Introduction to Colossus

Colossus was one of the first electronic digital computers anywhere. About nine were built in Britain during World War 2, to break German coded messages. Breaking the codes helped the Allies understand secret messages between Hitler and his generals. Colossus is a semi-programmable computer that could work on code patterns all day and night. The computer we see here is a reconstruction of Colossus Mark 2 which took 15 years of volunteer time to rebuild.

Input is from holes in a paper tape that the computer can read at 5000 characters per second. The punched tape is run in a continuous loop. Colossus is programmed by switches. Most of the 2500 glass valves make up the processor. It gets very hot. Output is from a series of lights, which give a score. The readable results are also output on to paper via a teleprinter. Colossus has no RAM so it keeps reading the data on the tape over and over again. It keeps reading the paper tape over and over again.

Several power switches need to be turned on the get Colossus started. The valves need to be heated slowly to prevent damage. The pulleys and frame system for the paper tape loop is called the 'bedstead' and the tape needs to be tensioned correctly before turning on. When the tape is running the high tension voltage switch can then be turned on to bring the whole of Colossus to life.

  • Introduction

    Running time: 01:27

  • Component parts

    Running time: 01:32

  • Starting up Colossus

    Running time: 02:50


The coded German message is put onto the paper tape, so that it can be read into the computer for statistical and logical analysis. The holes in the tape are read by an optical reader that shines light through the holes. The tape is read at 5000 characters per second. There is no mouse, keyboard or tablet for input.

If different types of cyphers were being decoded switches could be moved to change the kind of algorithm being used, making the computer semi-programmable. The computer was used for one specific purpose, to break the German cyphers.

Colossus has only one 'embedded' (dedicated) program, like a washing machine today. It can analyse how good the match is for different keys and it can do this in different ways. Lights give a score to show how well Colossus is doing at matching the input message. If it gets a significant score it will output to the teleprinter the settings to break the cypher. It could take 4 to 5 hours to break a single code.

  • Input

    Running time: 00:41

  • Programming

    Running time: 00:33

  • Programming part 2

    Running time: 01:15









User Interface


Impact on Society


Images of the Colossus Computing System

  • Colossus output display

  • Glass valve

  • The 2500 glass valves make up the processor on Colossus

  • The Colossus power supply

  • The readable results are output on to paper via a teleprinter

  • Work began on reconstructing Colossus in 1994 by Tony Sale