Computer History

First electronic computer (1943) : the building of Colossus

By designing a huge machine now generally regarded as the world's first programmable electronic computer, the then Post Office Research Branch played a crucial but secret role in helping to win the Second World War. The purpose of Colossus was to decipher messages that came in on a German cipher machine, called the Lorenz SZ.

The original Colossus used a vast array of telephone exchange parts together with 1,500 electronic valves and was the size of a small room, weighing around a ton. This 'string and sealing wax affair' could process 5,000 characters a second to run through the many millions of possible settings for the code wheels on the Lorenz system in hours - rather than weeks.

Both machines were designed and constructed by a Post Office Research team headed by Tommy Flowers ¹ at Dollis Hill and transported to the secret code-breaking centre at Bletchley Park, near Milton Keynes, where it was demonstrated on December 8, 1943. We have to fast forward nearly thirty years to 1972 for the arrival of the first desktop all-in-one computer, which are more familar to us today. That honour falls to the HP9830. But unfortunately few people got to hear about it because Hewlett Packard marketed it primarily to scientists and engineers - by nature very quiet people!

Colossus (1941) : inside the machine

During the Second World War the Germans used a Lorenz encoding teleprinter to transmit their high-command radio messages. The teleprinter used something called the 5-bit Baudot code, which enciphered the original text by adding to it successively two characters before transmission. The same two characters were applied to the received text at the other end to reveal the original message.

Gilbert Vernam had developed this scheme in America, using two synchronised tapes to generate the additional random characters. Lorenz replaced the tapes with mechanical gearing - so it wasn't a genuinely random sequence - just extremely complex.

But in August 1941 the Germans made a bad mistake. A tired operator sent almost the same message again, using the same wheel settings. It meant the British were able to calculate the logical structure inside the Lorenz.

Colossus was then built to find the Lorenz wheel settings used for each message, using a large electronic programmable logic calculator, driven by up to 2,500 thermionic valves. The computer was fast, even by today's standards. It could break the combination in about two hours - the same as today's modern Pentium PC.

Colossus Mk II (1944) : a bigger better Colossus

Without the contribution of the codebreaking activity, in which Colossus played such a major part, the Second World War would have lasted considerably longer.

By the time of the Allied invasion of France in the early summer of 1944, a Colossus Mk II (using nearly twice as many valves to power it) was almost ready.

The head of the Post Office Research Team, Tommy Flowers, had been told that Colossus Mk II had to be ready by June 1944 or it would not be of any use. He was not told the reason for the deadline, but realising that it was significant he ensured that the new version was ready for June 1, five days before D-Day.

It was in the build-up to D-Day and during the European campaign that followed that Colossus proved most valuable, since it was able to track in detail communications between Hitler and his field commanders.

Top secret : the ultimate Chinese walls

Colossus weighed around 35 tonnes in Mark II form. Its 2,500 valves, consuming 4.5 Kwatts, were spread over two banks of racks 7 feet 6 inches high by 16 feet wide spaced 6 feet apart. Thus the whole machine was around 80 feet long and 40 feet wide.

This huge machine was also one of the most closely guarded secrets of the war yet required dozens of people to build, many of them outside the military establishment in the Post Office.

Tommy Flowers was one of the very few entrusted with the overall plan - and even he didn't know the full details of the German codes.

In order to ensure security, Colossus was broken down into modules - each given to a separate Post Office team at Dollis Hill. The teams were kept apart - each having no idea of the overall shape of the ground breaking machines they were creating.

The building of SIGSALY (1943) : pioneer digital telephone system

Another secret wartime computer whose existence was finally revealed many years later was SIGSALY - the secret 'scrambling' system devised to protect the security of high level Allied telephone traffic.

SIGSALY - originally codenamed Project X - was also known as 'Green Hornet'. It was the first unbreakable speech coding system, using digital cryptography techniques, with one time digital keys being supplied by synchronised gramophone discs.

SIGSALY was built in the USA, though using pulse code modulation (PCM) digital encoding techniques invented in 1937 by the English engineer Alec Reeves ².

The first priority was to protect the hotline between the Cabinet War Room bunker under Downing Street and the White House in Washington D.C. The 50-ton London terminal was shipped over in 1943 and housed in the basement of the Selfridges annexe in Oxford Street, under tight guard.

1. Tommy Flowers (1905-1998) : creating the colossus

Tommy Flowers built the first computer, as a codebreaking device during the Second World War.

Flowers was a Londoner with a passion for electronics. Having gained his degree in electronic engineering he went to work for the Post Office. His dream was to try to convert Britain's mechanical telephone system into an electronic one, but opinion was against him.

During the Second World War he was drafted into Bletchley Park to join the ranks of mathematicians and cryptographers who were trying to crack Germany's code system.

He used his telephone experience to turn his fellow experts' ideas into an electronic codebreaking device named 'Colossus', which was in effect the first electronic computer. By the end of the war his team had built ten machines, each one improving on the one before.Most were dismantled afterwards and the plans destroyed for security reasons.

2. Reeves, Alec Harley (1902-1971) : a peaceful man with a rapid pulse

Alec Reeves devised Pulse-Code Modulation, the first digital coding system, which liberated bandwidth.

Reeves, a natural tinkerer, grew up in the Home Counties. He glided through his electrical engineering degree to take a job developing long-wave transatlantic radio communications in the 1920s. He also helped develop short-wave and microwave radio systems.

Reeves became acutely aware of the shortcomings of analogue communication and this led him to develop Pulse-Code Modulation in 1937. It was a long time before the work was fully appreciated, but in 1969 he received the CBE - and a postal stamp commemorating PCM was issued.

Reeves was peace-loving and reluctant to work on offensive weapons, so during the Second World War he developed pinpoint bombing aids, which helped reduce civilian casualties, for which he received an OBE.

He became head of research on electronic switching systems at Standard Telecommunication Laboratories until he retired. Reeves dedicated his private life to helping others, particularly in youth and community projects and rehabilitating prisoners.