English: Cutaway drawing of a crystal detector used as a demodulator in early crystal radios. It consists of crystalline pieces of two different minerals held in brass cups with their surfaces pressed against each other by the thumbscrew. The two crystals form a crude semiconductor diode. Different combinations of crystals could be used: the most common was zincite and chalcopyrite, called a "Perikon" detector, but the text suggests a combination of zincite and tellurium was a more sensitive detector. The detector was built as a plugin unit so different detectors could be quickly switched in and out to find the most sensitive. The crystal receivers which were used before vacuum tube receivers superseded them around 1920 did not have the ability to amplify the radio signal, so the sensitivity of the crystal detector mainly determined the reception range. Many different crystals were tried in an effort to improve sensitivity.