English: Inductively coupled crystal radio receiver circuit, a very simple radio that runs on the power it receives from the antenna. This type uses a resonant transformer, T-1, consisting of two magnetically coupled coils of wire, to select the station to be received. Each coil acts as a tuned circuit; the lefthand (primary) coil resonating with the antenna capacitance, the righthand (secondary) coil resonating with the tuning capacitor, CV which is variable and used to tune in different stations. Such a resonant transformer has a much sharper, narrower bandwidth than the single coil used in many circuits.