How do crystal radios work without batteries?

A crystal radio typically uses an antenna, a tuning circuit, a detector diode, and earphones. The antenna picks up radio waves, and the tuned circuit selects one station by resonating at a specific frequency. The diode rectifies the alternating radio signal into a signal that the headphones can turn into sound. Because it harvests only a tiny amount of power from the air, a crystal radio usually works best with very strong local stations and sensitive high-impedance headphones. It was one of the earliest practical radio receiver designs in the early 20th century.