Introduction to WITCH - What it did and why

The Harwell Dekatron computer was used as a giant calculator. It was used for the maths, science and engineering calculations for research into using atomic energy in the early 1950s. The computer was reliable and accurate. It kept working for 25 years until it was retired in 1974. In 1957 it was renamed the Wolverhampton Instrument for Teaching Computing from Harwell (WITCH) when it was moved from the Atomic Energy Research Establishment at Harwell in Oxfordshire to Wolverhampton University to be used for teaching computer science to students.

Starting the WITCH

After turning on the power it is reset and ready to work. Two instructions in the computer tell it to read the punched paper tape used for input.

  • Introduction

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  • Starting the WITCH

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  • Component Parts

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The program and data are punched onto paper tape ahead of time and input into the computer through tape readers. The arithmetic logic unit processes the instructions and does the calculations, like a modern processor. It then stores the data in Random Access Memory (RAM). Output is on paper tape or on a printer. The computer is open with all the parts showing: there are no panels or metal cabinets. It is very big, weighing 2.5 tons (2540kg).

Input and programming

Programmers only had 16 instructions available to perform all the required calculations. The instructions would be strung together, converted into numbers and punched onto tape to be fed into the computer. Several tape readers could be used in conjunction with each other to allow a separation of data, program, and continuous loops with repeating sub-routines. The computer could take days to perform some calculations.

Input and operating system

This computer has no operating system and it only does numerical calculations. The input reader can read 3 or 4 characters per second from the paper tape.

  • Input 1

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  • Input 2 and programming

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  • Input 3 and operating system

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The output from the computer is on a Friden Teletype printer, like an electric typewriter. It prints the results on paper in numbers and text but has no graphics. It could also punch the results on paper tape, which could then be used again in the future as input for another program.

  • Output

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Storage - RAM and counters

The storage devices used for RAM are glass tubes. These are called Dekatrons and are actually decimal counters. They are cooler than the glass valves seen in the Colossus as they do not have heater filaments. The counter has electrodes (short metal rods) arranged in a circle, one electrode for each of the numbers 0 through 9. The counter could be stopped at any one of 10 positions, from 0 to 9, to store that number. The counters were arranged in 90 rows and can be used to store positive numbers, negative numbers and decimal places. The RAM has 90 stores for numbers or instructions. The computer's memory works in base 10 (decimal) and not in binary.

  • Storage 1 RAM and counters

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Memory - RAM expansion

The RAM was expanded from 80 stores to 90 by adding 10 extra rows of counters to give it more capacity.

  • Storage 2 RAM expansion

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Processing - Relays and ALU

The processor at the heart of the computer uses small relays, which can be switched on and off to process the instructions. Removing the cans reveal more relays, the key element of the arithmetic logic unit (ALU).


Although the Dekatron (later known as the WITCH computer) could take 17 days to process 100,000 calculations (i.e. it was slow) its key strength was its accuracy and reliability and this was critical to its use in the design of atomic power stations.

  • Processing 1 Relays and ALU

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  • Processing 2 Reliability

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User Interface


Impact on Society - World's oldest original computer

The original Dekatron computer was restored, not recreated, at The National Museum of Computing between 2009 and 2012, making it the oldest original functioning electronic stored program computer in the world.

Why the computer was needed

Scientists, engineers and mathematicians building the first atomic power stations in the UK in the early 1950s could have used mechanical hand calculators in the planning process. They realised the results could be inaccurate because of human error. The Dekatron computer allowed them to automate the process and produce accurate calculations. But they also relied on good quality data to begin with to start the calculations. Results could only be as good as the data put in. The 1950's phrase "Garbage in - garbage out" (GIGO) is still true today in computer science.

  • World's oldest original computer

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  • Why the computer was needed

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Images of the WITCH computer system

  • Front of the WITCH computer

  • Back of the WITCH computer

  • The handheld calculator the WITCH computer replaced

  • WITCH Computer back panel wiring

  • WITCH Computer output - a teleprinter

  • WITCH Computer power supply

  • WITCH Computer system wiring

  • WITCH Display and control panel

  • WITCH Punch paper taper reader

  • WITCH Relay switches

  • WITCH Relays and Counters

  • WITCH Relays and counters

  • WITCH Relay covers to offer some sound proofing and protection