At MIT a 60 watt light bulb is lit from two metres away. Note the obstruction in the lower image.

Credit: Science

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SYDNEY: A new way of transmitting electricity wirelessly has been discovered by U.S. researchers. It could pave the way for the wireless charging of portable electronic devices, rendering power cords obsolete in the process.

Devised by physicists at the Massachusetts Institute of Technology (MIT), in Boston, the new technology is detailed today in the U.S. journal Science.

At the turn of the 20th century, famed physicist and inventor Nikola Tesla devoted considerable effort towards achieving large scale wireless power transfer. Success proved elusive, however, and the demand for such schemes declined.

Now, the revolution in consumer technology has re-ignited the interest of researchers.

Surfeit of batteries

"Over the past several years, portable electronic devices, such as laptops, cell phones, iPods and even household robots have become widespread – all of which require batteries that need to be recharged often," said study co-author John Joannopoulos.

The researcher team's vision of wireless electricity, which they call 'WiTricity', is based on using coupled resonant objects. Two resonant objects of the same magnetic resonant frequency tend to exchange energy efficiently, while interacting weakly with other nearby objects, like metal, wood or people.

As an example, consider a room full of wine glasses, each filled up to a different level so they all have different resonant frequencies. If an opera singer hits the right note loudly, a glass of the corresponding frequency could gain enough energy to explode, while the others would be entirely unaffected.

Rather than acoustic resonance, Joannopoulos and colleagues used magnetism to transfer power between two strongly coupled induction coils. And remarkably, using this method, they were able to power a 60 watt light bulb from a distance of two metres, despite no physical connections.

Magnetic fields interact weakly with most common materials, meaning WiTricity flows even in the presence of obstacles between the power source and the light bulb. "The fact that magnetic fields interact so weakly with biological organisms is also important for safety considerations," said co-author Andre Kurs.

Unplug and play

At first glance, such a power transfer is reminiscent of magnetic induction, the method used in power transformers and electric toothbrushes. These devices consist of an electric current running in a sending coil, which induces another current in a receiving coil.

However, the difference is in the distance power can be transferred. The usual non-resonant magnetic induction would be almost 1 million times less efficient than this particular system, said the authors.

The technology has the potential to make power cables redundant, at least when it comes to the growing array of cordless devices we use. The researchers believe WiTricity might even be used to bypass the batteries needed by some of these devices to operate.

"Efficient wireless power transfer may help reduce our reliance on batteries — which are bulky, expensive and contain toxic elements — by running the devices directly on the transferred power," said Kurs.

John Pendry, a theoretical physicist at Imperial College in London, England, said he first suggested the idea of magnetic resonances to the MIT team.

"How many times has your laptop run out of battery power? How often has your mobile phone run out of power?" said Pendry. "Imagine if your mobile phone or laptop started to charge immediately you stepped inside your office without being plugged into a socket."

"The power line will be the last wire to be cut before we can achieve true wireless devices," he added.

The MIT researchers say the technology is very near the point at which it could be applied for practical applications.