Invisible Women: The Six Human Computers Behind The ENIAC

Until the late 20th century, a ‘computer’ was not a machine like the one you may be reading this on now, but a job title — literally, someone who makes computations. This term can be found all the way back to the 17th century, but one of the most important eras for the human computer occurred during World War II.

With a depletion in the male workforce, it follows that a large majority of these human computers — largely tasked with calculating bullet and missile trajectories — were women. Six of these women then went on to become the programmers of the ENIAC, the world’s first computing machine; their names were Kay McNulty, Betty Holberton, Marlyn Meltzer, Ruth Teitelbaum, Jean Bartik, and Fran Spence.

All photos from Wikimedia Commons

Human Computers

During World War II, a call went out for women with degrees in mathematics, or similar experience, and it was this advertisement that our six programmers all answered. They worked out of the Moore School of Engineering, and for the first part of the job were largely tasked with performing mundane — but precise — calculations. All six women came to the job from vastly different backgrounds.

Kay McNulty, about whom the most has been written out of the six original ENIAC programmers, was born in Ireland during the tumultuous Irish War of Independence. Her father, an IRA training officer, was arrested and taken away on the night of McNulty’s birth, where he was imprisoned for two years. On his release, the family emigrated to the United States — even though McNulty at the time didn’t speak any English, only Gaelic. Throughout her studies in the USA, McNulty took as many mathematics courses as she could, both through high school and college.

Out of a class of 92 at her women’s college, she was one of only a few to graduate with a degree in mathematics in 1942 — along with Frances Spence, one of the other women who ended up on the ENIAC team. During her education, Spence switched from Temple University to Chestnut Hill College after being awarded a scholarship at the latter school. She majored in mathematics with a minor in physics, also graduating in 1942.

McNulty wanted to work in mathematics — without being a schoolteacher — and spent much of her third year of college looking for relevant jobs. As it turns out, it was only a week or two after graduating that she saw the US Civil Service ad calling for women who were educated in mathematics. She called two of her fellow mathematics majors to apply with her — one of whom was busy on the day, the other was Frances Spence.

Both Spence and McNulty were hired as ‘computers’ without delay, settling for the low pay grade for the chance to both use their degrees and be able to help the war effort at the same time. At the Moore School of Engineering, around ten ‘girls’ (as they were called at the Moore School) and four men worked in a former classroom to calculate bullet and missile trajectories, using mechanical desk calculators and huge sheets of columned paper. Four more of these ‘girls’ would end up working with them on the ENIAC.

Jean Bartik was another mathematics graduate, studying at Northwest Missouri State Teachers College and graduating in 1945 as the only mathematics major. Growing up as the sixth of seven children in a farming family that had been hit hard by the Great Depression, Bartik was determined to escape farm life — and her higher education was the way to do it. It was in the same year that she seized upon the opportunity to travel to Philadelphia, joining the team as a human computer, despite the warnings of her adviser that she would be just another “cog in a wheel”.

Betty Holberton experienced the sexism that all these women would constantly encounter throughout their life — starting on her very first day of classes at the University of Pennsylvania. Holberton’s maths professor asked her if she wouldn’t be better off at home raising children. Instead, Holberton ended up studying journalism — as one of the few fields open to women in the 1940s — as it would let her travel, and write about any field that interested her. She ended up back in mathematics, of course — as a human computer in the Moore School of Engineering.


The last two women ended up working in the same special area of the ENIAC project, both coming from very similar backgrounds. Marlyn Meltzer graduated from Temple University in 1942 with a major in mathematics and a minor in business machines. Thanks to her minor, she was hired by the Moore School of Engineering to perform weather calculations, and from there she was moved to the team calculating ballistics trajectories with the rest of the girls. Ruth Teitelbaum graduated from Hunter College with a B.Sc. in mathematics, and soon joined the Moore School group with the rest of the women.

Computing At Moore School


“The human computer is supposed to be following fixed rules; he has no authority to deviate from them in any detail.”


So said Alan Turing of the position in 1950. It was not a glamorous role, but rather one of rote and incredibly precise calculations. Kay McNulty and Frances Spence both came into the Moore School somewhat unprepared for their work calculating trajectories. They were unfamiliar with the numerical integration methods used to compute the trajectories, and the textbook they were given to learn from was almost useless. They soon learned how to perform their calculations despite these roadblocks through the advice of Lila Todd, a well-liked supervisor at the school.

Kay McNulty, Alyse Snyder and Sis Stump use the differential analyser

Around 75 young female computers were employed at the Moore School during this time, doing complex and time-consuming work. Each gun required its own firing table of around 1800 trajectories, with each trajectory requiring around 30 to 40 hours of work with a desk calculator.

Kay and Fran were eventually moved to work on the differential analyser in the basement of the Moore school, which was the largest and most complicated analogue mechanical calculator of the time. There were only 3 in the United States, and 5 or 6 in the entire world — the one at the Moore School had been lent to them for the duration of the war.

Using this advanced analyser had the ability to compress around 40 hours of work on a mechanical desk calculator into 50 minutes, but this promotion had its disadvantages for the girls. The analyser staff worked six days a week, with their only official holidays falling on Christmas and the Fourth of July. This experience did allow them to move on to bigger and better things, however.


The ENIAC is immortalised in history as the world’s first electronic general purpose computer. It was initially developed to do the same jobs as our human computers at the Moore School, by calculating bullet trajectories. While the ENIAC was designed and built by a team of male scientists from the University of Pennsylvania, all the programmers — who at the time were called ‘operators’ — were women.

