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How tesla electricity can create wireless power | The Economist

302166 / Jun 16, 2010

Nikola Tesla built a tower to broadcast electric power. It failed. Soon, sending power through the air might be the norm

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The inventor, Nikola Tesla, dreamt of wireless power. Among his work in the realm of electricity he built a coil, later named the Tesla coil, which could illuminate lamps from across a room and throw the occasional bolt of lightning at the nearest conductor. Tesla coils remain popular today, though often for their ability to put on a fantastic lightning show.

Nikola Tesla believed in wireless power with such an enthusiasm that with the financing of JP Morgan, he constructed a giant apparatus, the Wardenclyffe Tower, at his lab in Shoreham Long Island in 1901. This was before the world was wired.

The idea? To send wireless power around the globe.

Here's how it works. Run an electric current through a coil of copper wire and the coil will produce a short-range magnetic field. Place a second coil within this field and an electric current will flow through it. The magnetic field has transferred electrical power from one coil to the other. This principle is called induction, and it has been understood for more than a century. Induction is what charges a wireless electric toothbrush, for example, and it works well over extremely short distances. Pull the coils apart and the power transfer ceases.

It turns out that the trick to longer distance power transfer is the same principle an opera singer uses to shatter a wine glass from across the room; it's called resonance. For the opera singer, when the frequency of the sound wave matches the unique resonant frequency of a glass, the acoustic energy is converted to kinetic energy at the highest possible efficiency. The energy then builds inside the glass until it shatters.

The coils that Witricity uses to transfer power wirelessly are magnetic resonators. First, a rapidly oscillating electric current is applied to a coil at its specific resonant frequency. This creates a magnetic field in the region around the coil. Tune a second coil to the same resonant frequency as the source and it will couple, resonating anywhere within that region and converting the oscillating magnetic field into an electrical current within the second coil. This response is called highly coupled magnetic resonance, and it hasn't been done before. By attaching the second coil to a device such as the battery of an electric car, or a mobile phone, this current can be made to do useful work. The source can be either centimetres or meters away from the device being powered and can deliver power through walls or around metal obstacles. The power can even be distributed across multiple devices at once. So by a simple trick of physics, power is transferred wirelessly. Nikola Tesla would be proud.

Witricity is developing a system, not a specific product, and as a result they have many different platforms on display at once. In Witricity’s demonstration rooms a flat-screen television is powered using a resonator hidden in its base. Laptops, with their batteries removed and replaced by resonators, flicker on, and flash lights glow when placed next to a source concealed behind a bulletin board. The system can even be extended beyond the range of a single source using passive resonators. By this method, many cabinet lights are lit well beyond the expected range of the single source below.

The applications of wireless power in a wired world are endless and rethinking infrastructure may be decades away. A simple first step could be removing costly batteries from things as mundane as computer accessories and, instead, placing a small wireless resonator in the computer itself, tackling waste by centimetres at first.

Wireless power has in fact been available for decades, just waiting for a clever user to snatch it out of the air. It exists in radio and television signals and is available 24 hours a day. Intel labs in Seattle, Washington, are pointing their devices at television antennas and powering small but useful gadgets solely off of the energy that carries TV programs.

Josh Smith says wireless ambient radio power harvesting might yield a milliwatt or so. To a device, that's not very much. It would require 20 milliwatts to keep a mobile phone in standby mode, but it might be enough to perform some useful low-power functions.

Nikola Tesla's tower was torn down in 1916. Mr Morgan, his financier, was uninterested in broadcasting electricity that people could so easily harvest for free. Instead we strung wires and built meters. Tesla knew that wireless power transfer was possible but he never saw his dream realized - it seems that soon enough we will.

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