Restoring the Z3: How Relay Clicks Brought a Lost Computer Back to Life ✛ ✎
Berlin, December 1943. A bomb flash, and the first fully automatic programmable computer in history, the Z3, vanishes in flames. Its creator, Konrad Zuse, doesn’t even try to rebuild the hardware: after the war, he constructs new machines. But the idea of the Z3—like a lost painting, with only sketches and memories left — awaits its moment. Restorers, armed with schematics and modern components, reconstruct the logic of the masterpiece, restoring its voice — the steady click of relays.
The first ‘restoration’ happened in 1962, when Zuse himself built a replica for a patent dispute. In it, the original bulky relays were replaced by smaller equivalents, the wiring was simplified, like a copyist slightly altering the brushstrokes but keeping the composition. In 2001, a hybrid was created in Berlin: control was handed to a microcontroller, clock signals were assigned to electronics, while memory and logic circuits remained relay-based — a bridge between eras. The Z3r project (2010) turned the calculator into a teaching tool: each of the 2500 relays got an LED, and 60 toggle switches under the console allowed manual bit manipulation. Then in 2024, Swiss engineers presented the most authentic reconstruction yet: scarce relays were replaced with diode assemblies, but they preserved the angled stepping switches, electromagnetic compatibility, and even the visible rotation of shafts.
At the heart of all versions is a 22-bit floating-point word, with separate storage for sign, exponent, and mantissa. The formula for a number is (−1)^s × 1.m × 2^(e−64), where s is the sign bit, m the mantissa, and e the exponent. The processor reads commands from punched tape, a long paper strip with holes, and executes them strictly sequentially. Step by step, relays open and close, registers shift, numbers normalize. In the Swiss version, the stepping switches rotate openly, and the discrete computing machine comes to life: you can see bits moving through the circuits like blood through veins. Even numerical simulation on modern processors can’t convey this kinetic beauty.
These restorations are not museum mummies. They’ve become a research playground: historians analyze the recreated logic, engineers test the fault tolerance of relay circuits. Public demonstrations in Solothurn draw crowds who listen to the clicks of 80-year-old technology and realize: computing power isn’t measured only in gigahertz. Interest in relay computers is growing in education: the slow, visual execution of operations is ideal for explaining computer architecture. Perhaps future replicas will get a tactile interface — visitors will be able to program manually through patch cords.
Eighty years after the bombing, Zuse could walk into a hall where stepping switches rotate and relays click, and recognize his creation. His Z3, destroyed but not vanished, has found new life in restorations that preserve not only the principle but the soul of the first programmable automaton. In that steady rhythm lies not just history, but an anticipation of our digital world.
The original Z3 weighed about a ton, yet performed only 3–4 additions per second — slower than the calculator in a 1980s digital watch. Yet its 22-bit floating-point arithmetic was decades ahead of its time: mainstream processors didn't adopt it until the 1980s.
It recalls the alternate history from William Gibson’s novel 'The Difference Engine,' where the world runs on mechanical computers. The Z3’s relay logic is a living embodiment of steampunk, except it was built in earnest in 1941.