The ENIAC looked nothing like a computer as we know it today. It was a huge machine full of black panels and switches, containing 17,468 vacuum tubes, 7200 crystal diodes, 1500 relays, 70,000 resistors, 10,000 capacitors and approximately 5,000,000 hand-soldered joints. It weighed more than 30 short tons, occupied 167m2 and consumed 150 kW of electricity. Its huge power requirement led to a rumour that the lights across Philadelphia would dim every time the ENIAC was switched on.


“Since we knew both the application and the machine, we learned to diagnose troubles as well as, if not better than, the engineer.”


Kay McNulty was one of the first women to be chosen to join the ENIAC team, along with Holberton, Meltzer, Teitelbaum, and another woman called Helen Greenman, who declined to go to Aberdeen for training and was hence replaced with Bartik. McNulty’s friend Fran Spence would join the project later.

At the time it was being built, the ENIAC was a classified project, and the programmers were not at first allowed into the room to actually see the machine. Instead they had to study the blueprints to work out their programs before they even got their hands on the machine.

Instead of using a programming language, programming the ENIAC involved rerouting switches and plugs to create a route to the appropriate bank of electronics in parallel progression. The ballistics calculations which once took hours could now be done by the machine in about ten seconds, but it could also take a few days for the programmers to set the computer up for a new set of programs.

Betty Holberton appears on the right

Betty Holberton was known as being particularly skilled at the complex brand of problem-solving that programming the ENIAC required, figuring out the best path for guiding complicated calculations through the huge labyrinth of the ENIAC’s internal workings. Often, these ideas came to her overnight, leading fellow programmer Jean Bartik to say of her: “Betty had an amazing logical mind, and she solved more problems in her sleep than other people did awake.”

The ENIAC was known for being particularly unreliable, with tubes blowing out a couple of times a day during the early stages of its operation. This left the huge computer unable to operate almost half the time. While they eventually stabilised the rate of failure to a more acceptable rate, it was the programmers who became adept at finding and replacing broken tubes and joints. This would often involve literally crawling inside the machine to find the source of the problem. “Since we knew both the application and the machine, we learned to diagnose troubles as well as, if not better than, the engineer,” said Jean Bartik, speaking about the role of the female programmers.

Bartik also described the first public demonstration of the ENIAC, proving the entire team’s great excitement for the event:

“The day ENIAC was introduced to the world was one of the most exciting days of my life. The demonstration was fabulous. ENIAC calculated the trajectory faster than it took the bullet to travel. We handed out copies of the calculations as they were run. ENIAC was 1,000 times faster than any machine that existed prior to that time. With its flashing lights, it also was an impressive machine illustrating graphically how fast it was actually computing.”

This demonstration came in 1946 — too late to be used in the war, though the technology opened the door to many further breakthroughs in the field of computing. In mid-1947, the ENIAC was moved to Aberdeen Proving Grounds, and Kay McNulty, Ruth Teitelbaum and Frances Spence moved with the machine to continue their work, whereas the other women preferred to settle down into family life or find other jobs, rather than move to life on an army base.

Invisible Women

Left: Jean Bartik, Right: Frances Spence

Despite their importance in programming the world’s first computer, the role that the female team played in the ENIAC’s first years was largely downplayed. There existed at the time — some of which still exists today — a strong stigma that women were not interested in technology, and the general lack of public knowledge about the ENIAC’s programmers did not help the cause, only furthering the perceived disconnect between women and the field of computing.

When the ENIAC was unveiled in 1946, the US Army completely failed to mention the names of the programmers who had facilitated such complicated calculations, and photos of the women working on the computer itself usually went uncredited when published in the newspapers.

Interestingly enough, the role of programmer — or operator — was considered a clerical role. Despite the women’s strong mathematical backgrounds, they were still not publicly acknowledged for the vital roles they played in programming the ENIAC. Since then, all six women have been recognised for their contribution to computing of course, with many of them going on to make further advances in the field of computing and programming after they left the program.

Life After Eniac

Kay McNulty ended up marrying the co-inventor of the ENIAC, John Mauchly. She resigned from her role at Aberdeen to move in with him, having five children with Mauchly and settling into married life. That wasn’t the end of her programming career, however, as she later worked on software designs for the BINAC and UNIVAC I: both computers with hardware that had been designed by her husband. Jean Bartik had earlier been poached by McNulty’s husband Mauchly for his company, and thus she was also part of the team who worked on the BINAC and the UNIVAC.

Betty Holberton was one of the most celebrated women of the original team — and the only one to have received the Augusta Ada Lovelace Award, the highest award given by the Association of Women in Computing. After ENIAC, she continued to work in computers. She helped to develop the UNIVAC, designing control panels that put the numeric keypad next to the keyboard — as we are used to having on keyboards today. She was also behind the decision to replace the UNIVAC’s black casing with the grey-beige colour that for a long time was the universal colour for computers. She ended up working with Mauchly to develop the C-10 instruction set for BINAC, long considered the predecessor of modern programming languages.

All these women were inducted into the Women in Technology International Hall of Fame in 1997, despite the lack of recognition for their work during the time when they were actually doing it. While the work of the female team behind the ENIAC formed the basis of some of the technology we now use almost every day, the stigma around women in technology still exists, albeit in a less overt way. Even today the stories of these pioneering women are not widely known, although the legacy of their work lives on.

These Are Your Numbers is a new Lifehacker series where we profile great minds that have made significant contributions to robotics and computing.

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