A crystal radio consists of an L-C oscillator, an antenna, a germanium diode, and an earphone. There is no battery or other internal power source. The inductor has a ferrite core that can slide in and out of the coil, which modifies the inductance and therefore the oscillation frequency.


Here is a schematic diagram of the crystal radio circuit:


If the inductor-capacitor combination is tuned to oscillate at 810 kHz, the 810 kHz radio signal is maintained in the oscillating circuit, whereas signals from other stations fade out due to wire resistance. The diode converts the amplitude-modulated AM radio signal into an audio signal that can be heard in the earphone.

The energy for the radio comes entirely from the transmitted radio signal; no battery is needed. Only a tiny amount of power from the radio signal is captured by the antenna, so only one person can listen to the faint audio signal through the earphone.

LC Oscillator and the Hydraulic Analogy

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Crystal radio, Wikipedia
Good info on crystal radio operation and history

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