ENIAC: The 30-Ton Computer That Launched the Digital Age

In a large room at the University of Pennsylvania's Moore School of Electrical Engineering, workers completed the installation of the Electronic Numerical Integrator and Computer — ENIAC — in late 1945. The machine occupied 1,800 square feet of floor space, roughly the size of a medium-sized house. It weighed about 30 metric tons. It contained 17,468 vacuum tubes, 70,000 resistors, 10,000 capacitors, 1,500 relays, and 6,000 manual switches. Running it required 150 kilowatts of electrical power — about the amount needed to power 150 modern homes. When it was switched on for demonstrations, legend holds that lights dimmed across West Philadelphia.

Why Vacuum Tubes and Why They Were So Problematic

ENIAC's vacuum tubes were the key to its speed and also the source of its most persistent engineering headaches. A vacuum tube is a sealed glass envelope from which most of the air has been evacuated, containing metal electrodes. When heated, one electrode emits electrons that flow to another electrode through the vacuum, and by varying the voltage on a third electrode, the tube can amplify or switch electrical signals. This makes it function as a transistor's bulkier, hotter, more fragile predecessor.

The problem with vacuum tubes is that they fail. At ENIAC's scale, with nearly 18,000 tubes, the statistical failure rate meant that on average a tube would burn out every few days. Finding the failed tube in a system of that complexity required methodical searching through thousands of components. The engineers who operated ENIAC developed systematic diagnostic procedures for locating failures, and they kept a large stock of replacement tubes on hand. One technique was to run the machine gently for a time before important calculations — a kind of warm-up that would cause any near-failing tubes to burn out before the serious work began, so they could be replaced.

What ENIAC Was Built to Calculate

ENIAC was originally designed and funded by the U.S. Army's Ballistics Research Laboratory for a very specific purpose: calculating artillery firing tables. During World War II, targeting artillery accurately required tables listing the correct elevation and azimuth for a given range, taking into account the shell's ballistic properties, wind speed, air density, and other variables. These tables were calculated by human "computers" — typically women with mathematics degrees working through differential equations by hand. A single trajectory calculation could take 20 hours by hand; ENIAC could perform it in about 30 seconds.

The war ended before ENIAC became operational, but its first significant calculation was something even more consequential: the feasibility study for the hydrogen bomb, performed at the request of physicist John von Neumann and his colleagues on the Manhattan Project. ENIAC ran for nine days performing over a million operations to help determine whether a thermonuclear weapon was physically achievable. The calculations that contributed to shaping the nuclear age were performed on a machine that had to be programmed by physically rewiring cables.

Programming by Rewiring

ENIAC had no stored program memory in the modern sense. Programming it meant physically connecting its units with cables and setting thousands of switches to define the sequence of operations. Changing from one type of calculation to another could take a team of operators days to rewire. The programmers of ENIAC — primarily a team of six women including Jean Jennings Bartik, Frances Bilas Spence, Kathleen McNulty Mauchly, Ruth Lichterman Teitelbaum, Marlyn Wescoff Meltzer, and Betty Snyder Holberton — developed the discipline of electronic computer programming essentially from scratch, without documentation, manuals, or predecessors to consult.

These women are now recognized as computing pioneers in their own right. For years after ENIAC's public debut, photographs of the machine appeared in newspapers while its human programmers went uncredited — visible in the backgrounds of photographs, their work unacknowledged. Efforts by historians beginning in the 1980s recovered their stories and established their central role in the birth of electronic computing.

ENIAC operated until 1955, when it was finally decommissioned. Parts of it are preserved at the Smithsonian Institution and the University of Pennsylvania. Its 18,000 vacuum tubes, which filled a room and burned out regularly, have been replaced by chips containing billions of transistors small enough to pass through a hypodermic needle — but the machine that inspired them still stands as a monument to what human ingenuity can accomplish when the stakes are high enough.