English: Galena cat's whisker detector, an antique radio component, showing parts. This example is a modern reproduction. It was used as the detector in crystal radios from about 1906 to the 1940s, to rectify the radio signal, extracting the audio signal from the radio frequency carrier wave. It consists of a crystal of the mineral galena whose surface is touched by a fine metal wire. The device is extremely sensitive to the position of the wire on the crystal and can be disrupted by the slightest vibration, so it is made adjustable. The user finds a "sweet spot" on the crystal by trial and error before each use by dragging the wire across the crystal face until the radio begins working. (top left) Galena crystal and capsule used to hold it, (top right) "cat's whisker" wire contact on adjustable arm, (bottom) assembled device