The ENIAC was the first general-purpose electronic computer, completed in 1946

The ENIAC (Electronic Numerical Integrator and Computer) became operational in February 1946 at the University of Pennsylvania. It was the first machine that could be reprogrammed to solve different problems without rewiring its circuits — the key feature that made it a true computer rather than a specialized calculator.

ENIAC weighed 30 tons, occupied 1,800 square feet of floor space, and used 18,000 vacuum tubes to process information. It consumed 150 kilowatts of electricity and generated so much heat that the room housing it required dedicated air conditioning. Despite its size, ENIAC could perform about 5,000 additions per second, which was revolutionary for its time.

The machine was built during World War II to calculate artillery firing tables for the U.S. Army Ballistic Research Laboratory. By the time it was finished, the war had ended, but the Army kept it running for scientific and military calculations for nearly a decade.

Key Takeaways

  • ENIAC, completed in 1946, was the first general-purpose electronic computer because it could be reprogrammed to solve different types of problems.
  • Earlier machines like the Analytical Engine (1837) and Colossus (1943) existed, but they were either mechanical designs never fully built or single-purpose machines designed for one specific task.
  • ENIAC's vacuum tubes made it possible to process information electronically, but the machine was enormous, expensive to operate, and required constant maintenance.
  • The invention of the transistor in 1947 and later the integrated circuit made computers smaller, faster, and practical for wider use beyond military and research institutions.

Earlier machines that led to the first computer

Before ENIAC, mathematicians and engineers designed machines that performed calculations, but none combined all the features needed to be called a true computer. Charles Babbage designed the Analytical Engine in 1837, which had all the theoretical components of a modern computer — a processor, memory, and the ability to be programmed with punch cards. However, the machine was never fully built during his lifetime because the mechanical precision required was beyond what 19th-century manufacturing could achieve.

During World War II, the British built Colossus to break German military codes. Colossus used vacuum tubes and was electronic, making it faster than any mechanical machine, but it was designed to solve only one problem: decrypting a specific type of enemy message. Once the war ended, the machines were dismantled and kept secret for decades.

ENIAC differed from Colossus because it was reprogrammable. Changing what ENIAC calculated required rewiring some connections and resetting switches, but the same physical machine could solve artillery problems one week and physics equations the next. This flexibility is what made it the first general-purpose computer.

How ENIAC worked and what made it different

ENIAC used vacuum tubes as electronic switches. Each tube could turn on and off thousands of times per second, allowing the machine to represent numbers and perform logic operations at speeds no mechanical device could match. The tubes were fragile and burned out frequently — operators had to replace burned-out tubes constantly, and the machine was down for repairs much of the time.

Programming ENIAC meant physically plugging cables into a plugboard and setting thousands of switches by hand. A single program could take days to set up. The first program, written by Betty Jennings and Fran Bilas, took three days to load into the machine. Despite this cumbersome process, ENIAC could complete in hours calculations that would have taken human mathematicians weeks.

The machine operated on decimal numbers (0 through 9) rather than the binary system (0 and 1) that later computers used. This made ENIAC closer to how humans think about numbers, but it required more vacuum tubes and made the machine less efficient than it might have been.

What happened after ENIAC

ENIAC proved that electronic computing was practical, and other institutions quickly built their own machines. EDVAC (Electronic Discrete Variable Automatic Computer), also at the University of Pennsylvania, was completed in 1949 and used binary numbers, making it faster and more efficient. UNIVAC (Universal Automatic Computer), delivered to the U.S. Census Bureau in 1951, was the first computer sold commercially, though "sold" meant leasing it for about $15,000 per month.

The invention of the transistor in 1947 at Bell Labs began the shift away from vacuum tubes. Transistors were smaller, more reliable, and used far less power. Computers built with transistors in the mid-1950s were smaller and faster than ENIAC, though still room-sized machines that only large organizations could afford.

The integrated circuit, developed in the late 1950s, allowed thousands of transistors to be placed on a single chip of silicon. This invention made possible the miniaturization that led to personal computers in the 1970s and the devices we use today.

Why ENIAC gets the credit for "first computer"

Historians and engineers credit ENIAC as the first computer because it combined three essential features: it was electronic (not mechanical), it was general-purpose (reprogrammable for different tasks), and it actually worked and was used for real calculations. Earlier designs had some of these qualities but not all three together.

The definition of "computer" matters here. If you mean any machine that calculates, then mechanical calculators from the 1600s count. If you mean a programmable machine, then Babbage's Analytical Engine from 1837 counts in theory, even though it was never completed. If you mean an electronic machine, then Colossus from 1943 counts. But if you mean a machine that is electronic, general-purpose, and actually operational, ENIAC in 1946 is the answer.

The people who built ENIAC

John Mauchly and J. Presper Eckert led the team that designed and built ENIAC. Mauchly was a physicist interested in using electronics for calculation, and Eckert was an engineer who solved the practical problems of building a reliable electronic machine. The project employed about 50 people over its three-year development, including mathematicians, engineers, and technicians.

The contributions of the women programmers who wrote ENIAC's first programs — Betty Jennings, Fran Bilas, Ruth Lichterman, Marlyn Meltzer, and Jean Jennings Bartik — were often overlooked in historical accounts. These programmers had to figure out how to use the machine with almost no documentation, essentially inventing computer programming as they worked.

Frequently Asked Questions

Was there a computer before ENIAC?

Machines that performed calculations existed before ENIAC, including mechanical calculators and the British Colossus, which was electronic. However, ENIAC was the first general-purpose electronic computer that could be reprogrammed to solve different types of problems. Earlier machines were either mechanical, single-purpose, or both.

How much did ENIAC cost to build?

ENIAC cost about $500,000 to build, which is roughly $8 million in equivalent dollars from that era. The U.S. Army funded the project during World War II as part of the war effort. The cost was enormous for the time, but the machine's speed made it valuable for military and scientific calculations.

Could ENIAC run multiple programs at the same time?

No. ENIAC could run only one program at a time. Switching between programs required physically rewiring parts of the machine and resetting switches, a process that took hours. Modern computers handle multiple programs simultaneously through operating systems and processors designed for that task.

How long did ENIAC stay in operation?

ENIAC ran from 1946 until 1955, when it was shut down. During those nine years, it performed calculations for the military, universities, and research institutions. By the time it was retired, faster and more reliable computers using transistors were already in use.

Why did computers need to be so large in the 1940s?

Vacuum tubes, the electronic switches that made ENIAC work, were large and generated significant heat. Thousands of tubes were needed to store and process information, and each tube required wiring, cooling, and space for maintenance. The transistor, invented in 1947, was much smaller and more efficient, allowing computers to shrink dramatically over the following decades.