Wireless electricity transmission works by passing alternating current through one coil to which another coil corresponds and thus is coupled. At least this is the way WiTricity explains it. They also say that it's very much like mechanical resonance and that's why it's more efficient over greater distances than just ordinary induction. What I don't get is how can two coils be coupled like that? Is there some eigenmagnetic frequency that every coil has which allows them to be coupled when the frequency of the magnetic field hits that eigenfrequency? If it's so straightforward why the technology isn't widely implemented yet?