{MAKE YOUR way around the busy warehouses of Torrance, near the twin ports of Los Angeles and Long Beach, and you will find an unexpected sight. Dozens of electrified 18-wheelers belonging to Maersk, a Danish logistics firm, are hooked up to fast chargers. The lorries are tapping into electricity from linear generators, which are more efficient, and thus greener, than traditional rotating ones. These generators are the product of Mainspring, a Californian startup that has raised over $250m this year.}