British Tabulating Machine Company

The British Tabulating Machine Company (BTM) designed and built the Bombes. They manufactured and sold Hollerith unit record and other data-processing equipment. The company was formed in 1902 as The Tabulator Limited, after Robert Porter obtained the rights to sell Herman Hollerith‘s patented machines from the US Tabulating Machine Company (later to become International Business Machines or IBM). By 1909, the company had been renamed the British Tabulating Machine Company Limited. In 1920, the company moved from London to Letchworth in Hertfordshire, hence the Bombes they built were known as ‘Letchworth Bombes’. It was also at this point that they started manufacturing their own machines, rather than simply reselling Hollerith equipment.

In 1959 BTM merged with former rival Powers-Samas to become International Computers and Tabulators Limited (ICT), which in turn later became part of ICL (International Computers Limited). Bombes of several types were produced from 1940 to 1945 at Letchworth – 33 miles from Bletchley Park. Final assembly took place in the main BTM factory in Icknield Way.

The Bombes were built under the leadership of Harold ‘Doc’ Keen, so-called because he usually carried a doctor’s Gladstone bag. This factory later became known as 1/1 but was demolished to make way for a housing estate. Sub-assemblies for the Bombes, including the frames, were produced at other Letchworth factories – drums at Spirella and other parts in the basement of the Ascot, Government Training Centre in Pixmore Avenue. Interestingly, the Spirella building, first built to house the manufacture of ladies corsets, was formally opened by Prince Charles following its total restoration at the end of January 1999. This work was put in place by the Letchworth Garden City Heritage Foundation at a cost of £10m.

At Letchworth the Bombes were known as 6/6502 or CANTAB machines. By early 1945 the Bombe programme was winding down rapidly, and to mark the occasion, a celebration dinner was held at Letchworth, and those who had worked on the project were given diplomas. A CANTAB logo was produced and appeared on these diplomas and on the menu card produced for the celebration dinner.


This logo was adopted by the rebuild project. Bletchley Park was also known as Station X, but at BTM’s London head office it was referred to as Bureaux B. The first Bombes took six months to construct, but later, when built in batches of six, one came off the assembly line each week.  Security had to be as good at Letchworth as it was at Bletchley Park, and although literally hundreds of people were involved in the Bombe production, the secret was kept. In fact, security was deliberately kept very low key.

New Bombes were collected from the main factory loading ramp, visible from the main road, by a single soldier with an army lorry. He then set off to Bletchley Park or one of the Bombe out-stations at Eastcote, Stanmore, Wavendon, Adstock and Gayhurst without any escort! The basics behind BTM equipment were recording numerical and alphabetical information through square-shaped punched holes in Hollerith cards. These comprised 80 vertical columns, each able to hold an alphabetical or numerical character whose value were represented by the vertical position of a hole or holes. Fields of card columns could hold data while other columns held the category or type of information.

The holes, in repeated mechanical cycles, were sensed by small metal brushes which allowed timed electrical clock pulses of current to pass through electro-mechanisms which were activated via the punched cards. The pulses were distributed within or between mechanisms by commutators with circular rows of contact segments and rotating sensing brushes. Electro-magnetic relays were used as a means of control, or as switches. It was this technology which was adapted and used by BTM to construct the Bombes. Flexibility in design of providing pluggable connections where possible allowed for future changes and development of the Bombe, such as the Diagonal Board. If new Reflector wiring had ever been introduced by the Germans, then the BTM plugboards would have enabled simple alterations by re-plugging instead of rewiring each Bombe.