided by GCHQ [5]. Crown copyright.
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The console in front of Colossus in 1963. Image kindly provided by GCHQ [5]. Crown copyright.
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Colossus in 1963, seen from the rear. Image kindly provided by GCHQ [5]. Crown copyright.

Colossus rebuild
Immediately after WWII, most Colossus computers were ordered to be demolished. They were either destroyed or dismantled and the components were reused. Two machines were kept for future use by GCHQ during the Cold War. In 1959 and 1960, when newer technologies had emerged, these two machines were finally decomissioned and were eventually also dismantled.

In 1991, a team led by Tony Sale started playing with the thought that it might be possible to rebuild a fully operational Colossus computer.

In 1993, he set out to start the rebuild project, based on the very limited information that he had gathered. The first parts of Colossus were switched-on on 6 June 1996, in the presence of HRH the Duke of Kent and the original designer Tommy Flowers. The latter passed away in 1998 at the age of 92. In November 2007, the rebuild project was completed and Colossus was staged in an international Cipher Challenge contest.
  

The team then started to convert Colossus – since renamed Colossus-I – into Colossus Mark II. As the Mark II version contains about 1000 valves more than the the Mark I, this project would, no doubt, keep Tony and his team busy for the next couple of years. Sadly, Tony Sale passed away unexpectedly on 28 August 2011 at the age of 80, leaving behind the Colossus legacy. Luckily though, his achievement was well-documented so that his team could continue his work.

Colossus in September 2013
Colossus, with Kevin Coleman explaining it to the public.
Teleprinter in front of Colossus
Close-up of Colossus
Colossus seen from the right
Colossus seen from the front right
A view trough the bedstead
Rear view of Colossus
Colossus rear view
Colossus detail
Close-up of the valves
Dust settled down on the thermionic valves
Valve close-up
Stepping relays and valves inside Colossus
Rack detail
Status lamps on top of Colossus
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Colossus in September 2013
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Colossus, with Kevin Coleman explaining it to the public.
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Teleprinter in front of Colossus
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Close-up of Colossus
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Colossus seen from the right
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Colossus seen from the front right
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A view trough the bedstead
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Rear view of Colossus
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Colossus rear view
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Colossus detail
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Close-up of the valves
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Dust settled down on the thermionic valves
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Valve close-up
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Stepping relays and valves inside Colossus
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Rack detail
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Status lamps on top of Colossus

Colossus today
Tony's legacy has been preserved well. In 2012, the Colossus display in H-Block at Bletchley Park has been refurbished completely. The ceiling has been painted black and the floor has received a new carpet. More importantly, there is now much more room for the audience, who can walk all around Colossus now. The image above shows Colossus in September 2013 during a presen­tation by volunteer Kevin Coleman. At the far left is Chief Colossus Engineer Phil Hayes.


As it is now possible to walk around Colossus, you can 'view through' the machine and see the many, many wires, stepping relais, and thermionic valves (vacuum tubes) behind the front panel.

Real life visit
If you are interested in electronics, computer technology, history of WWII, etc., Colossus is a must see. Only when you are in the same room with Colossus, can you appreciate the efforts of its designers, its builders, its operators and also of its re-builders. You can see it in action, feel the heat generated by the valves and sense the typical smell of working electronics. When visiting Bletchley Park, you must take some time to visit The National Museum of Computing (TNMOC) in Block H. The volunteers will be more than willing to tell you all you want to know about Colossus.

 Visit the TNMOC website
Colossus in 2004
Below are some photographs of Colossus that we took in November 2004. We apologize for the rather poor quality, but at the time we were still using an old analog camera.

The bedstead of Colossus
Detail of the power supply
Close-up of some of the valves
Using a modern oscilloscope to check the operation of the optical tape reader
Walking through Colossus
Looking at a valve rack inside colossus
The optical tape reader seen from the rear
The programming panel
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The bedstead of Colossus
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Detail of the power supply
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Close-up of some of the valves
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Using a modern oscilloscope to check the operation of the optical tape reader
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Walking through Colossus
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Looking at a valve rack inside colossus
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The optical tape reader seen from the rear
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The programming panel

Enigma and the Bombe
It is sometimes assumed, even by respected writers, that Colossus was used to break the German Enigma codes. This was not the case however. Colossus was used to break the codes of the far more advanced Lorenz SZ-42, used by Hitler's High Command (Oberst Kommando des Heeres).

Enigma instead, was broken with help of an electro-mechanical machine, called the Bombe, designed by Alan Turing and Gordon Welchman at Bletchley Park and built by Harold (Doc) Keen at the British Tabulating Company (BTC).

Over 200 of these machines were installed at Bletchley Park and its outstations, attacking the German Enigma messages on a daily basis.

 More about the Bombe
 More about Enigma
  

References
  1. B. Jack Copeland, Colossus, Breaking the German Tunny Code at Bletchley Park
    An illustrated history. The Rutherford Journal, Volume 3, 2010.

  2. Tony Sale et al., Colossus Rebuild Project
    Date unknown. Retrieved March 2011.

  3. Tony Sale, Colossus 1943-1996,
    And How it Helped to Break the German Lorenz Cipher in WWII.
    ISBN 978-0947712365.

  4. South Beds News, Photograph of Tony Sale in front of Colossus
    Website The Telegraph. 31 August 2011.

  5. News: GCHQ celebrates 80 years of Colossus
    GCHQ website, 18 January 2024.
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© Crypto Museum. Created: Friday 23 November 2012. Last changed: Thursday, 18 January 2024 - 13:50 CET.
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