This page contains images of the Harwell Dekatron Computer, known from 1957 onwards as WITCH. This computer was designed and built over the course of two years between 1949 and 1951, at the Atomic Energy Research Establishment in Oxfordshire, UK. For more details have a look at its Wikipedia entry or at the page dedicated to the computer at the Computer Conservation Society website. At the time of its retirement in 1974 it was the oldest computer in the world still in continuous use, and was proclaimed by Guiness World Records to be the “Most durable computer in the World”.

Unlike the computers that we use every day, WITCH computed in base 10 using Dekatrons. These Dekatrons formed the volatile memory (RAM) of the computer. The cathode in a Dekatron which was glowing allowed one simply to read the value (between 0 and 9) being stored in that Dekatron, making it an ideal early computer for teaching. These Dekatrons were arranged into lines of 9, the first representing the parity (plus or minus) of a stored number and the following eight its digits. The numbers that WITCH could store were therefore between -9.9999999 and 9.9999999, even in its heyday this was not the most powerful of machines; its attraction was that it was incredibly reliable. Whilst other computers in the fifties needed continuous nursing, WITCH was left for days at a time, running merrily whilst people went on Christmas holidays and the like.

My grandfather, Cecil Ramsbottom, was in charge of the running and maintenance of WITCH when it moved to the University of Wolverhampton (Wolverhampton and Staffordshire Technical College at the time) in 1957. Here it was renamed to WITCH, Wolverhampton Instrument for Teaching Computing from Harwell. Here’s a photo of him (on the right) with the machine:

WITCH at Wolverhampton

Today, WITCH is being restored to its former glory, after several decades in storage, at The National Museum of Computing at Bletchley Park. Our family (my grandfather’s four children and their children) visited a few days after Christmas 2009 to hand over the key to Tony Frazer, who’s leading the project to restore the computer.

There were formerly a number of keys in existence but all the others appear to have been misplaced and they “thought we were going to have to hotwire it”.

Below are some of the photos that I took on the day. Clicking on a photo takes you to the full resolution (2808x1872) version. All photos are under a Creative Commons Attribution-Noncommercial 2.0 Generic license (i.e. do whatever you want as long as it’s not for commercial purposes and mention me).

Fool that I am I didn’t get a photo of the complete machine, but there are several such pictures on the CCS website.

One of the Dekatrons on test (not part of the machine).

Dekatrons in the memory store.

One memory store. Each row of nine Dekatrons stored one number, each bank stored ten numbers.

Four memory stores. There were nine stores in total (as you can see in the photo with the key in it).

Looking down WITCH.

The accumulators. These did the actual number crunching.

The accumulators from the front.

Behind one of the memory stores (in my grandfather’s writing), the names of the people who designed, built and used the computer at the Atomic Energy Research Establishment.

Inside one of the relay banks.

The six tape readers were how programs and data were input to WITCH.

You can’t do computing unless you’ve got power!

The key fitted and everyone breathed a sigh of relief.

Cable Routing Photos

The cable routing at the back of the computer was incredible.

© Matthew Badger 2014