English: A medium wave crystal radio receiver, manufactured by Heliogen in Germany, 1935. The coil of wire (left) is the tuning coil. It can be unplugged and replaced with different coils to cover different radio bands. It is wound in a "basket-weave" pattern to reduce resistance at radio frequencies. In front of it in the glass tube is the cat's whisker detector, consisting of a crystal of galena touched by a fine wire, which extracts the audio signal from the radio frequency carrier wave. In front of that is the tuning knob, connected to an adjustable capacitor which is used to tune in different stations.