The power of induction: cutting the last cord could resonate with our increasingly gadget-dependent lives

Science News,  July 21, 2007  by Davide Castelvecchi

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Matin Soljacic was understandably nervous. The young physicist was about to give his first public presentation of an idea that sounded almost too good to be true. There was no telling how his audience, at a Berkeley, Calif., symposium, would receive his daring proposal. Design two antennas to be as inefficient as possible at transmitting radio waves, Soljacic began.

Separate the antennas by a few meters and, with some fine-tuning, you can safely and efficiently transfer electricity from one to the other--without wires. Put this system inside your home, and you would have a wireless network for electrical power. You could recharge your laptop or turn on a light without plugging anything in.

The crucial bit would be the fine-tuning: The two antennas would have to be tweaked so that one would create a pulsating magnetic field with a specific frequency and geometry, which the other would then transform into an electric current.

When Soljacic first presented the principle, it was unproved. All he could show were his calculations. "I expected that some people would think I was a crackpot," says Soljacic, a physicist at the Massachusetts Institute of Technology (MIT). "This was pretty far out."

Perhaps it also didn't help that the participants at the symposium--a celebration of the 90th birthday of Charles Townes, who pioneered the laser in the 1950s--included 18 Nobel prize winners and dozens of other luminaries. Much to Soljacic's relief, he sold the scientists on his presentation.

A year and a half later, a bulb lit up in an MIT lab--unplugged. Soljacic and his collaborators had demonstrated a new way of coaxing magnetic fields into transferring power over a distance of several meters without dispersing as electromagnetic waves. The demonstration ushered in a technology that might eventually become as pervasive as the gadgets it could power. Laptops, cell phones, iPods, and digital cameras might someday recharge without power cords. With the proliferation of wireless electronics, perhaps it was just a matter of time before power transmission would go wireless, too.

The device that Soljacic and his collaborators put together had a disarming simplicity. On one side of the room, hanging from the ceiling, was a ring-shaped electrical circuit, about half a meter across, plugged into the wall. Hanging adjacent to the circuit, but with no physical connection to it, was a slightly larger copper coil looking like an oversize mattress spring. A few meters away hung a similar system with an ordinary lightbulb attached to the circuit. When the physicists sent power through the first circuit, the bulb lit up.

As expected, some energy was lost on its way to the lightbulb. However, a surprising amount reached its destination, the team reports in the July 6 Science. "The efficiency was 40 percent at the biggest distance we probed [more than 2 meters]," Soljacic says. At shorter distances, the efficiency was much higher.

The coils of this demonstration device would be too big to fit inside a laptop, let alone a cell phone. But this was only the first and simplest of several prototypes that the physicists have in mind. More tests are to come. The MIT team and other physicists say that in principle they see no obstacle to making such devices more compact and more efficient.

MAKING NO WAVES The idea of transmitting energy wirelessly isn't new. For almost two centuries, scientists have known that rapidly changing magnetic fields, such as those produced by an alternating current flowing through a wire, can induce an electric current in another wire. That's how the coils inside power transformers transmit energy from one coil to another without touching. But this form of induction usually works efficiently only when the two coils are very close to each other.

In the early 1900s, long before the power grid made electricity widely available, electricity pioneer Nikola Tesla devised a grand scheme to transfer large amounts of power over long distances from a tower 20 stories tall, to be built on Long Island in New York. To this day, historians puzzle over how Tesla's system was supposed to work, or whether it could have worked at all, says Bernard Carlson, a historian of science at the University of Virginia in Charlottesville who is writing a biography of the great engineer. "We can't even begin to understand what he was doing with this power stuff," Carlson says.

The project died when Tesla's financial backers pulled the plug, possibly because Tesla seemed unclear as to how to bill customers receiving wireless power. Ironically, Tesla also invented the alternating current (AC) system of power production, transmission, and distribution that would become the standard for the modern grid.

But electromagnetic radiation can indeed carry energy through air or empty space and over large distances. One familiar example is the energy we receive from the sun, mostly as visible light. Another is radio waves, first detected by Heinrich Hertz in 1888. An electromagnetic wave is a synchronized dance of an electric field and a magnetic field. Because an oscillating magnetic field generates an oscillating electric field, and vice versa, the two fields sustain each other as the wave propagates.