the junction by a battery and potentiometer. The voltage was adjusted with the potentiometer until the sound was loudest in the earphone. The bias moved the operating point to the curved "knee" of the device's current–voltage curve, which produced the largest rectified current. Silicon detector Patented and first manufactured in 1906 by Pickard, this was the first type of crystal detector to be commercially produced. Silicon required more pressure than the cat whisker contact, although not as much as carborundum. A flat piece of silicon was embedded in fusible alloy in a metal cup, and a metal point, usually brass or gold, was pressed against it with a spring. The surface of the silicon was usually ground flat and polished. Silicon was also used with antimony and arsenic contacts. The silicon detector was popular because it had much the same advantages as carborundum; its firm contact could not be jarred loose by vibration and it did not require a bias battery, so it saw wide use in commercial and military radiotelegraphy stations. Crystal-to-crystal detectors Another category was detectors which used two different crystals with their surfaces touching, forming a crystal-to-crystal contact. The "Perikon" detector, invented 1908 by Pickard was the most common. Perikon stood for "PERfect pIcKard cONtact". It consisted of two crystals in metal holders, mounted face to face. One crystal was zincite (zinc oxide, ZnO), the other was a copper iron sulfide, either bornite (Cu5FeS4) or chalcopyrite (CuFeS2). In Pickard's commercial detector (see picture), multiple zincite crystals were mounted in a fusible alloy in a round cup (on right), while the chalcopyrite crystal was mounted in a cup on an adjustable arm facing it (on left). The chalcopyrite crystal was moved forward until it touched the surface of one of the zincite crystals. When a sensitive spot was located, the arm was locked in place with the setscrew. Multiple zincite pieces were provided because the fragile zincite crystal could be damaged by excessive currents and tended to "burn out" due to atmospheric electricity from the wire antenna or currents leaking into the receiver from the powerful spark transmitters used at the time. This detector was also sometimes used with a small forward bias voltage of around 0.2V from a battery to make it more sensitive. Although the zincite-chalcopyrite "Perikon" was the most widely used crystal-to-crystal detector, other crystal pairs were also used. Zincite was used with carbon, galena,