Penn’s ENIAC, the world’s first electronic computer, turns 80

Computing now powers nearly every facet of modern life and industry. And the birthplace of the modern computer was not in Silicon Valley but at the University of Pennsylvania.

This new era in human problem-solving was launched in 1946, when Penn scientists J. Presper Eckert and John Mauchly built ENIAC, the world’s first general-purpose, electronic, programmable computer.

One of ENIAC’s most revolutionary contributions was its ability to be reprogrammed to solve complex numerical problems. Specifically, what set it apart from other machines of the time was that it could be rewired to perform different calculations rather than being fixed for a single task. ENIAC could execute “conditional branches”—operations that allowed it to switch between different calculations based on intermediate results. In other words, ENIAC could perform “if this, then that” operations.

Earlier machines were sophisticated but limited, relying on clunky gears and rotating shafts that had to physically mesh to perform a calculation. This reliance on the physical world limited their speed.

ENIAC, however, harnessed the flow of electrons. Using more than 17,000 vacuum tubes as near-instant switches, it was able to perform calculations far faster than mechanical relays, accelerating the development of the hydrogen bomb (known as the “Super”) and ushering in the digital age.

Although ENIAC, which is celebrating its 80th anniversary this week, no longer operates, four of its original 40 nine-foot-tall panels are on display at Penn Engineering’s Moore School Building—the site where modern computing began.

The architecture of choice

ENIAC grew from a collaboration between Eckert, a Penn alumnus and engineer who solved the technical challenges of vacuum tube reliability, and Mauchly, a physicist and professor in the Moore School of Electrical Engineering who envisioned a shift from slow mechanical parts to high-speed electronics to solve the Army’s backlog of complex ballistics calculations.

Programming this flexibility required what historians have described as a “physical hack” of the hardware, and the work fell to six pioneering women: Frances Bilas Spence, Jean Jennings Bartik, Ruth Lichterman Teitelbaum, Betty Snyder Holberton, Kay McNulty Mauchly Antonelli, and Marlyn Wescoff Meltzer. As the first digital-age programmers, they translated logic into electronic signals for ENIAC to interpret.

Before ENIAC could compute, it required a massive physical input.

One major test case involved the “Super.“ Scientists at Los Alamos provided one million IBM punch cards, which were fed into a card reader. The reader converted the holes in the paper into electrical pulses, which ENIAC interpreted as numbers that it then loaded into one of its 20 accumulators—its equivalent of short-term memory.

Because ENIAC lacked internal program storage, the programmers had to configure each computation manually. They stood between the machine’s 30-ton panels, rewiring plugboards and setting switches to route electric signals (pulses). They then “patched” the accumulators together with heavy cables, manually routing the flow of pulses—for example, from a multiplier to an accumulator and finally to a card puncher—to solve an equation.

Unlike earlier machines built for a single mathematical task, ENIAC was a blank slate. Its general-purpose design allowed it to contribute to diverse scientific fields, ranging from cosmic-ray and thermonuclear ignition research to wind-tunnel design and random-number studies.

The past that powers what comes next

Over eight decades, the purpose of this technology has migrated far from ENIAC’s wartime role. What began as a tool for calculating projectile trajectories, decoding secret messages, and modeling explosions has evolved into handheld and wearable devices, wireless networks, and artificial intelligence—technologies capable of aiding human well-being in ways the ENIAC’s creators could scarcely have conceived.

Yet, the core concept remains unchanged: the power of the electronic decision. Every time a smartphone filters a spam call or an AI model suggests a workflow optimization solution, it has a lineage traceable back to the Moore School Building, where an electronic machine was first taught to think and choose.

This story includes information from the University Archives and Records Center and Penn Engineering.

ENIAC is featured in Visit Philadelphia’s 52 Weeks of Firsts, in celebration of America’s 250th anniversary.