English: On the left a cat's whisker detector, an antique radio component used in crystal radio receivers beginning in the first decades of the 20th century, until about the 1940s. It consists of a semiconducting mineral crystal, often galena (PbS) in a metal cup (bottom), whose surface is touched by a fine metal wire attached to an adjustable arm (top). It forms a crude semiconductor diode which rectified the radio signal in the receiver, extracting the audio signal from the radio frequency carrier wave. Only certain sites on the crystal surface functioned as rectifying junctions, so it had to be adjusted to find a good contact spot by trial and error before each use. The device was very sensitive to the pressure of the wire contact and was easily disrupted by the slightest vibration. If it gets out of adjustment and stops working, it can be replaced by another cartridge. On the right an old type condensor, used in a crystal radio with capacity 200 cm, measured in cm as was usual in that time.