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The Hidden Figures Behind Bletchley Park’s Code-Breaking Colossus

“If anyone asked us what we did, we were to say that we…did secretarial work.” That’s how Eleanor Ireland described the secrecy surrounding her years at Bletchley Park. Ireland was decidedly not a secretary, but there was good reason for the subterfuge.

Ireland was one of 273 women recruited during World War II to operate Bletchley Park’s Colossus machines, which were custom built to help decrypt German messages that had been encoded using the sophisticated Lorenz cipher machines. (Bletchley Park’s more famous code-breaking effort, pioneered by Alan Turing, involved breaking the ciphers of the simpler Enigma machines.) Because of the intense, high-stakes, highly classified work, the women were all required to sign statements under the Official Secrets Act. And so their contributions, and Colossus itself, remained state secrets for decades after the end of the war.

In 1975, the U.K. government began slowly declassifying the project, starting with the release of some photos. The historian Brian Randell, who had been lobbying the government to declassify Colossus, was given permission to interview engineers involved in the project. He was also allowed to write a paper about their work, but without discussing the code-breaking aspects. Randell presented his findings at a conference in Los Alamos, N.M., in June 1976.

In 1983, Tommy Flowers, the electrical engineer chiefly responsible for designing the machines, was permitted to write about Colossus, again without disclosing details about what Colossus was used for. In 1987, IEEE Spectrum’s Glenn Zorpette wrote one of the first journalistic accounts of the code-breaking effort [see “Breaking the Code,” September 1987]. It wasn’t until 1996, when the U.S. government declassified its own documents from Bletchley Park, that the women’s story finally started to emerge.

Beginning in 1943, a group of Wrens—members of the Women’s Royal Naval Service—who either excelled at mathematics or had received strong recommendations were told to report to Bletchley Park. This Victorian mansion and estate about 80 kilometers northwest of London was home to the Government Code and Cipher School.

There, the Wrens received training in binary math, the teleprinter alphabet, and how to read machine punch tapes. Max Newman, head of the section responsible for devising mechanized methods of code breaking, initially led these tutorials. After two weeks, the women were tested on their knowledge and placed into jobs accordingly. Eleanor Ireland landed the plum assignment of Colossus operator.

Colossus was the first digital electronic computer, predating ENIAC by two years. Tommy Flowers, who worked on switching electronics at the Post Office Research Station in Dollis Hill, designed the machine to help decipher the encrypted messages that the Nazi high command sent by radioteleprinter. The Germans called their radioteleprinter equipment Sägefisch, or sawfish, reportedly because of the radio signals’ sawtooth wave. The British accordingly nicknamed the German coded messages “fish,” and the cipher that Colossus was designed to break became “Tunny,” short for tuna fish.

The Tunny machine was known to the Germans as the Lorenz SZ40, and it operated as an attachment to a standard teleprinter. Like the Enigma, the Lorenz had a set of wheels that encrypted the message. But where the Enigma had three wheels, the Lorenz had 12. Because of the Lorenz’s significantly stronger encryption, the Germans used it for their highest-level messages, such as those sent by Hitler to his generals.

The Bletchley Park code-breakers figured out how to break the Tunny codes without ever having seen a Lorenz. Each of the 12 wheels was imprinted with a different number of two-digit numerals. The code breakers discovered that the wheels consisted of two groups of five—which they called the psi wheels and the chi wheels—plus two motor, or mu, wheels. The chi wheels moved forward in unison with each letter of a message. The psi wheels advanced irregularly based on the position of the mu wheels. Each letter of a message was the sum of the letters—that is, the sum of the numbers representing the letters—generated by the chi and psi wheels.

The initial function of Colossus was to help determine the starting point of the wheels. Colossus read the cipher’s stream of characters and counted the frequency of each character. Cryptographers then compared the results to the frequency of letter distribution in the German language and to a sample chi-wheel combination, continually refining the chi-wheel settings until they found the optimal one.

Eventually, there were 10 Colossi operating around the clock at Bletchley Park. These room-size machines, filled with banks of vacuum tubes, switches, and whirring tape, were impressive to behold. The official government account of the project, the 1945 General Report on Tunny, used words such as “fantastic,” “uncanny,” and “wizardry” to describe Colossus, creating a mental image of a mythic machine.

