They require no outside power source.
Crystal radios are simple devices.
There are many variations, as you will see. These radios receive AM medium wave frequencies, and in some cases, the higher shortwave frequencies as well. This crystal radio is the classic design with a variable inductor, a tuning capacitor, a germanium diode detector, and a resistor. The radio is hooked up to a 30' or longer wire for the antenna, and is also grounded. The radio is capable of receiving most strong local AM stations during the day, and can also receive more distant stations and even shortwave stations at night.