Relay Interpolator

The order came from American defence research in 1943: gunnery data for anti-aircraft directors needed smoothing and interpolating, and doing it by hand was slower than the aircraft. Stibitz answered with about 440 relays driven by paper tape, known afterwards as Bell Labs Model II. The real contribution was the number representation. Each decimal digit was encoded as exactly two ones among five positions — a two-out-of-five code. A stuck relay produces three ones or one, and the machine halts instead of computing confidently on garbage.

The consequence was practical rather than theoretical. Because faults produced a stop and not a wrong answer, the interpolator could run unattended through the night; the operator arrived in the morning to see whether it had finished or halted, and did not have to check the arithmetic. The idea that a machine should catch its own errors instead of trusting its components to behave is still what ECC memory and check digits are doing.