Charles Petzold

Reading Copeland’s “Colossus”

June 22, 2008
Roscoe, N.Y.

The Collected Works of Alan Turing are published by Elsevier, an Amsterdam-based publishing house. The four volumes reproduce Turing’s 30-odd papers and articles, as well as other material, mostly in facsimile with valuable introductory essays. The only drawback is the price: the individual volumes range from $165 to $190.

An excellent low-cost alternative is the The Essential Turing (Oxford University Press, 2004), edited by B. Jack Copeland, Professor of Philosophy at the University of Canterbury in New Zealand and Director of the Turing Archive for the History of Computing, The Essential Turing includes all of Turing's major papers on mathematical logic (including “On Computable Numbers, with an Application to the Entscheidungsproblem” that is the subject of my recent book) and artificial intelligence, along with essays written by Jack Copeland and others. In particular, I found the essay “Corrections to Turing’s Universal Computing Machine” by Donald W. Davies to be indispensable in analyzing Turing's paper for my book.

Another Turing-related book edited by Jack Copeland was published by Oxford University Press in 2005. Alan Turing’s Automatic Computing Engine: The Master Codebreaker's Struggle to Build the Modern Computer is all about the ACE computer project that Turing worked on after World War II. Prior to the announcement of this book, the title of my book-in-progress was Alan Turing’s Computing Machine, but Copeland's title was so similar I felt obliged to change mine!

Another somewhat-Turing-related book edited by Jack Copeland, Colossus: The Secrets of Bletchley Park's Codebreaking Computers was published by Oxford University Press two years ago, but somehow I missed this one. I spent much of today reading big chunks of it, and found it very illuminating. The book is a collection fo 26 essays, 12 technical appendices, and 16 pages of photographs threaded into a coherent narrative concerning the purpose and design of the 1943 Colossus computer. A good chunk of the book is written by veterans of Bletchley Park.

As is well known now, Alan Turing and about 10,000 other people spent the years during World War II at Bletchley Park, a large estate and mansion purchased by the British government to consolidate code-breaking activity. Bletchley is very conveniently located: From the Bletchley train station you can go south to London, east to Cambridge, or west to Oxford. Turing worked mostly on cracking the German Enigma encryption machine and designed a parallel simulation device called the Turing Bombe to help work out possible combinations of Enigma settings.

One of the primary motivations of Colossus is to correct a common misconception that Turing also designed the Colossus computer at Bletchley . He did not. That honor belongs to Thomas H. Flowers who contributed two essays included in this book before his death in 1998, and who appears in the prominent frontispiece photograph.

Tommy Flowers worked at the Post Office Research Branch at Dollis Hill since 1930, and from his work there in telephone switching networks, he pioneered the use of vacuum tubes (called "valves" in England) to replace slower electromechanical relays. The common wisdom at the time was that tubes were too prone to burnout to be used in large quantities in critical applications. Flowers knew different, and he applied that knowledge in building the Colossus, which is now regarded as the first functional electronic (that is, using tubes rather than relays) digital computer. "Flowers was (as he himself remarked) possibly the only person in Britain who realised that valves could be used reliably on a large scale for high-speed digital computing." (73)

The Colossus was not always awarded its proper place in computing history due to the secrecy that surrounded its implementation. Although ten Colossi were built, all but two were dismantled and destroyed after the end of the war. For some 30 years it remained a state secret that an electronic computer had actually been built at Bletchley Park!

The Colossus was built specifically to help break coded messages transmitted by the Lorenz Schlüsselzusatz 40 or SZ40. The code-breakers at Bletchley Park gave the different German code machines names of fish, and this one was known as Tunny (tuna). Unlike the Enigma (which required that coded messages be manually transcribed from flashing lights and sent by Morse Code), Tunny directly transmitted five-bit Baudot-Murray code by radio, and printed unencrypted messages on a teletypewriter. A series of 12 wheels generated a five-bit key that changed from character to character, and was combined with the text using a bitwise exclusive-or operation. Applying the same key again decrypted the text.

Information about the Tunny was only declassified by the British government in 2000; a 500-page Report on Tunny is the result. The release of that document freed up Bletchley Park veterans to talk about the Tunny, which made Colossus the book possible.

Colossus contains a wealth of information about Tunny and Bletchley Park, but some bonuses as well, including more biographical information about Maxwell Herman Alexander Newman than I've ever seen. Max Newman taught the Foundations of Mathematics course at Cambridge that inspired Turing to write his "On Computable Numbers" paper, and he helped guide the paper to publication after it became obvious that Turing had been scooped by Alonzo Church. Newman was also at Bletchley Park and then at the University of Manchester where he ran the Mark I computer project. Colossus includes a short biography of Max Newman by his son William. (I was very surprised to learn that William Newman coauthored one of the classic books on computer graphics that I've owned for many years, Principles of Interactive Computer Graphics (McGraw-Hill, 1973), known popularly as "Newman and Sproull.")

Catherine Caughey, one of the Wrens (Women's Royal Naval Service) who worked at Bletchley Park, tells this story about Max Newman:

Catherine Caughey later talks about the strong emphasis on secrecy at Bletchley Park and for decades afterwards: "My great sadness is that my beloved husband died in 1975 without knowing what I did in the war." (171) It's a common refrain among the people who worked at Bletchley Park that their triumphs were completely unknown to the general public for decades afterwards.

Alan Turing is not a major player in the story of the Colossus but he makes frequent appearances in the pages of this book. Few people can resist telling their Alan Turing anecdotes. Here's Max Newman's son William writing about Turing during the University of Manchester days:

I was familiar with some of the Turing anecdotes, but here's one told by Peter Hilton that was new to me:

Copeland’s biography on the dust jacket of Colossus indicates that his next book is entitled Turing’s Machines. I'm looking forward to it.

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