Besides being used to power up drones, chargers that worked this way could be shaped into bowls, to accommodate things like smart watches, or simply built invisibly into desktops. That permits power to be transferred over distances up to the diameter of the coils, or even slightly farther. When such circuits are tuned to the same resonant frequency, they exchange energy more efficiently. Wibotic’s answer is to use tuned electrical circuits in place of simple transmitting and receiving coils. They would be impractical, though, for a drone trying to hover over a charging station. Inductively coupled systems like this thus work well for things like repowering electric toothbrushes. Separate the coils by more than a few millimetres however, or wiggle them around, and the efficiency of the process drops off quickly. If the transmitting and receiving coils are close together and aligned in the same direction, almost all of the power will be transmitted. That field then induces another alternating current in a second, nearby coil. In this process an alternating current passing through a coiled wire creates an oscillating magnetic field. Lasers aside, the principle behind most wireless power-transmission is a piece of basic physics known as induction. IHS, a market-research firm, estimates that sales of such machines, now half a billion dollars a year, will grow 30-fold over the next decade.
The technology Tesla pioneered is already being used to charge mobile phones, and researchers are working on similarly wirelessly powered kitchen appliances, military equipment such as heads-up displays, and medical devices ranging from heart pumps to brain monitors. (Tesla was a good scientist, but also an inveterate showman.) More modest remote power-transmission is, however, now attracting attention again. That failed, as most other physicists of the time predicted it would. He claimed to believe it possible to broadcast power around the world using a system of towers and balloons, and even convinced J.P. And that was a mere fleabite compared with his grander ambitions. In 1893 Nikola Tesla (pictured), who was one of the pioneers of industrial-scale electricity, illuminated light bulbs across a room in a demonstration at the Columbian Exposition in Chicago. The idea of wireless power-transmission of this sort goes back more than a century. It is developing a system designed to replenish the batteries of drones that are still aloft, using lasers and photovoltaic cells. LaserMotive, another Seattle-based company, is even more ambitious. A ’bot whose batteries were low would simply manoeuvre itself to within half a metre or so of a recharging station to top them up. In May he started a company called Wibotic that plans to recharge drones (and also earthbound robots) without them having to establish an awkward physical connection with a plug. Joshua Smith, a computer scientist at the University of Washington, in Seattle, hopes to change that. Those who talk up these possibilities, though, often neglect to mention the drawbacks of such robot aircraft-one of which is that most cannot fly for more than a quarter of an hour before they need to find a human being to swap their batteries for them or plug them into an electrical socket. DRONES may one day transform the way parcels are delivered, crops monitored and suspects apprehended.
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