Cut the cord

Demonstrating the ability to turn on a lightbulb is not usually the sort of thing to elicit a gasp of amazement from the world's technophiles.

But when Justin Rattner, computer giant

technology chief, flicked the switch on a 60W bulb at the firm's recent developers' forum, that was precisely the reaction he achieved.

For while lightbulbs usually dangle from a ceiling fitting or are screwed into a lamp, this one was powered wirelessly by a strange looking device placed several metres away. It was, claimed Rattner, the latest development in a journey that could one day complete our transition to a 'wireless world', enabling us to say goodbye to the power cable and charge everything from our mobile phones to our laptops simply by walking into a room.

Intel's so-called Wireless Resonant Energy Link (WREL) exploits the phenomenon of resonant coupling, where an object is caused to vibrate when energy of a certain frequency is applied. The concept is analogous to the effect that enables opera singers to shatter glasses of corresponding frequencies, while others remain intact.

The physics behind WREL has been understood and studied for centuries, most notably by 19th century physicist Nikola Tesla, who experimented with long-distance wireless energy transfer and even built a 29m tower that he hoped to use to demonstrate the concept.

For financial reasons, Tesla's tower was abandoned, but thanks largely to the ever-increasing popularity of portable gadgets, Intel believes the business case is somewhat more compelling than it was in Tesla's day.

And the company is not alone. Last year a team at MIT demonstrated a strikingly similar system that it calls Witricity. Like Intel, the group used the system to power a 60W lightbulb from a source placed more than 2m away and, as with Intel's system, Witricity is based on resonant coupling.

researcher Andre Kurs explained that the technology uses two coupled electromagnetic resonators. One of these ring-shaped copper coils is attached to the mains, and the other to a device that is being powered.

At first glance, the principles behind the technology are not unlike magnetic induction, where an electric current running in a sending coil induces another current in an adjacent receiving coil. The big difference is that resonant coupling enables the coils to be placed further apart — at distances where conventional magnetic induction would be millions of times less efficient.

'The distance between the two antennas is larger than the characteristic size of the antennas,' said Kurs. 'In a typical inductor, for example a transformer, the distance between the two coils is very close — they overlap. we've reversed that ratio to make the distance larger than the size of the object.'

Another key development is that while systems that use electromagnetic radiation usually scatter energy in all directions, MIT's system fills the space around it with a non-radiative magnetic field that prevents the energy escaping until it comes into close proximity with a receiving coil resonating at the same frequency. The non-radiative field helps to ensure that most of the power not picked up by the receiving coil remains bound to the vicinity of the sending unit, instead of being radiated into the environment. Kurs explained that this helps to minimise the extent to which the system will affect other objects that may be nearby.

MIT is continuing to refine its technology, and although Kurs said it is too early to reveal the results, there are perhaps two obvious areas of improvement.

In the current design the resonant coils are more than half a metre in diameter. Clearly, any commercial application would require the receiving unit at least to be shrunk down to a usable size. Kurs said that while smaller receivers would certainly reduce the range, it should be possible to develop a receiving coil small enough to fit in a laptop or phone, but still powerful enough to receive power from across the other side of a room.

There are also potential gains to be made by improving the efficiency of the system over longer distances. 'There's a trade-off between efficiency and distance,' said Kurs. 'in our original paper, at a metre of separation the efficiency was nearly 90 per cent, at 2m it was 50 per cent, but at 2.25m it was somewhere between 35 and 40 per cent.'

Kurs said that he and his team have spent the last year further refining the system, and although it could be many years away from commercialisation he reports a great deal of early-stage interest from numerous corporations.

Intriguingly, as well as the obvious applications in consumer electronics — charging mobile phones and the like — Kurs believes that wireless power will be perfect for a number of more exotic niche applications, such as recharging sensors positioned in hard-to-reach locations.

But not everybody is convinced. For some, the prospect of beams of energy bouncing around the home is an irresponsible and dangerous dream. Prof Denis Henshaw, head of the Human Radiation Effects group at

, believes the technology has serious health implications. 'it's a dead duck as far as serious power transmission is concerned, and the health protection agency would endorse that,' he said.

'It's useful for remotely charging your mobile phone or low-power devices where the device you want to charge is right next to this transit. But the idea of transmitting power over large distances when people are going to be there is ridiculous and they ought to forget it.'

Henshaw believes that applications will be limited to systems that exploit the same principles without requiring a physical space between the charger and the device. Indeed, a number of companies are already marketing such devices.

In the UK,

of Cambridge has developed a wireless recharging pad that uses electromagnetic induction. And in the US, Colorado-based

has developed an adapter that replaces a mobile phone's back cover and receives a charge from a specially-developed conductive pad.

Though impressed by such developments, MIT's Kurs believes technology that beams power across a room will be far more useful. 'I think it would be more convenient if you just had one source in the corner of the room and used it to power all of the devices in the vicinity,' he said. Unlike Henshaw, Kurs does not believe that such technology should carry a major health warning. 'It's something we're looking at very carefully,' he said. 'we're looking into the standards from all the regulatory authorities, but we're cautiously optimistic. it seems from what we've done so far that this technology could be applied to a wide range of applications and will be safe for human use.'

Ultimately, for Kurs, the commercial case is simply too compelling for wireless power to be condemned to another 200 years of solitude. 'there are all these portable devices around and wireless is all the rage. there's Bluetooth and Wi-Fi, but at some point you still have to recharge your batteries.'