received, intercepted by the antenna. Therefore, the sensitivity of the detector was a major factor determining the sensitivity and reception range of the receiver, motivating much research into finding sensitive detectors. In addition to its main use in crystal radios, crystal detectors were also used as radio wave detectors in scientific experiments, in which the DC output current of the detector was registered by a sensitive galvanometer, and in test instruments such as wavemeters used to calibrate the frequency of radio transmitters. Types The crystal detector consisted of an electrical contact between the surface of a semiconducting crystalline mineral and either a metal or another crystal. Since at the time they were developed no one knew how they worked, crystal detectors evolved by trial and error. The construction of the detector depended on the type of crystal used, as it was found different minerals varied in how much contact area and pressure on the crystal surface was needed to make a sensitive rectifying contact. Crystals that required a light pressure like galena were used with the wire cat whisker contact; silicon was used with a heavier point contact, while silicon carbide (carborundum) could tolerate the heaviest pressure. Another type used two crystals of different minerals with their surfaces touching, the most common being the "Perikon" detector. Since the detector would only function when the contact was made at certain spots on the crystal surface, the contact point was almost always made adjustable. Below are the major categories of crystal detectors used during the early 20th century: Cat whisker detector Patented by Karl Ferdinand Braun and Greenleaf Whittier Pickard in 1906, this was the most common type of crystal detector, mainly used with galena but also other crystals. It consisted of a pea-size piece of crystalline mineral in a metal holder, with its surface touched by a fine metal wire or needle (the "cat whisker"). The contact between the tip of the wire and the surface of the crystal formed a crude unstable point-contact metal–semiconductor junction, forming a Schottky barrier diode. The wire whisker is the anode, and the crystal is the cathode; current can flow from the wire into the crystal but not in the other direction. Only certain sites on the crystal surface functioned as rectifying junctions. The device was very sensitive to the exact geometry and pressure of contact between wire and crystal, and the contact could be