Colossus design started in March 1943. By December 1943 all the various circuits were working and the 1,500 valve Mark I, built at Dollis Hill, was dismantled, shipped up to Bletchley Park, and assembled in F Block over Christmas 1943. The Mark I was operational in January 1944 and successful on its first test against a real enciphered message tape.

Colossus reduced the time to break German Lorenz messages from weeks to hours, and came into service just in time to decipher messages giving vital information to Eisenhower and Montgomery prior to D-Day. After D-Day the French resistance and the British and American Air Forces bombed and strafed all the telephone and teleprinter land lines in Northern France, forcing the Germans to use radio communications, and so the volume of the intercepted messages suddenly went up enormously. The Mark I had been rapidly succeeded by the Mark II Colossus in June 1944, and eight more were quickly built to handle the increase in messages. The Mark I was upgraded to a Mark II and there were thus ten Mark II Colossi in the Park by the end of the war.

 

After VJ-Day, suddenly it was all over. Eight of the ten Colossi were dismantled in Bletchley Park. Two went to Eastcote in North London and then to Government Communications Headquarters at Cheltenham. These last two were dismantled in about 1960, and in 1960 all drawings of Colossus were burnt, and its very existence was kept secret.

The campaign to save Bletchley Park from demolition by property developers started in 1991. In 1993 Tony Sale gathered all the information available. This amounted to no more than eight 1945 wartime photographs of Colossus plus the 1980s papers of the design engineers and some fragments of circuit diagrams which some engineers had kept, quite illegally, as engineers always do.

Tony Sale spent nine months poring over the wartime photographs using a sophisticated modern CAD system on his PC to recreate the machine drawings of the racks. He found that sufficient wartime valves were still available as were various pieces of Post Office equipment used in the original construction.

Colossus is not a stored-programme computer. It is hard-wired and switch-programmed, just like the ENIAC. Because of its parallel nature it is very fast, even by today's standards. The intercepted message punched onto ordinary teleprinter paper tape is read at 5,000 characters per second. The sprocket holes down the middle of the tape are read to form the clock for the whole machine. This avoids any synchronisation problems, as whatever the speed of the tape, that's the speed of Colossus. At 5,000 cps the interval between sprocket holes is 200 microseconds. In this time Colossus will do up to 100 Boolean calculations simultaneously on each of the five tape channels and across a five character matrix.

The rebuilt Colossus can now be seen working in the Museums at Bletchley Park.

From 'Colossus 1943 - 1996' by Tony Sale.

 

What is a valve in computers and radios? What is its job and how does it work? Why are valves no more in use? Illustrate your answer with a sketch.

What event is referred to by D-Day? What was the codeword for this event? What were the achievements of Eisenhower and Montgomery?

After completion of the Mark I the team around Tony Sale carried on rebuilding a Colossus Mark II. When was the Mark II rebuild completed? Explore Tony Sale's home page! Click on the last link below.