Obituary: Allen Coombs

Allen William Mark Coombs, electronics engineer: born 23 October 1911; married 1945 Vera Barella (four daughters); died Yealmpton, Devon 30 January 1995.

Allen Coombs was one of Britain's early computer hardware designers, one of a small band of brilliant Post Office engineers and technicians who during the Second World War designed and built a series of now famous but for many years highly secret electronic computers. He was a senior member of T.H. (Tommy) Flowers's team from the Post Office Research Station at Dollis Hill which designed and created the equipment for the codebreakers at Bletchley Park. In "Colossus" they created the world's first electronic digital computer, which preceded "Eniac" (to which that credit is usually given) by several years.

Coombs went to Bletchley in late 1943, just as Colossus Mark I was being commissioned. The team had been brought in to improve the machines Bletchley Park had devised for deciphering top secret German teleprinter messages. The devices incorporated a few electronic valves but they were mainly electromechanical in nature, and were slow and unreliable. Flowers decided to try to build an alternative codebreaking machine. It would need what was for the time a huge number of valves; Bletchley Park was not convinced that such a machine could ever work. With the backing from the PO Research management, however, Flowers's team went ahead. The prototype machine of Colossus became operational in December 1943 and revolutionised the codebreaking process.

About February 1944 the team was asked to produce some 12 more machines by May - which was quite impractical, but they agreed to try to deliver one by that date. The prototype was modified: the Mark IIs were to operate at a five times higher speed by using shift registers for parallel operation, they were to be more flexible, and to use standardised modular circuits made to non-critical design standards. By a superhuman effort the first Mark II was installed at Bletchley and operational by the early morning of 1 June -just in time for D-Day.

Coombs took over leadership of the whole Colossus project shortly afterwards when Tommy Flowers took up other work. By the end of the war some 10 or 12 Colossi were in operation. Coombs had the task of designing the electronic circuits. These made the Colossi reliable and adaptable machines, with an effective reading speed of 25,000 characters per second. In addition he produced designs for similar smaller devices, "Proteus", "Aquarius" and "Salamander" among others.

The technical significance of the Colossus achievement is best understood when one realises that each machine was similar in size and complexity to several of the post-war digital computers, all of which took several years to design and build. For the rest of the war, the Colossi made it possible for Bletchley Park to obtain a continuous stream of priceless intelligence, long strategic messages by teleprinter, especially to and from the German High Command. Military historians now estimate that the intelligence gained from those and other more operational messages, typically in the "Enigma" cypher, led to the war's ending one or even two years earlier than it might have done, and to countless lives' being saved.

After the war Coombs continued working in the computer field. He collaborated with Alan Turing working on the "Ace" pilot machine. He led the work on "Mosaic", a specialised computer for the Ministry of Supply unique in its use of a large bath of mercury for delayed storage. These machines demonstrated that, given good design, thermionic valves could be successfully used to produce electronic computers. However, the transistor, then only just emerging from the laboratory, was to add another dimension.

After the laying of the first transatlantic telephone cable in 1956 it was realised that the apparent cable capacity could be increased if, in a two-way conversation, an outgoing circuit was assigned only to the person actually speaking at the time. Coombs was given the job of evaluating the possibility of this unique packet switching of analogue signals - Time Assignment Speech Interpolation (Tasi). He contributed a notable paper on the theory of caller queuing which influenced the operation of the US equipment that was eventually installed.

Later he was assigned to the task of developing an optical character recognition (OCR) system for the postal service. Unlike other OCR projects, no control was possible over the fonts or the ink and paper quality used by the customers. He developed the concept of a multi- dimensional "caltrop". The idea was that the many questions that should be asked of a shape to determine its identity should be equally powerful and equally independent of each other. If represented in space by equal- length vectors, they would have a common origin and be separated from each other by the same angle. Such a concept is most easily visualised in three dimensions, the categories being four points in space as the ends of four limbs or vectors (the questions) with a common origin and of equal length (their power), all separated by the same angle (or independence). Pictorially this represents a caltrop, a medieval device for impeding cavalry. When the number of categories in the full set (N) of alphanumeric characters is considered, N vectors are needed, and thus (N-1) dimensions. This idea of a multi- dimensional caltrop is difficult to visualise, but feasible in mathematics.

Allen Coombs's working life started when he was accepted into the Post Office Engineering Department as an open examination entrant to the post of Assistant Engineer in 1936. He was assigned to the Post Office Research Station at Dollis Hill, in north-west London, where he was to spend the rest of his career. Here he was known as "Doc" to distinguish him from a colleague with the same name, and the nickname stuck. He retired in 1973 but then took up a part-time career in teaching.