The National Museum of Computing is a UK-based museum that is dedicated to collecting and restoring historic computer systems, and is home to the world's largest collection of working historic computers. The museum is located on Bletchley Park in Milton Keynes, Buckinghamshire. It opened in 2007 in Block H – the first purpose-built computer centre in the world, having housed six of the ten Colossus computers that were in use at the end of World War II.
As well as first-generation computers including the original Harwell Dekatron computerthe world's oldest working digital computer and mainframes of the 1950s, 60s and 70s, the Museum houses an extensive collection of personal computers and a classroom full of BBC Micros. It is available for corporate, group, school, and individual visitors.
Although located on the Bletchley Park campus, The National Museum of Computing is an entirely separate registered charity with its own admission fee. It receives no public funding and relies on ticket sales and the generosity of donors and supporters. The museum has its own cafe and gift shop. In 2024 it was awarded full accreditation as a Nationally-styled museum by Arts Council England.
Origins The Bletchley Park estate was threatened with demolition and redevelopment in the late 1980s and early 1990s. It was saved in 1993 thanks to the efforts of the Bletchley Park Trust (BPT), which had been established in the previous year. One leading member – and secretary to the Trust – was a scientist with electronics and computer engineering skills named Tony Sale (1931–2011). He had worked for MI5 and later at the Science Museum alongside Doron Swade on a series of projects to restore some of the Science Museum's computer holdings to working order. Sale became the first curator of the Bletchley Park Museum, which in its early days was supplemented by more than a score of collections varying from WWII memorabilia to model railways. One of these centred around the history of computing and contained many historic computers, several of which were maintained in working order by enthusiastic volunteers, many of whom were members of the Computer Conservation Society.
In 1993, Tony Sale and a group of volunteers started to rebuild a Colossus (a 'rebuild' as it contains parts from an original) in Block H. By June 1996 they had a prototype machine working, which was formally switched on by the Duke of Kent in the presence of Tommy Flowers who built the wartime Colossi. When in 2004 Block H came under threat of demolition, Sale and colleagues were able to protect it by obtaining Grade II listed building status for it. This led to the detachment of the computing collection from the Bletchley Park Trust museum, and the establishment in 2005 of the Codes and Ciphers Heritage Trust, which became the National Museum of Computing in 2007. Between 1994 and 2007 a group of volunteers led by John Harper built a working replica of a Turing-Welchman Bombe (used to help decipher Enigma–coded messages) in the BPT museum. This was relocated to Block H in 2018.
Exhibits The exhibits on display in the museum represent only a fraction of the collection, but are chosen to tell the story of computing developments in Britain. There are a number of galleries which can be visited in a broadly chronological sequence, starting with the working replicas of WWII machines that were developed and used by Bletchley Park codebreakers.
Bombe Gallery
This gallery tells the story of Cryptanalysis of the Enigma. Enigma machines were used by the Germans before and during WWII for sending secret messages. Alan Turing further developed, and Gordon Welchman enhanced, an idea implemented by Polish codebreakers, of a machine to assist in decrypting Enigma messages. This gallery houses a fully working replica of a Bombe machine, a working replica Enigma and various related artefacts.
The replica Bombe was built by a team led by John Harper following the release in 1995 to the Bletchley Park Trust of some 2,000 BTM documents and drawings relating to the Bombes that they had built during the war. The replica is owned and managed by the Turing-Welchman Bombe Rebuild Trust, which provides and trains the volunteers who run and demonstrate the machine to visitors on a regular basis.
Tunny and Colossus Galleries
Separate from the Enigma story is the less well-known endeavour of the diagnosing and deciphering of messages produced by the more secure 12-rotor Lorenz SZ teleprinter cipher attachments, which is told in these two galleries. The Tunny galley exhibits one of the very few Lorenz SZ42 machines still in existence — something that nobody in the Allied side saw until after Nazi Field Marshal Albert Kesselring surrendered in May 1945, shortly before VE-day.
'Tunny' was the name given to the messages, to the unseen cipher machine and to the British-built emulator of it. The gallery contains a reproduction of part of the original Lorenz listening station at Knockholt in Kent, with its multiple RCA AR-88 radio receivers, pen recorders (undulators) and the sort of paper tape and teleprinter equipment that was used to record the messages and transmit them to Bletchley Park. Also on display is a working replica of a British Tunny machine that exactly emulated the Lorenz machine and a working replica of the Heath Robinson machine, the forerunner of Colossus.
The Colossus gallery houses the fully working rebuild of a Colossus Mark 2. During his work to save Bletchley Park, Tony Sale recognised the pioneering nature of the ten Colossus machines that had been designed and built during WWII to assist in breaking messages enciphered by the Lorenz machines. He and his team spent 14 years from 1993 in building this machine.
