English: A carborundum crystal detector, a device used as a demodulator in crystal radio receivers from about 1906 to the 1920s. This 1911 example was sold to hobbyists and radio amateurs in the back of American radio magazines for a price of 50 cents. It consists of a crystalline pebble of silicon carbide (then called carborundum) clamped between two electrical contacts. 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 carrier wave. 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. Carborundum detectors required a forward bias voltage of a few volts applied across the crystal by a battery and potentiometer.