Bell Labs Model V

George Stibitz's group at Bell Labs built six relay machines between 1939 and 1946, and the Model V was the end of the line: around 9,000 telephone relays, ten tons, two units. The first went to NACA's laboratory at Langley in 1946, the second to the Ballistic Research Laboratory at Aberdeen the following year. Unlike its predecessors it was general-purpose, computed in floating point, and had two arithmetic units that could work on separate problems or gang together on one.

What set the Bell machines apart from the electronic builds was their attitude to failure. Digits were encoded two-out-of-five, so every valid character has exactly two ones and a sticking relay produces an invalid digit rather than a plausible wrong number. The machine noticed, retried the computation, and if necessary skipped to the next problem in the queue. That is why it could be left running unattended overnight, something no ENIAC operator would have risked. A multiplication took a bit over a second, and that was the price.