English: A carborundum crystal detector, a device used in wireless receivers and later crystal receivers from about 1906 to the 1920s. It consists of a crystalline pebble of carborundum (silicon carbide) held in a metal cup, with a metal contact pressed against its surface by the thumbscrew (center). It was a crude semiconductor diode, conducting electric current in one direction but not in the opposite direction. In a radio receiver its function was to rectify the radio signal, extracting the audio (sound) signal from the radio frequency signal. Because of the heavier construction of carborundum detectors, which didn't require a delicate "cat's whisker" contact like other crystal detectors, they were used in shipboard and military radio receivers where vibration could be expected.