When Jennings showed up in March 1945, at age twenty, there were approximately seventy women at Penn working on desktop adding machines and scribbling numbers on huge sheets of paper. Captain Herman Goldstine’s wife, Adele, was in charge of recruiting and training. “I’ll never forget the first time I saw Adele,” Jennings said. “She ambled into class with a cigarette dangling from the corner of her mouth, walked over to a table, threw one leg over its corner, and began to lecture in her slightly cleaned up Brooklyn accent.” For Jennings, who had grown up as a spirited tomboy bristling at the countless instances of sexism she faced, it was a transforming experience. “I knew I was a long way from Maryville, where women had to sneak down to the greenhouse to grab a smoke.”25
A few months after she arrived, a memo was circulated among the women about six job openings to work on the mysterious machine that was behind locked doors on the first floor of Penn’s Moore School of Engineering. “I had no idea what the job was or what the ENIAC was,” Jennings recalled. “All I knew was that I might be getting in on the ground floor of something new, and I believed I could learn and do anything as well as anyone else.” She also was looking to do something more exciting than calculating trajectories.
When she got to the meeting, Goldstine asked her what she knew about electricity. “I said that I had had a course in physics and knew that E equaled IR,” she recalled, referring to Ohm’s Law, which defines how a flow of electrical current is related to voltage and resistance. “No, no,” Goldstine replied, “I don’t care about that, but are you afraid of it?”26 The job involved plugging in wires and throwing a lot of switches, he explained. She said that she wasn’t afraid. While she was being interviewed, Adele Goldstine came in, looked at her, and nodded. Jennings was selected.
In addition to Jean Jennings (later Bartik), the others were Marlyn Wescoff (later Meltzer), Ruth Lichterman (later Teitelbaum), Betty Snyder (later Holberton), Frances Bilas (later Spence), and Kay McNulty (who later married John Mauchly). They were a typical squad thrown together by the war: Wescoff and Lichterman were Jewish, Snyder a Quaker, McNulty an Irish-born Catholic, and Jennings a lapsed Church of Christ Protestant. “We had a wonderful time with each other, mainly because none of us had ever been in close contact with anyone from one of the others’ religions,” according to Jennings. “We had some great arguments about religious truths and beliefs. Despite our differences, or perhaps because of them, we really liked one another.”27
In the summer of 1945, the six women were sent to Aberdeen Proving Ground to learn how to use IBM punch cards and wire up plug boards. “We had great discussions about religion, our families, politics, and our work,” McNulty recalled. “We never ran out of things to say to each other.”28 Jennings became a ringleader: “We worked together, lived together, ate together, and sat up until all hours discussing everything.”29 Since they were all single and surrounded by a lot of single soldiers, there were multiple memorable romances nurtured over Tom Collins cocktails in the booths of the officers’ club. Wescoff found a Marine who was “tall and quite handsome.” Jennings paired up with an Army sergeant named Pete, who was “attractive but not really handsome.” He was from Mississippi, and Jennings was outspoken in her opposition to racial segregation: “Pete told me once that he would never take me to Biloxi because I was so outspoken in my views on discrimination that I’d be killed.”30
After six weeks of training, the six women programmers consigned their boyfriends to memory archives and returned to Penn, where they were given poster-size diagrams and charts describing ENIAC. “Somebody gave us a whole stack of blueprints, and these were the wiring diagrams for all the panels, and they said, ‘Here, figure out how the machine works and then figure out how to program it,’ ” explained McNulty.31 That required analyzing the differential equations and then determining how to patch the cables to connect to the correct electronic circuits. “The biggest advantage of learning the ENIAC from the diagrams was that we began to understand what it could and could not do,” said Jennings. “As a result we could diagnose troubles almost down to the individual vacuum tube.” She and Snyder devised a system to figure out which of the eighteen thousand vacuum tubes had burned out. “Since we knew both the application and the machine, we learned to diagnose troubles as well as, if not better than, the engineers. I tell you, those engineers loved it. They could leave the debugging to us.”32
Snyder described making careful diagrams and charts for each new configuration of cables and switches. “What we were doing then was the beginning of a program,” she said, though they did not yet have that word for it. They wrote out each new sequence on paper to protect themselves. “We all felt that we’d be scalped if we ruined the board,” said Jennings.33
One day Jennings and Snyder were sitting in the second-floor classroom they had commandeered, staring at rolled-out sheets containing the diagrams of ENIAC’s many units, when a man came in to inspect some construction. “Hi, my name is John Mauchly,” he said. “I was just checking to see if the ceiling’s falling in.” Neither woman had met the ENIAC visionary before, but they were not the least bit shy or intimidated. “Boy are we glad to see you,” Jennings declared. “Tell us how this blasted accumulator works.” Mauchly carefully answered the question and then others. When they finished, he told them, “Well, my office is next door. So anytime I’m in my office, you can come in and ask me questions.”
Almost every afternoon, they did. “He was a marvelous teacher,” according to Jennings. He pushed the women to envision the many things ENIAC might someday do, in addition to calculating artillery trajectories. He knew that in order to make it a true general-purpose computer, it would need to inspire programmers who could coax various tasks out of the hardware. “He used to always try to get us to think of other problems,” said Jennings. “He would always want us to invert a matrix or something like that.”34
Around the same time that Hopper was doing so at Harvard, the women of ENIAC were developing the use of subroutines. They were fretting that the logical circuits did not have enough capacity to compute some trajectories. It was McNulty who pushed a solution. “Oh, I know, I know, I know,” she said excitedly one day. “We can use a master programmer to repeat code.” They tried it and it worked. “We began to think about how we could have subroutines, and nested subroutines, and all that stuff,” recalled Jennings. “It was very practical in terms of doing this trajectory problem, because the idea of not having to repeat a whole program, you could just repeat pieces of it and set up the master programmer to do this. Once you’ve learned that, you learn how to design your program in modules. Modularizing and developing subroutines were really crucial in learning how to program.”35
Shortly before she died in 2011, Jean Jennings Bartik reflected proudly on the fact that all the programmers who created the first general-purpose computer were women: “Despite our coming of age in an era when women’s career opportunities were generally quite confined, we helped initiate the era of the computer.” It happened because a lot of women back then had studied math, and their skills were in demand. There was also an irony involved: the boys with their toys thought that assembling the hardware was the most important task, and thus a man’s job. “American science and engineering was even more sexist than it is today,” Jennings said. “If the ENIAC’s administrators had known how crucial programming would be to the functioning of the electronic computer and how complex it would prove to be, they might have been more hesitant to give such an important role to women.”36