Pilot ACE was a preliminary version of the full ACE, which had been designed by Alan Turing. After Turing left NPL (in part because he was disillusioned by the lack of progress on building the ACE), James H. Wilkinson took over the project, Harry Huskey helped with the design. The Pilot ACE ran its first program on May 10, 1950 and was demonstrated to the press in December 1950.
Although originally intended as a prototype, it became clear that the machine was a potentially very useful resource, especially given the lack of other computing devices at the time. After some upgrades to make operational use practical, it went into service in late 1951, and saw considerable operational service over the next several years. One reason the ACE was useful is that it was able to perform Floating point arithmetic necessary for scientific calculations. Wilkinson tells the story of how this came to be. The pilot ACE was built without hardware for multiplication or long division, in contrast to other computers at that time. The ACE started out using fixed-point multiplication and division implemented as software. It soon became apparent that fixed point arithmetic was a bad idea because the numbers quickly went out of range. It only took a short time to write new software so that the ACE could do floating-point arithmetic. After that, James H. Wilkinson became an expert and wrote a book on rounding errors in floating-point calculations, which eventually became a big seller.
It had approximately 800 vacuum tubes, and used mercury delay lines for its main memory. The original size of the latter was 128 32-bit words, but that was later expanded to 352 words; a 4096-word drum memory was added in 1954. Its basic clock rate, 1 megahertz, was the fastest of the early British computers. The time to execute instructions was highly dependent of where they were in memory (due to the use of delay line memory). An addition could take anywhere from 64 microseconds to 1024 microseconds.
The Pilot ACE was shut down in May, 1955, and was given to the Science Museum, where it remains today.
See also 
- "History of Computing in the Twentieth Century" edited by Gian-Carlo Rota et al, Academic Press (1980).
- J.H.Wilkinson, "Rounding Errors in Algebraic Processes", reprinted by Dover (1994).
- James H. Wilkinson, Turing's Work at the National Physical Laboratory and the Construction of Pilot ACE, DEUCE and ACE (in Nicholas Metropolis, J. Howlett, Gian-Carlo Rota, (editors), A History of Computing in the Twentieth Century, Academic Press, New York, 1980)
- Martin Campbell-Kelly, Programming the Pilot ACE (in IEEE Annals of the History of Computing, Vol. 3 (No. 2), 1981, pp. 133–162)
- B. Jack Copeland (editor), Alan Turing's Automatic Computing Engine. Oxford University Press, 2005 (ISBN 0-19-856593-3)
- Michael R. Williams, A History of Computing Technology. IEEE Computer Society Press, 1997. ISBN 0-8186-7739-2. Chap. 8.3.4.
- How Alan Turing's Pilot ACE changed computing BBC News, 15 May 2010
Further reading 
- Simon H. Lavington, Early British Computers: The Story of Vintage Computers and The People Who Built Them (Manchester University Press, 1980)
- David M. Yates, Turing's Legacy: A History of Computing at the National Physical Laboratory, 1945–1995 (Science Museum, London, 1997, ISBN 0-901805-94-7)
- Oral history interview with Donald W. Davies, Charles Babbage Institute, University of Minnesota. Davies describes computer projects at the U.K. National Physical Laboratory, from the 1947 design work of Alan Turing to the development of the two ACE computers. Davies discusses a much larger, second ACE, and the decision to contract with English Electric Company to build the DEUCE -- possibly the first commercially produced computer in Great Britain.
- The Pilot ACE at the Science Museum
- How Alan Turing's Pilot ACE changed computing
- The world's first multi-tasking computer