Drones have arrived in US airspace, and now they are multiplying. By 2022, 700,000 of the little unmanned aircraft could be exploring American skies, according to the FAA, delivering packages, monitoring traffic, inspecting bridges, and filling other yet to be discovered niches. To do that work, every last one will need electricity to spin its rotors and run its sensors. Most will get it from batteries they take with them to work. Some might pull from the grid directly, using tethers.

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