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The Manchester Baby, also known as the Small-Scale Experimental Machine (SSEM), was the world's first electronic stored-program computer. It was built at the University of Manchester, England, by Frederic C. Williams, Tom Kilburn, and Geoff Tootill, and ran its first program on 21 June 1948, seventy-one years ago.

The first design for a program-controlled computer was Charles Babbage's Analytical Engine in the 1830s. A century later, in 1936, mathematician Alan Turing published his description of what became known as a Turing machine, a theoretical concept intended to explore the limits of mechanical computation. Turing was not imagining a physical machine, but a person he called a "computer", who acted according to the instructions provided by a tape on which symbols could be read and written sequentially as the tape moved under a tape head. Turing proved that if an algorithm can be written to solve a mathematical problem, then a Turing machine can execute that algorithm.

The ENIAC (1946) was the first machine that was both electronic and general purpose. It was Turing complete, with conditional branching, and programmable to solve a wide range of problems, but its program was held in the state of switches in patchcords, not in memory, and it could take several days to reprogram. Researchers such as Turing and Zuse investigated the idea of using the computer's memory to hold the program as well as the data it was working on, and it was mathematician John von Neumann who wrote a widely distributed paper describing that computer architecture, still used in almost all computers.

Turing joined the National Physical Laboratory (NPL) in October 1945, by which time scientists within the Ministry of Supply had concluded that Britain needed a National Mathematical Laboratory to co-ordinate machine-aided computation. A Mathematics Division was set up at the NPL, and on 19 February 1946 Alan Turing presented a paper outlining his design for an electronic stored-program computer to be known as the Automatic Computing Engine (ACE). This was one of several projects set up in the years following the Second World War with the aim of constructing a stored-program computer. At about the same time, EDVAC was under development at the University of Pennsylvania's Moore School of Electrical Engineering, and the University of Cambridge Mathematical Laboratory was working on EDSAC.

The government department responsible for the NPL decided that, of all the work being carried out by the TRE on its behalf, ACE was to be given the top priority. NPL's decision led to a visit by the superintendent of the TRE's Physics Division on 22 November 1946, accompanied by Frederic C. Williams and A. M. Uttley, also from the TRE. Williams led a TRE development group working on CRT stores for radar applications, as an alternative to delay lines. Williams was not available to work on the ACE because he had already accepted a professorship at the University of Manchester, and most of his circuit technicians were in the process of being transferred to the Department of Atomic Energy. The TRE agreed to second a small number of technicians to work under Williams' direction at the university, and to support another small group working with Uttley at the TRE.

For use in a binary digital computer, the tube had to be capable of storing either one of two states at each of its memory locations, corresponding to the binary digits (bits) 0 and 1. It exploited the positive or negative electric charge generated by displaying either a dash or a dot at any position on the CRT screen, a phenomenon known as secondary emission. A dash generated a positive charge, and a dot a negative charge, either of which could be picked up by a detector plate in front of the screen; a negative charge represented 0, and a positive charge 1. The charge dissipated in about 0.2 seconds, but it could be automatically refreshed from the data picked up by the detector.

Although Newman played no engineering role in the development of the Baby, or any of the subsequent Manchester computers, he was generally supportive and enthusiastic about the project, and arranged for the acquisition of war-surplus supplies for its construction, including GPO metal racks and "the material of two complete Colossi" from Bletchley.

Each 32-bit word of RAM could contain either a program instruction or data. In a program instruction, bits 012 represented the memory address of the operand to be used, and bits 1315 specified the operation to be executed, such as storing a number in memory; the remaining 16 bits were unused. The Baby's 0-operand instruction set|single operand architecture meant that the second operand of any operation was implicit: the accumulator or the program counter (instruction address); program instructions specified only the address of the data in memory.

Geoff Tootill wrote an amended version of the program the following month, and in mid-July Alan Turing who had been appointed as a reader in the mathematics department at Manchester University in September 1948 submitted the third program, to carry out long division. Turing had by then been appointed to the nominal post of Deputy Director of the Computing Machine Laboratory at the university, although the laboratory did not become a physical reality until 1951.

In 1998, a working replica of the Baby, now on display at the Museum of Science and Industry in Manchester, was built to celebrate the 50th anniversary of the running of its first program. Demonstrations of the machine in operation are held regularly at the museum. In 2008, an original panoramic photograph of the entire machine was discovered at the University of Manchester. The photograph, taken on 15 December 1948 by a research student, Alec Robinson, had been reproduced in The Illustrated London News in June 1949.