US factories got no gain from electric dynamos until they rebuilt around unit drive

19th-century factories ran on one central power source — a steam engine or waterwheel — turning a single large shaft that ran the length of the building, with a maze of belts and pulleys distributing power down to every machine on the floor. That system dictated the entire factory's physical layout: every machine had to sit close to the shaft it drew power from, workflow arranged around mechanical proximity to power rather than around the logical sequence of production.

Electric dynamos became commercially available in the 1880s, and the obvious first move for factory owners was the obvious one: replace the steam engine with a single central electric dynamo, keeping the exact same shaft-and-belt distribution system and the exact same factory layout it required. Economist Paul David later found that this substitution alone delivered almost no productivity gain for roughly four decades — electrification looked, on paper, like a technology that had failed to live up to its promise.

Gave each machine its own small motor, killing the central shaft and letting factories be laid out around workflow, not power proximity.

Unit drive eliminated the central shaft, so machines no longer needed to be near a shared power source.

Factories could then be laid out around production flow, enabling lighter, modular, single-story buildings.

Independent motors allowed running machines individually, increasing flexibility and efficiency.

Productivity gains only appeared after enough factories rebuilt, which took decades due to capital costs.

When a new technology underperforms, check if you're still using it inside a system built for the old one; redesign around its real capabilities.

David found that widespread unit-drive adoption didn't reach roughly 50% until the 1920s, some 40 years after commercially available electric dynamos, and that measurable productivity acceleration from electrification only appeared once that threshold was crossed — with electrification then accounting for roughly half of all US manufacturing productivity growth during the 1920s, a delayed but eventually enormous payoff once factories redesigned around the technology's real advantage rather than its literal substitution for the old one.