ENIAC

In simple terms

ENIAC (Electronic Numerical Integrator and Computer), completed in 1945, was one of the first machines that could be called a real electronic computer. It filled a large room, weighed 30 tons, and used about 18,000 vacuum tubes instead of mechanical parts — which made it astonishingly fast for its time. But it had no software in the modern sense: to give it a new problem, operators physically rewired it by plugging cables and flipping switches, a process that could take days. ENIAC sits at the hinge between mechanical calculators and the programmable computers we know today.

More detail

ENIAC was built at the University of Pennsylvania by John Mauchly and J. Presper Eckert for the US Army, mainly to compute artillery firing tables. What made it historically pivotal:

• Electronic, not mechanical — using vacuum tubes for switching, it ran roughly a thousand times faster than the electromechanical machines before it, performing about 5,000 additions per second.
• General-purpose — unlike single-task calculators, it could be configured for many different computations.
• Programmed by wiring — it had no stored program; “programming” meant rewiring the machine. This very limitation motivated the next breakthrough: the stored-program (von Neumann) architecture, where instructions live in memory alongside data — the model essentially all computers use today.

ENIAC’s programmers were a team of six women — including Jean Bartik and Betty Holberton — who figured out how to set up its computations, foundational work that was long under-credited.

Vacuum tubes were also its weakness: they burned out frequently, so keeping ENIAC running was a constant battle — a problem the transistor would later solve.

Why it matters

ENIAC proved that large-scale electronic computation was practical, marking the transition from the mechanical era to the electronic one. Just as importantly, its clumsy rewiring-based programming directly inspired the stored-program concept that defines modern computer architecture. It’s a milestone where you can see the shape of all later computers beginning to emerge — and a reminder of how recently (within a single human lifetime) the computer age began.

Real-world examples

• ENIAC’s original job — computing artillery firing tables for the military — was the kind of tedious numeric work that motivated building it.
• Its rewiring nightmare led directly to the stored-program architecture every computer uses now.
• The ENIAC programmers, six women whose work was long overlooked, are now recognized as among the first programmers.

Common misconceptions

• “ENIAC was the first computer ever.” It was among the first general-purpose electronic computers, but it had important predecessors and contemporaries (the Atanasoff–Berry Computer, Colossus, Zuse’s Z3); “first” depends on your definition.
• “ENIAC had software.” It had no stored program — reprogramming meant physically rewiring it, which is exactly the limitation that drove the next leap in design.

Learn next

Place ENIAC in the broader timeline via history of computing; the transistor is what replaced its failure-prone vacuum tubes.