The Harwell computer, later known as the Wolverhampton Instrument for Teaching Computing from Harwell (WITCH), or the Harwell Dekatron Computer, is an early(1949) British dekatron and relay based computer. It is currently on display at The National Museum of Computing at Bletchley Park, where it is described as "the oldest original functioning electronic stored program computer in the world".
The computer was built and used at the Atomic Energy Research Establishment in Harwell, Oxfordshire. Construction started in 1949, and the machine became operational in April 1951. It was handed over to the computing group in May 1952 and remained in use until 1957.
It used dekatrons for volatile memory, similar to RAM in a modern computer, and paper tape for input and program storage. Output was to either a Friden teleprinter or to a paper tape punch. The machine was decimal and initially had twenty 8-digit dekatron registers for internal storage, which was increased to 40 which appeared to be enough for nearly all calculations. It was assembled from components more commonly found in a British telephone exchange. Although it could on occasions act as a true stored-program computer, that was not its normal mode of operation. It had a multiplication time of between 5 and 10 seconds, very slow for an electronic computer.
In 1957, at the end of its life at Harwell, the Oxford Mathematical Institute ran a competition to award it to the college that could produce the best case for its future use. The competition was the idea of John Hammersley, who had worked at AERE previously. The competition was won by the Wolverhampton and Staffordshire Technical College (which later became Wolverhampton University) where it was used to teach computing until 1973. The computer was renamed as the WITCH, the Wolverhampton Instrument for Teaching Computing from Harwell.
The WITCH was donated to the Museum of Science and Industry, Birmingham in 1973. After the museum closed in 1997, the computer was disassembled and stored at the Birmingham City Council Museums Collection Centre.
From September 2009, the machine was loaned to The National Museum of Computing at Bletchley Park, where began to be restored to working order as a Computer Conservation Society project.
- An Electronic Digital Computer - Electronic Engineering - August 1951 - (Original)
- Partial List of Parts Used with datasheets
- General Count of Components used
- Harwell Computer Wikipedia Entry
- Computer Conservation Society Harwell Computer Page
- BCS Harwell Computer Conservation Project Page
- The Nation Museum of Computing Large Systems Page
- The Nation Museum of Computing Harwell Computer Restoration Updates Page
- Photos of Harwell Computer from Cecil Ramsbottom's Grandson Matthew Badger
- National Valve Museum
The Harwell WITCH serves as a unique and important step in computing history. With this in mind, the ability to study the engineering, math, logic, and computing pricinples used to create the WITCH should be part of educational experiences of todays students. Unfortunately, the WITCH does use extremely high voltages, is very large, and uses many components that are no longer available. In order to make the WITCH available to students, the project aims to replace logical component blocks of the original WITCH design with commonly available 7400 series Transitor-Transitor Logic(TTL) components. This allows for the creation of low cost and easy to assemble educational replicas of major portions of the WITCH. These educational replicas can be safely used in the class room to study many of the unique aspects of the WITCH that are some of the core pricinples of computing.
- Low cost - components and pcb needed for each replica block should not cost more than $75USD.
- Easy to Assemble - use of through hole and two layer pcbs for each replica block which can be ordered from multiple distributors
- Logical Block Replication - wave forms and timings should replicate the original design as closely as possible
- Stand Alone Operations - each replica block should operate as a stand alone education tool to demostrate at least one aspect of the design.
- Schematic of Simulated Dekatron
- Schematic of Transfer Unit
- Schematic of Carry Unit
- Schematic of Send/Receive Configuration