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The Harwell Computer
also known as
The 'WITCH' Computer
Wolverhampton Instrument for Teaching Computation from Harwell.
Information gathered together by Kevin Murrell - August 2007
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First envisaged at A.E.R.E. Harwell in 1949, this programable digital computer uses Dekatron tubes for storage and relays for sequence control. It was completed in 1951, handed over to the computing group in 1952, and ran until 1956. After its life at Harwell, the machine was made available, through the Oxford Mathematical Institute, to the college that could produce the best case for its future use. That competition was won by Wolverhampton and Staffordshire College of Technology from nine possible contenders. It ran there from 1957 until 1973 where it was known as WITCH or 'Wolverhampton Instrument for Teaching Computation from Harwell'. In 1973 it was transferred to Birmingham Museum of Science and Technology where it was displayed for many years. It is now in storage.
From Jack Howlett's memories of Computing at Harwell:
"However, as a step on the way Electronics Division offered to design and build for us an automatic calculator in which the switching was done by relays (as had been done in a classic series of machines built by Stibitz in Bell Labs.) but in which the decimal arithmetic and memory were electronic, using about 800 scale-of-ten Dekatron tubes. We got this in 1951 and housed it in the old control tower on the south-east corner of the airfield. It was, of course, slow, not much faster than hand calculation on single operations, but fully automatic, extremely reliable and utterly relentless.
One day E. B. Fossey, an excellent hand-computer (still with what used to be called the Atlas Laboratory), settled down beside the machine with his desk machine and attempted a race. He kept level for about half an hour working flat out, but had to retire, exhausted; the machine just ploughed on.
It took little power and could be left unattended for long periods; I think the record was over one Christmas-New Year holiday when it was all by itself, with miles of input data on punched tape to keep it happy, for at least ten days and was still ticking away when we came back. It was perhaps only just a computer, but granted that, it was certainly one of the earliest in serious and regular use in the country. There is an account in Bowden's Faster Than Thought. The subsequent history of this machine is interesting. We used it up to about 1958 and then, rather than scrap it, offered it as a prize to the educational institute which could give us the best reason for having it. This was the idea of J .M. Hammersley of Oxford and we conducted the operation in collaboration with the Oxford Extra-Mural Department. It went to Wolverhampton Polytechnic who used it for teaching and for real work for at least the next 15 years - an astonishing record; it is now in a museum in Birmingham, and I'm told it can still be made to work."
As opposed to other relay based machines of this period (the Imperial College ICCE computer and the Zuse Z3 computer) this is a stored program computer. It was quite possible to transfer a section of code from paper tape to Dekatron storage and executes orders from the store.
The picture above, from Wolverhampton, shows the machine itself and five paper tape readers in the corner of the room. (Missing from this picture is the paper tape perforator and the printer.) Typically one reader holds the program tape, another holds the data tape, and the three remaining readers can hold loops of tape containing common routines. On starting the machine, two hard coded instructions force the machine to search for block 1 on tape reader 1, and then transfer control to tape reader 1. Subsequent instructions are then read from that tape reader. Blocks of code on other readers can be searched for and then control transferred to that reader. There is no equivalent to a simple 'return from subroutine' instruction, but control would need to be passed back to a specified tape reader that would resume providing instructions. (A work around to overcome this is described in the Programming Manual.)
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