Recreating EDSAC


Information about EDSAC has been located in the libraries of the Cambridge University Library and the Cambridge University Computer Laboratory library, as well as in the possession of individuals.

The original EDSAC was scrapped in the late 1950s and only one ‘chassis’ remains out of a total of 120+ in the original. And no definitive record exists of the final ‘as built’ design, either in terms of the circuitry or the physical layout of the machine. The archive material that we have found so far spans the whole timescale of the original EDSAC project, from Wilkes’ first thoughts to final decommissioning.

EDSAC was repeatedly modified over its almost 10 year lifetime; our aim is to create a replica of EDSAC as it was on 6th May 1949, when it ran its first ever program successfully.

The rebuild project has required complex detective work (and will require still more) in order to come up with both the logical and physical designs that we can be reasonably confident reflect EDSAC as it was in May 1949.

Where possible, we will build the replica using authentic materials and components - with one major exception. EDSAC’s memory was constructed from ‘delay tubes’ filled with mercury; these exploited the relatively slow speed of a sound wave in the liquid metal. Expense and operating concerns will prohibit an authentic reconstruction of these tubes. An alternative is to use nickel magnetostrictive delay lines, a technology employed in computers that followed EDSAC, and which follow similar physical principles and method of operation as acoustic delay lines.

In the 63 years since EDSAC became operational, technology has changed out of all recognition. Because of this, original components (or even their later replacements) have become very hard to find. In the “You Can Help” section, we will post details of the less common components that we still need to obtain if we are to build a close replica of the original 1949 machine.

Location and Construction

The EDSAC replica will be built and housed at The National Museum of Computing (TNMOC) located on Bletchley Park, the UK’s WWII code-breaking centre and undoubtedly the spiritual home of computing in the UK.

It will be built almost exclusively by volunteers, many of them in their 60s or 70s, and many of whom have experience of old valve-based technology. Some of these older volunteers will train much younger volunteers, so that the EDSAC replica can be maintained in an operational state for many decades to come.

As is TNMOC’s policy, it will not be a mere static exhibit. During the latter stages of construction, visitors to the museum will be able to see volunteers actually building it. And once completed, it will be powered up and running as often as is practical. Particular efforts will be made to involve and inspire schoolchildren – the next generation of technologists and engineers!

The first meeting of the EDSAC Project volunteers