Pilot ACE was a preliminary version of the full ACE, which had been designed by Alan Turing. After Turing left NPL, James H. Wilkinson took over the project. Donald Davies, Harry Huskey and Mike Woodger were involved with the design. The Pilot ACE ran its first program on 10 May 1950, and was demonstrated to the press in November 1950. Although originally intended as a prototype, it became clear that the machine was a potentially 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 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 sold well. Pilot ACE used approximately 800 vacuum tubes. Its main memory consisted of mercurydelay lines with an original capacity of 128 32-bit words, which 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 on where they were in memory. An addition could take anywhere from 64 microseconds to 1024 microseconds. The machine was so successful that a commercial version of it, named the DEUCE, was constructed and sold by the English Electric Company. Pilot ACE was shut down in May 1955, and was given to the Science Museum, where it remains today.
