The First Electronic Computer: The Birth of ENIAC

February 14, 1946, University of Pennsylvania.

A group of reporters gathered before a huge room. Inside, countless lights flashed, fans hummed, and heat radiated outward.

“This is what we’re demonstrating,” an engineer said, “the world’s first electronic computer.”

Reporters stared dumbfounded at this behemoth: it had 18,000 vacuum tubes, 1,500 relays, weighed 30 tons, occupied 170 square meters. It consumed 150 kilowatts of electricity—enough to power a small town.

The engineer input a set of data, and the machine began working. A few seconds later, results appeared on the typewriter.

“What did it just do?” a reporter asked.

“It calculated a shell trajectory,” the engineer said proudly. “If calculated by hand, it would take 20 hours. ENIAC took only 30 seconds.”

This machine was called ENIAC (Electronic Numerical Integrator and Computer). It was the world’s first general-purpose electronic computer.

Why Was ENIAC Needed?

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ENIAC’s birth stemmed from a practical need: calculating artillery trajectories.

During World War II, the U.S. Army needed precise firing tables for artillery. Each type of gun, each type of shell, each firing angle required trajectory calculations. The trajectory of a shell is affected by gravity, air resistance, wind speed, and many other factors—extremely complex calculations.

At the time, these calculations were done by “human computers”—young women using mechanical calculators, calculating number by number. Calculating one trajectory took 20 hours, but on the battlefield, results were needed in seconds.

In 1943, John Mauchly and J. Presper Eckert at the University of Pennsylvania proposed to the Army: build an electronic computer using vacuum tubes.

The Army agreed, appropriating $400,000 (equivalent to about $6 million today).

ENIAC’s Design

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ENIAC’s design was revolutionary for its time.

Core Component: Vacuum Tubes

A vacuum tube is an electronic component that can control the flow of current. It acts like an electronic switch: apply voltage to the control grid, and current flows; remove voltage, and current is blocked.

With vacuum tubes, logic gates (AND, OR, NOT) could be built, and then adders, multipliers, and other computing circuits.

ENIAC used 18,000 vacuum tubes. Each vacuum tube was like a small light bulb, glowing and heating when operating. When ENIAC ran, the whole room was illuminated by countless tiny suns.

Problem: Vacuum Tubes Break Easily

Vacuum tubes had a lifespan of only a few thousand hours. With 18,000 vacuum tubes, if each worked 1,000 hours before failing, on average one would break every few minutes.

Eckert came up with a solution: reduce the operating voltage of vacuum tubes. Normal vacuum tubes operated at 6.3 volts; he reduced ENIAC’s vacuum tubes to 5.7 volts. Although performance decreased slightly, lifespan was greatly extended.

The result: ENIAC averaged only one vacuum tube failure every two days—a miracle at the time.

ENIAC’s Structure

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ENIAC consisted of 40 panels, each 2.4 meters high and 0.6 meters wide. The panels were covered with vacuum tubes, switches, sockets, and indicator lights.

Main components included:

• 20 accumulators: Each could store a 10-digit number and perform addition/subtraction
• Multiplier: Used combinations of accumulators to perform multiplication
• Divider/Square rooter: Performed division and square root operations
• Master control unit: Controlled the entire machine’s operation
• Card reader: Read input from IBM punched cards
• Typewriter: Output results

ENIAC had no keyboard, no display, no hard drive. Input came through punched cards, output through typewriters. Programming was done by plugging wires—engineers had to unplug and replug cables on panels, connecting different circuits.

ENIAC’s Programmers: The Forgotten Women

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ENIAC’s programmers were a group of young women whose names were long forgotten by history.

At the time, “programming” was considered low-level work, similar to secretarial work. Engineers designed and built the machine; women “operated” it.

But ENIAC’s programmers did far more than “operate.” They had to understand every circuit in the machine, understand how every instruction executed. They used hand-drawn diagrams to plan program flow. They used plugs and switches to write complex programs.

The six main ENIAC programmers:

• Jean Jennings: A farm girl from Missouri, later became a software engineer
• Marlyn Wescoff: A math teacher, later helped develop early programming languages
• Frances Spence: A mathematics graduate
• Kathleen McNulty: Daughter of Irish immigrants, later married ENIAC designer Mauchly
• Betty Snyder: Later helped develop the COBOL language
• Ruth Lichterman: A mathematics graduate

They wrote ENIAC’s first running program, proving the machine could work. Their work laid the foundation for modern software engineering.

But it wasn’t until decades later that they were recognized by history. In 2017, their story was made into the documentary The Computers: The Remarkable Story of the ENIAC Programmers.

ENIAC’s Achievements

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In 1946, ENIAC was officially demonstrated to the public.

It shocked the world.

Performance comparison:

ENIAC could execute 5,000 additions per second, 1,000 times faster than previous mechanical calculators.

It was used to calculate trajectories, weather forecasts, atomic energy research, cosmic ray analysis. It was even used to verify Einstein’s relativity calculations.

ENIAC’s Limitations

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But ENIAC also had obvious defects:

First, programming was too troublesome. Changing the program required rewiring, potentially taking days.

Second, small storage capacity. ENIAC could only store 20 10-digit numbers, unable to store large amounts of data.

Third, reliability issues. Although vacuum tube lifespan was extended, 18,000 vacuum tubes still meant frequent failures.

Fourth, huge power consumption. 150 kilowatts of power consumption, equivalent to 1,500 modern computers. Legend has it that every time ENIAC started, the lights in Philadelphia dimmed.

These problems were gradually solved in later computers. ENIAC’s designers Mauchly and Eckert later designed EDVAC, adopting von Neumann’s stored program architecture.

After ENIAC

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ENIAC ran for 10 years until it was retired in 1955.

It was disassembled, with some parts preserved in the Smithsonian Museum, the University of Pennsylvania, and other locations.

In 1996, to commemorate ENIAC’s 50th anniversary, the University of Pennsylvania created an “ENIAC-on-a-Chip” using modern technology—integrating the entire ENIAC’s functionality onto a chip the size of a fingernail.

The 30-ton behemoth of the past could now fit on a fingertip.

This is the speed of computer development.

The Beginning of the Computer Age

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ENIAC’s birth marked the official beginning of the computer age.

From then on, humanity had a new kind of tool: a machine that could automatically execute any computational task. It was no longer a specialized calculator, but a general-purpose information processor.

In the following decades, computers would evolve at astonishing speed:

• From vacuum tubes to transistors
• From transistors to integrated circuits
• From integrated circuits to microprocessors
• From room-sized to pocket-sized

Each evolution brought leaps in performance and drops in cost.

And it all began with that 30-ton behemoth.

Tomorrow, we’ll discuss the computer’s first revolution: From Vacuum Tubes to Transistors.

Today’s Key Concepts

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Vacuum Tube An electronic component that controls current flow in a sealed glass tube with vacuum inside. Vacuum tubes can amplify signals or act as switches. They were the core components of early electronic computers. They were large, power-hungry, and short-lived, later replaced by transistors.

Accumulator A register used to store intermediate results of operations. In ENIAC, accumulators were the basic computing units, capable of storing a number and performing addition/subtraction. Modern CPUs still have accumulators or registers with similar functions.

Discussion Questions

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1. ENIAC weighed 30 tons and occupied 170 square meters. Today’s smartphones weigh less than 200 grams but have computing power billions of times greater than ENIAC. What do you think made this progress possible?
2. ENIAC’s programmers were women, but they were long forgotten by history. Why do you think this happened?

Tomorrow’s Preview: The Transistor Revolution—how the vacuum tube’s replacement made computers smaller, faster, and more reliable?