CHAPTER 8

Turing and the origins

of digital computers

brian randell

I

n this chapter I describe my initial attempts at investigating, during the early 1970s, what

Alan Turing did during the Second World War. My investigations grew out of a study of

the work of Charles Babbage’s earliest successors—in particular, the Irish pioneer Percy

Ludgate—a study that led me to plan an overall historical account of the origins of the digital

computer. The investigation resulted in my learning about a highly secret programmable

electronic computer developed in Britain during the Second World War. I revealed that this

computer was named Colossus, and had been built in 1943 for Bletchley Park, the UK gov-

ernment’s wartime codebreaking establishment. However, my attempt to get the details of

the machine declassified were unsuccessful, and I came to the conclusion that it might be a

long time before anything more would become public about Bletchley Park and Colossus.^1

Introduction

Around 1970, while I was seeking information about the work of Charles Babbage and Ada

Lovelace to use in my inaugural lecture at Newcastle University, I stumbled across the work of

Percy Ludgate. In a paper he wrote about Babbage’s ‘automatic calculating engines’,^2 Ludgate

mentioned that he had also worked on the design of an Analytical Engine, indicating that he

had described this in an earlier paper in the Proceedings of the Royal Dublin Society.^3 From a

copy of that paper I learned that an apparently completely forgotten Irish inventor had taken up

and developed Babbage’s ideas for what would now be called a program-controlled mechani-

cal computer. Previously I had subscribed to the general belief that over a century had passed

before anyone had followed up Babbage’s pioneering 1837 work on Analytical Engines.^4 This

discovery led me to undertake an intensive investigation of Ludgate, the results of which I

published in the Computer Journal.^5