When I saw a headline about a new MIT scheme to transmit power without wires, I was concerned this might lead to more problems than interference from broadband over power lines. A bit more reading revealed that the research team is using induction, not RF, to transfer power over short distances. However, one example described in the paper introducing the technique used coupled dielectric disks at a frequency of 6.4 MHz to generate an electric field. The paper also references use of microwave frequencies.
Fortunately, capacitively loaded conducting wire loops, which generate near-field energy that's mostly magnetic, are preferred. Most common materials, as well as humans, are nonmagnetic. More objects affect electric fields, which are predominant in the near field of the coupled dielectric disks. The paper noted humans could sustain strong magnetic fields without undergoing any risk.
Of course, wireless transfer of power using induction is not that unusual. If you have an electric toothbrush, the batteries in it are likely charged using power coupled by induction through the holder. What are the MIT researchers doing that's different? How can they achieve power transfer over distances of meters instead of millimeters? The short answer is that both the power source and the receptor in the device being powered are resonant at the same frequency. This is what allows the increased efficiency and greater power transfer distance.
In the paper's conclusion, authors Aristeidis Karlis, J.D. Joannopoulos and Marin Soljacic outline some potential uses for the technology, ranging from transferring power to CMOS electronics or autonomous nano objects all the way to largescale projects such as powering electric buses by using a source-cavity in a "pipe" running above the highway.
If you want more details on how this method of power transfer works, see the paper Wireless Non-Radiative Energy Transfer. The paper includes numerous graphs, plots of electromagnetic fields and a rigorous mathematical analysis of the technique. For a more general explanation of the technique, see the American Institute of Physics release on Eurekalert, "Wireless energy transfer can potentially recharge laptops, cell phones without cords."
