Kay McNulty: The Mathematical Thinking Behind ENIAC’s Wires

Interpretive illustration of six women collaborating on early electronic programming; not portraits or an archival scene

At a computing anniversary in 1996, Kathleen “Kay” McNulty brought her original War Department hiring paper. It dated from 15 July 1942 and described her job as “assistant computer”. The computer on the document was a person.

Hero illustration: an original interpretation of the subject, not an archival image or exact reconstruction.

Her later work helped change the meaning of that word. McNulty became one of the six women who programmed ENIAC, a pioneering general-purpose electronic digital computer. Their task was to turn mathematical problems into sequences that a room-sized machine could actually execute.

From Donegal to calculating trajectories

McNulty was born near Creeslough, County Donegal, in 1921. She moved to the United States with her family in 1924 and grew up in an Irish-speaking household in the Philadelphia area. At Chestnut Hill College she studied mathematics, graduating in 1942.

The University of St Andrews biography places her recruitment within the enormous wartime demand for calculation. The U.S. Army needed firing tables: numerical guides describing the trajectories of projectiles under specified conditions.

Human computers worked through calculations with paper, tables and mechanical desk calculators. This required mathematical judgement as well as arithmetic. A person had to know which operation came next, carry information between steps and identify results that did not make sense.

McNulty also worked with a differential analyser, an analogue machine that helped solve differential equations mechanically. Her career crossed several forms of computation before ENIAC: mental and written reasoning, desk calculators and a large mechanical system.

Programming before a program could be typed

The original ENIAC was not programmed by typing lines into a screen. Its units had to be configured through switches and cable connections. Numbers could be held in electronic units called accumulators, but that did not mean the original machine stored its program as a modern computer does.

The Computer History Museum’s account explains why the distinction matters. Different functions were physically connected to implement a calculation. Later changes, operating in 1948, allowed coded instructions to be held in banks of switches. ENIAC had a history of development, not one unchanging method of programming.

What the programmer had to translate

  1. The mathematics: identify the operations needed to solve the problem.
  2. The sequence: work out dependencies, repetition and the order in which results were required.
  3. The machine: map that plan onto ENIAC’s available units and connections.
  4. The check: establish whether an unexpected result came from the plan, the setup or the hardware.

This is why photographs of people plugging in cables can be misleading. The visible movement was only the last part of a larger intellectual task. The cables embodied decisions about the calculation.

Blueprints, engineers and learning together

In her recorded 1996 recollections, McNulty described learning from circuit and block diagrams before ENIAC manuals had been written. She also remembered the engineers as available and helpful. Arthur Burks explained the accumulator; understanding one helped the programmers understand the machine’s twenty similar units.

That testimony complicates a familiar slogan about women who had “no instructions”. They faced a genuinely new task without a finished manual, but they were not working without diagrams, colleagues or assistance. Their achievement was to develop expertise within an unfinished technological project.

The University of Pennsylvania’s account of ENIAC recognises programming as essential to making its electronic speed useful. Fast arithmetic alone could not choose a mathematical method, set up a problem or decide that an answer was trustworthy.

Six names belong in the story

McNulty’s achievement was part of a team’s. The original group comprised:

  • Kathleen “Kay” McNulty, later Mauchly Antonelli
  • Betty Snyder, later Holberton
  • Jean Jennings, later Bartik
  • Frances Bilas, later Spence
  • Marlyn Wescoff, later Meltzer
  • Ruth Lichterman, later Teitelbaum

Remembering all six avoids replacing one incomplete history with another. McNulty was a pioneering programmer, not the sole inventor of programming, and ENIAC belonged to a broader development of computing in several countries.

Its public demonstration took place on 15 February 1946. The machine became famous more quickly than the women’s contribution. Later, Kathy Kleiman’s ENIAC Programmers Project recorded extensive interviews with surviving programmers and helped bring their work to a wider audience.

The story she eventually told in her own name

McNulty left paid computing work after marrying ENIAC co-designer John Mauchly in 1948. She remained close to his work, but the Dictionary of Irish Biography notes how difficult her later technical contributions are to trace. It is better to acknowledge that limit than to assign her undocumented inventions.

Recognition eventually became explicit. The six programmers entered the Women in Technology International Hall of Fame in 1997. During a visit to Ireland in 1999, McNulty returned to Creeslough for the first time since childhood. She died in 2006.

The hiring paper makes a fitting object through which to remember her. It records a time when computation was visibly human labour. McNulty helped carry that mathematical labour into electronic machinery, and her testimony helps us see the thinking behind the wires.

Read William Rowan Hamilton’s very different search for a mathematical language, or return to the Irish inventors and scientists guide.

Our McNulty surname history explores the family name separately. Sharing it does not establish descent from Kay McNulty. You can also browse our Irish heritage gifts.

Sources and further reading