Zuse Z2

The Z1 had demonstrated two things: that Konrad Zuse's logical design was sound, and that it could not be built out of metal plate. The sliding pins jammed and the machine stopped after a few minutes. So during 1939 and 1940 he rebuilt the arithmetic unit from around 600 second-hand telephone relays and left the memory as it was — sixteen words, still hand-sawn sheet metal. Clock speed was about 5 Hz, and numbers went back to 16-bit fixed point, a step down from the Z1's floating point.

None of that mattered, because the machine's purpose was a demonstration. It was given in 1940 to the DVL, the German aeronautical research establishment, and the machine held together for exactly as long as required. The DVL provided funding, Zuse built the all-relay Z3 the following year, and so the only Z machine never intended to do useful work did the one useful thing that counted.

The Z1, Z2 and Z3 were all destroyed in the bombing of Berlin in 1943–44. Replicas of the Z1 and Z3 have since been built; there is none of the Z2, which seems fair for a machine whose sole job was to convince a funding body.