Tommy Flowers: The Man Who Built Colossus

Tommy Flowers is one of histories most significant electrical engineers. His work on the first programmable computer, Colossus, has sealed his place in history.
Christopher McFadden

Tommy Flowers is one of the greatest heroes of World War II, though many may never have heard of him. Although his profile is not as prominent as the likes if Guy Gibson or Douglas Bader, Tommy nonetheless saved many thousands of lives. 

Tommy, with some help from William Tutte, designed and built the famous 'Colossus' computer. Historians have since argued that their combined work likely shortened the war by two years.


Early Years

Tommy Flowers was born at 160 Abbott Road, Poplar in London's East End on the 22nd December 1905. He was the son of a bricklayer. After taking an apprenticeship in mechanical engineering at Royal Arsenal, Woolwich, he would go on to earn an electrical engineering degree from the University of London. This degree was earned by taking evening classes until his eventual graduation.

In 1926, Tommy Flowers joined the telecommunications branch of the General Post Office (GPO). In 1930, Tommy joined their research center at Dollis Hill in Northwest London. From around 1935 onwards Tommy would explore the use of electronics in telephone exchanges.

In 1935, Tommy would marry Eileen Margaret Green. The couple would later have two children, John and Kenneth. He became obsessed with electronics and by 1939 had become convinced that an all-electronic system should be possible. An insight that would prove very useful indeed during the coming World War. 

Tommy Flowers: The Man Who Built Colossus
The GPO exchange at the Central Government War Headquarters aka "Burlington." Source: NJ/Wikimedia Commons

World War II

At the advent of World War II, the entire GPO research unit moved to Bletchley Park. It was believed the team's skill would be useful for the war effort. Alan Turing personally tasked Flowers with helping him build a decoder for the Bombe Machine. This device was used to decode German messages.

Despite failing in the task, Turing was impressed with Turing and introduced him to his friend Max Newman. Max was a brilliant mathematician who had closely followed the work of William Tutte on the German Lorenz SZ40/42.

This high-level German cipher was generated by a teletypewriter in-line cipher machine. The SZ40 and SZ42 were also called "Geheimschreiber" (secret writer) systems. The British would go on to nickname it "Tunny" or Tunafish. This was a far more complex system than Enigma. The decoding system involved was also far to complex to easily do by hand. 

Tommy Flowers: The Man Who Built Colossus
An Enigma machine in use in 1943. Source: Grupp/Wikimedia Commons

Sadly for Tutte, though his work had been vital to decoding it, it had proved slow. Newman had begun to think that it was never going to be possible to mechanize the process that Tutte and his team had done by hand. They did have a simple machine that had attempted to help with the process. It was nicknamed 'Heath Robinson' but it proved unreliable and would often break down.

Tommy was brought in initially to fix 'Heath Robinson'. However having spent some time studying it he believed he would be able to build a different, better version. He began the task in February 1943. And the job that would consume him until December of the same year.

How 'Colossus' was born

After several months of intensive work one of the greatest inventions of the 20th Century was born, 'Colossus'. 'Colossus' was one of the worlds first programmable computers and a masterpiece of engineering.

Tommy Flowers: The Man Who Built Colossus
Reconstruction works of a Colossus at Bletchley Park pictured here in 2006. Source: MaltaGC/Wikimedia Commons

Tommy Flowers' new machine worked off roughly 1,800 thermion valves. Many of his contemporaries believed that this was a great weakness of the machine as they were notorious for being unreliable. 'Colossus' also had one paper tape instead of two by generating the wheel patterns electronically.


Flowers knew better. His previous work as a GOP engineer showed him that the main reason for the valves failing was constantly switching the machine on and off. To this end, 'Colossus' was to be left on permanently, in what Flowers described as a "stable environment".

Tommy was confident of his hypothesis quoting that the British Telephone System comprises of thousands of valves and was stable because the circuitry was live all the time. Flowers' simile proved to be absolutely correct. 'Colossus' turned out to be a very reliable machine indeed. Prior to 'Colossus', the maximum number of valves in any single machine was a meager 150. Interestingly, the managers at Bletchley Park were highly skeptical of his proposal for the new machine.

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Tommy Flowers: The Man Who Built Colossus
Heath Robinson machine at the National Museum of Computing. Source: AlisonW /Wikimedia Commons

The personal cost of Colossus

Such was the level doubt that Flowers was required to fund a large part the project out of his own money as well as build it on his own time. 'Colossus' would prove to be an extraordinary machine for the time. Its predecessor, Newman's 'Heath Robinson', could read about 1000 characters a minute. Owing to its poor reliability this would often average a lot less. The entire process needed speeding up especially as D-Day was approaching. General Eisenhower and his team needed to know as much as possible about Nazi positions in northern France.

This is where 'Colossus' came into its own. It could read around 5000 characters and minute with excellent reliability.

