A crystal radio needs a device to convert the amplitude modulated radio signal into a signal that can drive headphones or a speaker. The conversion device was called a detector, but in modern terms is a point contact diode. There are many crystalline minerals that can be used to make a point contact diode. Galena, which is lead ore or lead sulfide, was one of the most common minerals used to make radio detectors. Galena has a very low band gap which make it very efficient at converting the AM radio signal into sound. Germanium also has a low band gap and was used in commercial diodes and transistors. A modern silicon diode could be used, but would not be as efficient as Galena or Germanium.
In early crystal radios a piece of Galena was mounted in a Wood’s Metal holder. Wood’s Metal has a very low melting point so the Galena is not damaged when the holder is made. A fine pointed wire (Cat’s Whisker) is pressed against the surface of the Galena to make a detector. It is quite a project to find a sensitive spot on the surface of the galena that will work as a detector, and to determine just the right amount of pressure on the wire contact for the best performance. Once found, the slightest vibration could disturb the settings and the search for a working spot would need to be repeated.
In addition to the detector we also need a bandpass filter to tune in a single AM radio station. The bandpass filter, tuned circuit, or resonant circuit is typically made from an inductor and a capacitor, an L-C circuit. Either the inductor, the capacitor, or both are adjustable to tune the circuit.
Since there is very little energy available to power the headphones they need to be very efficient. Speakers and headphones used in an amplified radio have an impedance of about 8 Ohms and need much more power than a crystal radio can provide. Crystal radios use special high impedance headphones, about 2000-8000 Ohms, to make a usable amount of sound with a small amount of power.
It can be very challenging to find a sensitive spot on the crystal with the cat’s whisker, and adjust the inductor and capacitor at the same time to tune in a single radio station. If you visit the Museum you can give it a try.