Power cables and plugs may soon become obsolete because scientists have developed a way to wirelessly charge electrical devices from a distance.

WiTricity can already power a lightbulb over a distance of 2m (7ft), and its inventors believe that within three to five years the system would be able to charge mobile phones, laptops, MP3 players and other electronic devices.

The appliances would only need to be within range of a WiTricity transmitter to work.

Professor Marin Soljacic, of the Massachusetts Institute of Technology, said that wireless electricity could eventually replace most power cables, in the same way that mobile and cordless phones had supplanted landlines.

“Once, when my son was about three years old, we visited his grandparents’ house,” said Professor Soljacic, whose results are published in the journal Science. “They had a 20-year-old phone and my son picked up the handset, asking, ‘Dad, why is this phone attached with a cord to the wall?’

“That is the mindset of a child growing up in a wireless world. My best response was, ‘It is strange and awkward, isn’t it?’ Hopefully, we will be getting rid of some more wires, and also batteries, soon.”

Professor Soljacic’s inspiration for WiTricity grew out of his frustration at having to find a plug to charge his mobile phone. “It was probably the sixth time that month that I was awakened by my cell phone beeping to let me know that I had forgotten to charge it. It occurred to me that it would be so great if the thing took care of its own charging.”

While there are already wireless devices that can charge batteries by electromagnetic induction – as used in most electric toothbrushes and pads on which mobile phones can be placed – these work over only very short distances.

Standard electromagnetic fields, which are used in wi-fi networks to send data, could feasibly transmit power too, but they spread radiation in all directions so most of the energy would be wasted. Lasers can be used to focus energy in one direction, but an uninterrupted line of sight is needed.

For WiTricity, Professor Soljacic turned to the concept of resonance, which allows the efficient transmission of energy between objects that resonate at the same frequency. The principle is the same when a soprano sings a loud single note – the note can make wine glasses vibrate and even break, so long as they are filled to a level that makes them resonate at the same frequency.

The WiTricity system consists of two copper coils, one sending power, the other receiving it. The receiver is designed to resonate at the same frequency as the magnetic field generated by the transmitter.

Professor Peter Fisher, a colleague of Professor Soljacic, said that the system worked efficiently and automatically, and did not require a clear line of sight between the transmitter and the electronic appliance.

“As long as the laptop is in a room equipped with a source of wireless power, it would charge automatically, without having to be plugged in,” he said. “It would not even need a battery to operate inside such a room.”

At present, the system uses fairly large coils, which are unsuitable for mobile devices, but Professor Soljecic said that it should be possible to miniaturise the system for commercial use within three to five years.

“One would like the distance between the source and the device to be a bit larger – four to five metres – the sizes of the coils to be somewhat smaller, to fit into a laptop, and the efficiency to be a bit higher. We have promising ideas as to how to achieve this so now is a good time to start seriously thinking about commercialisation.”

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