The setup is straightforward, explains Andre Kurs, an MIT graduate student and the lead author of the paper. Two copper helices, with diameters of 60 centimeters, are separated from each other by a distance of about two meters. One is connected to a power source–effectively plugged into a wall–and the other is connected to a lightbulb waiting to be turned on. When the power from the wall is turned on, electricity from the first metal coil creates a magnetic field around that coil. The coil attached to the lightbulb picks up the magnetic field, which in turn creates a current within the second coil, turning on the bulb.
This type of energy transfer is similar to a well-known phenomenon called magnetic inductive coupling, used in power transformers. However, the MIT scheme is somewhat different because it’s based on something called resonant coupling. Transformer coils can only transfer power when they are centimeters apart–any farther, and the magnetic fields don’t affect each other in the same way. In order for the MIT researchers to achieve the range of two meters, explains Soljačić, they used coils that resonate at a frequency of 10 megahertz. When the electrical current flows through the first coil, it produces a 10-megahertz magnetic field; since the second coil resonates at this same frequency, it’s able to pick up on the field, even from relatively far away. If the second coil resonated at a different frequency, the energy from the first coil would have been ignored.
The researchers’ approach, says Soljačić, also makes the energy transfer efficient. If they were to emit power from an antenna in the same way that information is wirelessly transmitted, most of the power would be wasted as it radiates away in all directions. Indeed, with the method used to transfer information, it would be difficult to send enough energy to be useful for powering gadgets. In contrast, the researchers use what’s known as nonradiative energy that is bound up near the coils. In this first demonstration, they showed that the scheme can transfer power with an efficiency of 45 percent