A system that can deliver power to devices without the need for wires has been shown off at a hi-tech conference.

一个高科技展会上，一套无线供电系统亮相。

Magnetic fields interact with everyday objects less than electric fields

The technique exploits simple physics and can be used to charge a range of electronic devices.

Eric Giler, chief executive of US firm Witricity, showed mobile phones and televisions charging wirelessly¹ at the TED² Global conference in Oxford³.

He said the system could replace the miles of expensive power cables and billions of disposable batteries.

"There is something like 40 billion disposable batteries built every year for power that, generally speaking, is used within a few inches or feet of where there is very inexpensive power," he said.

Trillions of dollars, he said, had also been invested building an infrastructure⁴ of wires "to get power from where it is created to where it is used."

"We love this stuff [electricity] so much," he said.

Mr Giler showed off a Google G1 phone and an Apple iPhone that could be charged using the system.

Witricity, he said, had managed to pack all the necessary components⁵ into the body of the G1 phone, but Apple had made that process slightly harder.

"They don't make it easy at Apple to get inside their phones so we put a little sleeve on the back," he said.

He also showed off a commercially available television using the system.

"Imagine you get one of these things and you want to hang it on the wall," he said. "Think about it, you don't want those ugly cords hanging down."

Good vibrations⁶

The system is based on work by physicist⁷ Marin Soljacic at the Massachusetts Institute of Technology (MIT).

It exploits "resonance⁸（共鸣，共振）", whereby energy transfer is markedly more efficient when a certain frequency is applied⁹.

When two objects have the same resonant¹⁰ frequency, they exchange energy strongly without having an effect on other, surrounding objects.

For example, it is resonance that can cause a wine glass to explode when a singer hits exactly the right tone.

But instead of using acoustic¹¹（声，声响） resonance, Witricity's approach exploits the resonance of low frequency electromagnetic waves.

The system uses two coils（镀锡卷板，卷带马口铁） - one plugged into the mains and the other embedded¹² or attached to the gadget¹³.

Each coil is carefully engineered with the same resonant frequency. When the main coil is connected to an electricity supply, the magnetic field（磁场） it produces is resonant with that of with the second coil, allowing "tails" of energy to flow between them.

As each "cycle" of energy arrives at the second coil, a voltage begins to build up that can be used to charge the gadget.

Mr Giler said the main coil could be embedded in the "ceiling, in the floor, or underneath¹⁴ your desktop¹⁵".

Devices using the system would automatically begin to charge as soon as they were within range, he said.

"You'd never have to worry about plugging these things in again."