As the war progressed and the volume of enemy encrypted radio intercepts increased, it became evident that the manual method then in use for deciphering this material was woefully inadequate. The Post Office Engineering Department at Dollis Hill in London was involved in many different projects designed to further the war effort, and an approach was made by Bletchley Park for assistance in devising equipment to speed up the decryption process.
Flowers had joined the staff at the Research Station at Dollis Hill in the mid-1930s. A team-player with unconventional technical ideas, he had established himself as a man of considerable foresight in the field of telephone exchange switching design, and was asked to find an answer to this problem. He devised a machine, called a "Robinson" (as he described it later, it was "a Heath Robinson affair held together with string and sealing wax").
The use of thermionic valves was felt by many of those concerned to be a weak point in the design of this kind of machine. Flowers made the revolutionary claim that a valve left on all the time, and not switched on and off as required, would have a very long life. The Staff Engineer in charge at DH had faith in Flowers, and backed him. A small design team was formed in great secrecy to work on the first and subsequent machines, the last of which were the 10 Colossus machines, having 2,500 valves apiece, which were so successfully employed at Bletchley Park.
Colossus was designed to deal with the complex intelligence known as Fish, with messages sometimes 10,000 characters in length sent between Hitler's High Command and commanders in the field, and containing vital information on troop dispositions, ration strengths, and even details of leave arrangements for generals. The Morse-transmitted Enigma code, on the other hand, although very complex, used messages of a purely tactical nature, which were never more than 250 characters long. This work had no association with Colossus.
Colossus was engaged on the statistical analysis of the enemy teleprinter intercepts. A separate machine to use this data to decipher each message was needed, and Flowers arranged for me, a former member of the DH staff, then in the Army in North Africa engaged on another DH-backed project, to be posted home to join his team. Busy though he was, Flowers made a daily visit to his small band working in a small laboratory to observe progress.
The new machines, codenamed "Tunny", were to be fed with an enciphered message tape, and after being set up according to the Colossus data, were to produce a printed output on a teleprinter in clear German. To facilitate the testing of Tunny, one of the design engineers devised a certain set of code wheel patterns, and on his next visit Flowers was invited to type in the message "Now is the time for all good men to come to the aid of the party". His delight at seeing the machine print out "I wandered lonely as a cloud that floats on high o'er vales and hills" was reward enough for the small band, who like Flowers, did a 12- or 14-hour day without complaint.
Born in the East End of London in 1905, Flowers gained a scholarship to technical college, enabling him to stay on until 16. He served a four- year mechanical apprenticeship at the Royal Arsenal in Woolwich, gained a London University degree in Engineering through evening classes, and then joined the Post Office as an electrical engineer in 1926, aged 20. Besides his involvement with electronic telephone transmissions, he also did sterling work in the forward line of the DH hockey team in the immediate pre-war years. After the war, he continued his work at Dollis Hill, applying his expertise of electronics to telephone switching and signalling systems.
He was appointed MBE for his contribution to the war effort along with a rather meagre pounds 1,000 award, but, since the codebreaking activities at Bletchley Park were kept secret for 30 years, Flowers remained largely unknown to the public. Only later did his work receive recognition - he was presented with an honorary doctorate from Newcastle University in 1977, and another in 1993 from De Montfort University in Leicester. A reconstruction of Colossus is now on view at the Bletchley Park museum.
Thomas Harold Flowers, engineer: born London 22 December 1905; MBE 1943; married 1935 Eileen Green (two sons); died London 28 October 1998.