As its name implies, Colossus is a large machine which weighed five tonnes. It was designed and built for the single purpose of assisting with deciphering messages enciphered with the 'Tunny' machines. At the heart of the machine is a set of five counters that, for each transit of the looped paper tape containing the message, count the number of times that defined Boolean expressions deliver a specified value. These Boolean expressions were programmed by operating a panel of some 190 switches. The looped message tape would be run continuously, being read at 5000 characters per second. A cryptanalyst would specify different Boolean expressions for evaluation according to the results produced. With its 2,420 valves (vacuum tubes) and its programmability, the machine on display is a recreation of the world’s first large-scale, electronic programmable digital computer, albeit a special purpose, not a general purpose machine.
There are a number of related artefacts in this gallery.
First Generation Gallery This gallery continues the story of valve or tube-based computers and exhibits three large machines and many other related items. The three unique large machines are:
EDSAC – a replica nearing completion. The machine is owned and managed by the EDSAC Replica Project, which provides and trains the volunteers who are building it and, eventually, will run and demonstrate it.
The original EDSAC (Electronic Delay Storage Automatic Calculator) was constructed by the Cambridge University Mathematical Laboratory under Sir Maurice Wilkes. Wilkes had read John von Neumann's seminal paper First Draft of a Report on the EDVAC and attended the Moore School Lectures in Summer 1946. Starting in 1947, he designed and built the machine to serve a user community from many different departments of the university. The EDSAC ran its first programs on 6 May 1949 and is therefore claimed to be the first practical general-purpose stored-program electronic computer.
The vast increase in computing power that EDSAC and its successor EDSAC 2 supplied, contributed to the winning of three Nobel Prizes – John Kendrew and Max Perutz (Chemistry, 1962) for the discovery of the structure of myoglobin, Andrew Huxley (Medicine, 1963) for quantitative analysis of excitation and conduction in nerves and Martin Ryle (Physics, 1974) for the development of aperture synthesis in radio astronomy. All acknowledged EDSAC in their Nobel Prize speeches.
Harwell Dekatron (aka the WITCH) from 1951. The world’s oldest original working digital computer. Planned in 1949 to automate the tedious work performed by teams of bright young graduates using mechanical calculators. Simplicity, reliability and unattended operation were the design priorities. Speed was a lesser consideration. This pioneering computer first ran in 1951 and by 1952 was using 828 Dekatron tubes for program and data storage, relays for sequence control and valve-based electronics for calculations. When it was pitched against a human mathematician to check the machine’s operation, the human kept up with it for 30 minutes, but then retired exhausted as the machine carried on remorselessly. It once ran unattended for ten days over a Christmas/New Year holiday period.
It was used at AEA Harwell until 1957, when a competition was held for colleges to see who could make best use of it. The competition was won by Wolverhampton and Staffordshire Technical College (later becoming Wolverhampton University) and they gave it its second name of the WITCH (Wolverhampton Instrument for Teaching Computation from Harwell). The WITCH was used in computer education for over 15 years until 1973.
For a while the machine was on display at Birmingham Museum of Science and Industry, following which it was disassembled and put into storage at Birmingham City Council Museums’ Collection Centre. In 2009 the machine was spotted by TNMOC volunteers who recognised what it was, and made a plan to bring it to TNMOC for restoration in full public view. This was completed in 2012.
Hollerith Electronic Computer (HEC 1) from 1951 ( temporarily away from the museum). A relatively recent arrival, nearly as old as the WITCH, but not working.
HEC was based on an original design by Andrew Booth of Birkbeck College, London University. His design for a small scientific computer was adapted by Raymond Bird at BTM in 1951 to become a prototype commercial computer designed to work with the punched card equipment familiar to BTM's customers. The first production machine was delivered early in 1955, and the subsequent 1200 series of computers were highly successful.
The HEC and EDSAC had a huge bearing on the development of computing in the UK. In particular, EDSAC led directly to LEO, the world’s first computer to run a business. The WITCH had less influence on the development of computers but in the 1960s and 1970s, and again now, is a great educational tool.
Among the smaller items are several from the productive partnership between the Victoria University of Manchester and the electrical engineering company Ferranti. These include: A Williams–Kilburn tube the first random-access memory device that gave rise to the world's first stored program computer, the Manchester Baby. Parts from both a Manchester Mark 1 computer and an Atlas supercomputer which was a world first with virtual memory and which, for a time was the world's most powerful computer.
Large Systems Gallery
This gallery contains many machines of the 1960s, -70s and -80s and one or two from the 1990s. Many machines are in working order and include:
Marconi TAC. Designed in 1959/60 this was one of the first transistorised computers made in the UK. The machine on show was one of a pair of machines that were used as a monitoring system for the Wylfa nuclear power station on Anglesey. They ran live 24/7 from 1968 to 2004.
Elliott 803. one of the first UK all transistor machines from around 1962. Despite having spent about 15 years in a farm barn, it was restored to working order and it is now demonstrated playing music, drawing graphs on a Calcomp plotter and solving mathematical problems.