When volunteers began rebuilding one of the world's first fully operational general-purpose computers they had to painstakingly piece together the inner workings of the 65-year-old machine.
The original EDSAC - the Electronic Delay Storage Automatic Calculator - was built immediately after World War II at the University of Cambridge, where it aided research into areas including genetics, meteorology and X-ray crystallography. The machine's design was later used to create LEO, the world's first business computer.
With the historic EDSAC scrapped decades ago, the 20 volunteers recreating the computer had to scrape together information about the machine. For guidance the team consulted documents held by individuals and in libraries at the University of Cambridge, and examined the one surviving chassis used in the EDSAC.
Now the team working out of Bletchley Park in England, home to the famous World War II codebreakers that included father of computing Alan Turing, have had confirmation the rebuild is on the right track after stumbling across diagrams of the original machine.
The 19 detailed circuit diagrams, which came to light more than 60 years after they were drawn up, were delivered to the team by chance.
For decades the drawings sat in the home of John Loker, who worked as an engineer in the maths lab at the University of Cambridge and came across the diagrams in 1959, just after EDSAC had been decommissioned.
"In a corridor there was a lot of stuff piled up ready to be thrown away, but amongst it I spotted a roll of circuit diagrams for EDSAC. I'm a collector, so I couldn't resist the urge to rescue them," he said.
"It wasn't until I visited TNMOC [The National Museum of Computing at Bletchley Park] recently and learned about the EDSAC Project that I remembered I had the diagrams at home, so I retrieved them and gave them to the project."
Many of the diagrams, which date from between 1949 and 1953, were drawn after EDSAC had been constructed and are thought to have been an aid in refining the original machine and in designing its successor. They are believed to have been part of a much larger set of at least 150 drawings and are in remarkably good condition.
Andrew Herbert, leader of the EDSAC Project rebuild, said: "Thankfully, the documents confirm that the reconstruction we are building is basically correct, but they are giving us some fascinating insights about how EDSAC was built and show that we are very much in tune with the original engineers: both teams have been exercised by the same concerns.
"Importantly, the drawings clearly show that the aim of EDSAC's designer, Sir Maurice Wilkes, was to produce a working machine quickly rather than to create a more refined machine that would take longer to build. The refinements could come later -- and many did as the sequence of diagrams over the five-year period shows."
Above, from left, is EDSAC technical leader Chris Burton, with John Loker, the engineer who saved the diagrams, and leader of the EDSAC Project rebuild Andrew Herbert.
The National Museum of Computing