Despite the success of 'Colossus', the 'Heath Robinson' was still a valuable machine for solving certain problems. The final development of the concept was a machine called Super Robinson that was designed by Tommy Flowers. This one could run four (not just two) tapes and be used for running depths and "cribs" or known-plaintext attack runs.

Colossus gets the go ahead

Soon after Flowers was made a Member of the Order of the British Empire in June of 1943, he got the full backing from the Director of Dollis Hill, W. G. Radley. This backing provided Flowers and his team the highest priority for the acquisition of parts during wartime. The first fully functional machine was built with 11 months. The machines nickname is derived from the fact that the machine was very large. It seemed very fitting.

Colossus 1 performed extremely well compared to its predecessor. It turned out to be in the order of five times faster than Heath Robinson. The completed machine was finally delivered to Bletchley Park in January of 1944. Once there it was assembled and became operational in February 1944. Algorithms for Colossus were provided by Tutte and team of mathematicians. 

Flowers anticipated the need for more of his programmable computers. He was already working on the improved second version, the Mark 2. This would employ double the number of valves at around 2400. Thankfully for Eisenhower and Allied high command, Colussus and its sister machine 'Colossus' 2 were both ready for D-Day. Both machines were up and running by the 1st of June, 1944.

This provided Allied Supreme Commanders with nearly instant access to information detected over the Lorenz System. The Nazis had absolutely no idea their code had been broken and still held their system in high regard. The information gleaned proved invaluable to the planning of D-Day. 

Tommy Flowers: The Man Who Built Colossus
U.S. Soldiers disembark a landing craft at Normandy, France, June 6, 1944. Source: Jasonanaggie /Wikimedia Commons

Colossus probably saved thousands of lives

On one occasion, the speedy decoding of Axis communications just before D-Day on June 5th, 1944 showed that Hitler was convinced D-Day landings would occur at Pays de Calais. This comforted Allied Command that 'Operation Fortitude' had worked perfectly.

Hitler then refused to move troops to Normandy. This might well have saved many thousands of lives during the eventual Normandy landings.

At the end of Second World War, British Intelligence had built ten Colossus machines in total. All but two would be dismantled. These surviving machines were subsequently used at GCHQ but these too would be dismantled between 1959 and 1960. A fully functioning Colossus Mark II was rebuilt by a team of volunteers led by Tony Sale between 1993 and 2008It is currently on display at The National Museum of Computing at Bletchley Park.

Tommy Flowers: The Man Who Built Colossus
The fully-functional replica of a Colossus Mark 2, shown here and now on display at the National Museum of Computing (in H Block) Bletchley Park. Source: GeographBot/Wikimedia Commons

Post War

Tommy Flowers was awarded £1000 for his 'Colossus' invention post-war. This amount of money sadly did not cover the amount of money he had personally invested in the machine's construction. Rationing was still in effect and would be a day to day occurrence for some time. Tommy Flowers was said to be a very humble man.

This was perfectly demonstrated when he divided the money up amongst the team who had helped him. In the end, he was left with around £350. This was a decent amount of money for 1945 but not a huge amount for the man credited with inventing the modern computer. Amusingly enough, Tommy would later apply for a loan from the Bank of England to build a Colossus-like machine. His application was denied as the bank believed the machine could not work.

Flowers, still under the Official Secrets Act, was unable to argue the case or inform them that he had already successfully built one. The post-war insanity of the Cold War would see Tommy Flowers' work marked as highly classified. He remained under the Official Secrets Act for many years to come. It is a wonder whether he found it amusing or galling when the 'first computer' was announced by the Americans in 1948. Especially considering he'd created 'Colossus' some 5 years earlier.

Tommy Flowers: The Man Who Built Colossus

Frontal view of the reconstructed Colossus. Source: TedColes/Wikimedia Commons

Later years

Tommy Flowers would continue to work at the Post Office Research Station as the Head of the Switching Division. Tommy and his team would go on to pioneer work on all-electronic telephone exchanges. A basic design was in place by 1950 that would lead to the Highgate Wood Telephone Exchange. 

Tommy would also become heavily involved in the development of ERNIE. In 1964, he became the head of the advanced development of Standard Telephones and Cables Limited. Here he continued to work on and develop electronic telephone switching systems. This included pulse amplitude modulation exchange.

Tommy retired in 1969. Post-retirement, Flowers would keep working. In 1976 when he wrote and published an Introduction to Exchange Systems. In 1977 Tommy was also made an honorary Doctor of Science by Newcastle University.

Tommy would become the first recipient of the Martlesham Medal in 1980. This was in recognition of his achievements in computing. In 1993, he received a certificate from Hendon College, having completed a basic course in information processing on a personal computer.

It would not be for many years that the truth would come out. Even as late as 1982, he was still tied to the Official Secrets Act. In a lecture he was to give in 1982,  Flowers told American Students he needed to consult with the Ministry of Defence to see what he was allowed to cover. Even as late as 2011, some of Tommy Flowers' work still remains classified. 

