In the early 20th century, Serbian inventor Nikola Tesla dreamed of pulling limitless free electricity from the air around us. Ever ambitious, Tesla was thinking on a vast scale, effectively looking at the Earth and upper atmosphere as two ends of an enormous battery. Needless to say, his dreams were never realised, but the promise of air-derived electricity – hygroelectricity – is now capturing researchers’ imaginations again. The difference: they’re not thinking big, but very, very small. In May, a team at the University of Massachusetts (UMass) Amherst published a paper declaring they had successfully generated a small but continuous...