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Tiny, Morphing, Electricity-Stealing Spy Planes Developed
tkohler writes "The Air Force Research Lab is developing an Electric Motor-powered Micro Air Vehicle (MAV) that can 'harvest' energy when needed by attaching itself to a power line. It can also temporarily change its shape to look more like innocuous piece of trash hanging from the cable. For domestic spying, maybe it will morph into a pair of sneakers?"
Cute idea. What they're trying to do, it seems, is mooch a little power from the electrostatic field gradient around the wire. This is quite feasible if you have a wire with a few KV to ground. The classic demo is to light up a fluorescent lamp by placing it vertically below a high tension line. This works partly because air is not a perfect insulator. There's an electrical path to ground; it just has a high resistance.
If the thing lands on an 11KV power line that's 10m above ground, and has a conductive part that dips 10cm below the line, it should see a voltage difference of about 90 volts. You can't draw very much current before the voltage difference disappears, but you can draw a little.
It's also possible to extract some energy magnetically. See U.S. Patent #3,202,963, "Apparatus for Illuminating Power Lines". But that approach requires heavier parts than an electrostatic approach.