English: A galena cat whisker detector from around 1922, an antique radio component used in crystal radio receivers from 1906 to World War 2. It consists of a fine wire attached to an adjustable arm that touches the surface of a crystal of galena (lead sulfide). In this type, common in portable radios, the delicate cat whisker and crystal are sealed inside a glass tube to prevent contamination by dirt and fingerprints. It functioned as a crude semiconductor diode, rectifying the AC radio signal to extract the audio (sound) modulation signal from the radio frequency carrier wave. The galena crystal is embedded in a fusible metal alloy such as Wood's metal in a metal cup in the base of the detector. The cup and the cat whisker are connected to the binding posts, which are used to connect the device with the rest of the receiver.