When Konrad Zuse switched on the Z3 in his Berlin workshop in May 1941, he had just built the first programmable computer in history out of roughly 2,000 surplus telephone relays, and almost no one in the world knew it existed because the war buried the story for two decades. - Make Tech Easier

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On May 12, 1941, a 30-year-old civil engineer named Konrad Zuse stood in a cramped workshop at Methfesselstra&szlig;e 7 in Berlin-Kreuzberg and demonstrated the Z3, a refrigerator-sized cabinet of clicking telephone relays that could be programmed to perform any calculation its operator could describe in binary. His audience that day was a small group of scientists from the Deutsche Versuchsanstalt f&uuml;r Luftfahrt, the German Laboratory for Aviation, including the professors Alfred Teichmann and Curt Schmieden. The machine they watched run through its paces was the first fully functional, programmable, automatic digital computer ever built. Almost no one outside that workshop would learn about it for two decades. Within two years, an Allied bombing raid would reduce the only working Z3 to splinters and scorched wire, and the man who invented programmable computing would soon be hauling a successor machine across the Bavarian countryside in the chaos of the last weeks of the war.

The story of the Z3 is the story of an invention that arrived in the wrong country at the wrong moment, then got buried by a war it had nothing to do with winning.

The workshop on Methfesselstra&szlig;e

Zuse had quit his job as a stress analyst at the Henschel aircraft factory in 1935 because he was tired of doing the same arithmetic by hand, over and over, on tables of numbers that filled entire notebooks. Aircraft stress calculations in the 1930s meant pages of multiplication and division using slide rules and mechanical adding machines, and Zuse had decided there had to be a better way. He moved back into his parents&rsquo; apartment in Berlin and started building.

The first machine, the Z1, occupied most of the family living room. It was a mechanical computer made of thin metal sheets cut by hand, with sliding pins that represented binary digits. According to Hackaday&rsquo;s detailed reconstruction of his early work, Zuse and a small circle of friends financed the project themselves, cutting the parts with a jigsaw in the apartment. His mother kept feeding them. His father was retired and patient. The Z1 worked, sort of, but the mechanical linkages jammed constantly.

By the time he started the Z3, Zuse had outgrown the family living room. He had rented his own workshop at Methfesselstra&szlig;e 7 in the Kreuzberg district of Berlin, and it was there, not at his parents&rsquo; place, that the breakthrough machine took shape. The Z2 in between had replaced the arithmetic unit with electromechanical telephone relays salvaged from the German postal service. Those relays were the breakthrough. They were cheap, they were everywhere, and they switched cleanly between two states, which was exactly what binary arithmetic needed.

What 2,000 relays could do

The Z3, finished in 1941, used roughly 2,000 of those surplus relays. About 600 of them formed the arithmetic unit. The remaining 1,400 or so handled memory and control. (Some later sources put the number as high as 2,600. The figure varies depending on which parts of the machine are counted.) The Z3 ran on a 5-to-10 Hz clock, which means it could perform between five and ten operations per second. A multiplication took about three seconds. Modern processors do billions of operations in that same second.

But the Z3 had something none of its contemporaries had. It was fully programmable, using punched film tape, and it worked in binary floating-point arithmetic, a numerical format Zuse had designed himself years before anyone else implemented it in hardware. It could handle very large and very small numbers with the same machinery, a problem that bedeviled mechanical calculators of the era.

It was, by the strict definition computer scientists later applied, Turing-complete, though Zuse himself never used that language and the proof came decades...