The Legacy of Tommy Flowers

Tommy's vital work for the war effort during World War II was not revealed to the world until 1970's. Today, the place of 'Colossus' in computing history is assured, and Flowers is acknowledged as the principal architect of the machine.

His family was even in the dark for all those years. All they knew was that his efforts during the war were something 'secret and important'. Tommy Flowers, the man who invented the first programmable computer, died on October 28th, 1998. He was 92 years old.

Tommy was honored by London Borough of Tower Hamlets, where he was born. An Information and Communications Technology (ICT) center for young people, the Tommy Flowers Centre, opened there in November 2010. The center has closed but the building is now the Tommy Flowers Centre, part of the Tower Hamlets Pupil Referral Unit.

He has also had a street named after him, Flowers Close, near the site of the old research center for the GPO in Dollis Hill. This is now a housing estate. 

Tommy Flowers: The Man Who Built Colossus
ERNIE 1: The first premium bond number producing machine in the science museum. Source: geni/Wikimedia Commons


Random numbers are far from simple things. To most, it is difficult to believe that an entire computer would be needed to generate some. This was exactly the intended role for ERNIE when it was developed in 1957. And it was a beast of a machine.

In 1956 The British Government decided to help its people start saving money. Their solution was to an institutional lottery called Investment Premium Bonds. Unlike regular bonds, these had no guaranteed rate of return.

Instead each month a draw was conducted to see which winning 'ticket' had won the various prizes on offer. Unlike regular lotteries, however, this one had lots of small prizes and a few bigger ones to pick from. By the end of 1956 around 50 million bonds had been sold, each with a unique serial number. The existing system could accommodate around 100 million unique serials at the time.

To pick winners was going to take a lot more than some balls and tumbler machine. What they needed was an industrial scale random number generator machine. 

But random numbers come in two flavors: pseudo-random and real-random. Pseudo-random number generators are those that computers generally use to create random numbers. Because they are produced using algorithms they aren't technically random, but they come close. Due to the nature of lotteries, pseudo number generators are just not good enough. 

The need for randomness

What was needed was a real random number generator. It would need to have some form of physical randomness and not be predictable. 

In order to create a truly random number, you need to add something to the basic design of the machine to generate a random signal. In the case of ERNIE, Electronic Random Number Indicator Equipment, the source was a set of neon tubes. The very design of neon tubes added an element of randomness that would be ideal for the generation of numbers. The colliding electrons and atoms and the path taken by these particles within the tube tend to be very chaotic. This allows for a random current component leaving the tube.

In the computer, these randomised currents were amplified to make the noise dominate any steady current and then this was converted into pulses by a "slicing valve" - presumably a threshold device or level clipper. The output was then applied to a multivibrator circuit that cleaned the pulses up.

Tommy Flowers: The Man Who Built Colossus
The circuit of the noise generator. Source: i-programmer

On average, the system received 3000 pulses per second out of the neon tube. Each and every one of these was used drive a counter. The number of pulses per second also had a random component but wasn't truly random as it fluctuated about a minute. The trick to generating a random number was to use the lowest order digit.

By using the lowest order digit, a 'ring' counter was used to would in turn count mod 6, mod 10 or mod 24. To improve the randomness so that all digits were equally likely the counter was started off from different values each time - in practice just where ever it got to in the previous count.  

These counters constantly went around and around as the pulses were counted. It was in effect, an electronic version of a roulette wheel. 

The winning ticket

In the final machine, nine neon tubes were used in connection with nine counters. The ninth counter was arranged to display a value from 0 to 22 to further generate a letter. These digits each corresponded to one of the digits in the serial number of the premium bonds.

ERNIE is said to have cost £25,000 and it was completed in 1957. It took typically 52 days to complete a draw and was tested by the UK Government Actuary's Department each time to prove its randomness.

Today the prizes can be very large.

Honoring the man behind Colossus

In September of 2012, his wartime diary was put on display at Bletchley Park. 70 years after to the creation of 'Colossus' in 2013, Tommy's legacy was honored by British Telecom with a memorial. BT became the successor to Post Office Telephones after the war. A life-size bronze bust, designed by James Butler, was officially unveiled by Trevor Baylis at Adastral Park. BT also began a computer science scholarship and award in his name.

BT also opened the Tommy Flowers Institute in September of 2016. This institution is used for ICT training at Adastral Park to support the development of postgraduates transferring into the industry. It focusses on bringing ICT-sector organizations together. The idea is to boost academic research to help solve some of the challenges facing UK businesses. It also works heavily on cybersecurity, big data, autonomics and converging networks.

The launch event was attended by professors from Cambridge, Oxford, East Anglia, Essex, Imperial, UCL, Southampton, Surrey, and Lancaster as well as representatives from the National Physical Laboratory, Huawei, Ericsson, Cisco, ARM, and ADVA. 

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