But the actual task of operating Colossus was tedious, time-consuming, and stressful. Before the machine could even start crunching data, the punched paper tapes that stored the information had to be joined into a loop. The Wrens experimented with various glues and applications until they arrived at the ones that worked best given the speed, heat, and tension of the tape as it ran through the machine. Dorothy Du Boisson described the process as the art of using just the right amount of glue, French chalk, and a warm clamp to make a proper joint.

The operator then had to feed the tape through a small gate in front of the machine’s photoelectric reader, adjusting the tape’s tautness using a series of pulleys. Getting the right tension was tricky. Too tight and the tape might break; too loose and it would slip in the machine. Either meant losing valuable time. Colossus read the tape at thousands of characters per second, and each tape run took approximately an hour.

The cryptographers at Bletchley Park decided which patterns to run, and the Wrens entered the desired programming into Colossus using switches and telephone-exchange plugs and cords. They called this pegging a wheel pattern. Ireland recalled getting an electric shock every time she put in a plug.

During the first three months of the Colossus program, many of the Wrens suffered from exhaustion and malnutrition, and their living conditions were far from enviable. The women bunked four to a room in the cold and dreary servant quarters of nearby Woburn Abbey. Catherine Caughey reported that the abbey’s plumbing couldn’t keep up.

The rooms that housed the Colossi were, by contrast, constantly overheated. The vacuum tubes on the machines gave off the equivalent of a hundred electric heaters. Whenever a Wren got too hot or sleepy, she would step outside the bunkers to splash water on her face. Male colleagues suggested that the women go topless. They declined.

The Wrens worked in 8-hour shifts around the clock. They rotated through a week of day shifts, a week of evenings, and a week of nights with one weekend off every month. Women on different shifts were often assigned to the same dorm room, the comings and goings disrupting their sleep.

Those in charge of the Wrens at Woburn Abbey didn’t know what the women were doing, so they still required everyone to participate in squad-drill training every day. Only after women started fainting during the exercises were a few improvements made. Women in the same shift began rooming together. Those working the night shift were served a light breakfast before starting, rather than reheated leftovers from supper.

During their time at Bletchley Park, the computer operators knew very little about their successful contribution to the war effort.

Having signed the Official Secrets Act, none of the 273 women who operated the Colossi could speak of their work after the war. Most of the machines were destroyed, and Tommy Flowers was ordered to burn the designs and operating instructions. As a result, for decades the history of computing was missing an important first.

Beginning in the early 1990s, Tony Sale, an engineer and curator at the Science Museum, London, began to re-create a Colossus, with the help of some volunteers. They were motivated by intellectual curiosity as well as a bit of national pride. For years, U.S. computer scientists had touted ENIAC as the first electronic computer. Sale wanted to have a working Colossus up and running before the 50th anniversary of ENIAC’s dedication in 1996.

On 6 June 1996, the Duke of Kent switched on a basic working Colossus at Bletchley Park. Sale’s machine is still on view in the Colossus Gallery at the National Museum of Computing on the Bletchley estate, which is open every day to the public.

When the British government finally released the 500-page General Report on Tunny in 2000, the story of Colossus could be told in full. Jack Copeland captures both the technical detail and the personal stories in his 2006 book Colossus: The Secrets of Bletchley Park’s Codebreaking Computers (Oxford University Press), which he wrote in collaboration with Flowers and a number of Bletchley Park code breakers and computer historians.

And what of the women computer operators? Their stories have been slower to be integrated into the historical narrative, but historians such as Janet Abbate and Mar Hicks are leading the way. Beginning in 2001, Abbate led an oral history project interviewing 52 women pioneers in computing, including Eleanor Ireland. These interviews became the basis for Abbate’s 2012 book Recoding Gender: Women’s Changing Participation in Computing (MIT Press).

In 2017, Hicks published Programmed Inequality: How Britain Discarded Women Technologists and Lost Its Edge in Computing (MIT Press). In it she traces women’s work in the burgeoning computer field before World War II through the profession’s gender flip in the 1960s. The book documents the industry’s systematic gender discrimination, which is still felt today.

As for the computer operators themselves, Ireland took advantage of the lifting of the classification to write an essay about Colossus and the fellowship of the Wrens: “When we meet, as we do in recent years every September, we all agree that those were our finest hours.”

An abridged version of this article appears in the January 2020 print issue as “The Hidden Figures of Colossus.”

Part of a continuing series looking at photographs of historical artifacts that embrace the boundless potential of technology.

About the Author

Allison Marsh is an associate professor of history at the University of South Carolina and codirector of the university’s Ann Johnson Institute for Science, Technology & Society.