English: Zone refining of a bar of rare earth metal at University of Michigan in 1961. Zone refining, invented in 1953 by William Pfann at Bell Laboratories, is a technique for removing impurities from crystals, widely used in the semiconductor industry. In the picture a narrow bar of metal runs down the center of the glass tube. The coil of copper wire around the tube is attached to an electronic oscillator generating a radio frequency alternating current through the wire. This heats the bar by induction heating, melting a short length of the bar. The coil is moved slowly down the tube by the lead screw (right), so the molten zone moves down the bar. The impurities in the bar stay in the molten section and are moved to the end of the bar, which is cut off.