On 21 June 1948 at the University of Manchester, a small experimental machine nicknamed the “Baby” ran a program stored in its own electronic memory and computed the correct answer. Officially the Small-Scale Experimental Machine, it was built by Frederic Williams, Tom Kilburn, and Geoff Tootill primarily to test a new kind of memory - the Williams-Kilburn tube, which stored bits as charged spots on the face of a cathode-ray tube and could hold up to 1,024 bits. The Baby had just 32 words of memory and a tiny instruction set.
What made the moment historic was not the machine’s size but what it proved. Earlier electronic computers such as ENIAC were programmed by rewiring or by setting switches; their instructions lived outside the machine. The Baby instead held both its program and its data in the same read-write electronic store, so that a new task meant loading new numbers, not rebuilding the machine. This is the stored-program idea, set out a few years earlier in von Neumann’s EDVAC report, and the Baby was the first machine to demonstrate it working. The first program, which ran for about 52 minutes, searched for the highest proper factor of a number.
The Baby was a proof of concept that grew quickly into the full Manchester Mark 1, and the Williams-Kilburn tube became an early commercial memory technology. The occasion is often called the birth of software: from this point on, a computer was a general machine whose behavior was defined by a program you could change at will.