Removing the battery from an off-the-shelf drone, the researchers attached a copper ring to the drone's electronics. Then, a transmitter on the ground connected to a power supply made a magnetic field. When the drone flew into the field, hovering at a distance of no more than 4 inches above it, it was able to draw power from that field, and convert it into direct current to power the small flying machine. The technology is still constrained by the narrow gap between drone and charger; 4 inches away from the charger is better than being plugged into it, but it's not a ton of space. Still, the team at Imperial College London seems confident in the results, floating concepts for drones that charge over power lines, drones that charge other drones, and even inductive charging stations that can land on Mars to recharge tired robotic rovers.