Anyone whose mobile phone has ever run out of juice-which means, these days, more than half the world’s population-will like the idea of getting electrical power out of the air. The notion is far from new. A little over a century ago, the inventor Nikola Tesla drew up ambitious plans to transmit electrical power without wires. He carried out a series of experiments in which electric lights were illuminated via electrostatic induction, by connecting them to metal sheets suspended in a strong electric field produced by a distant transmitter. In 1898 he proposed a ‘world system