Many people have heard of and encountered light-emitting diodes, also called LEDs; they are used often in electronic toys and readouts. The first diodes were invented to serve as rectifiers--that is, they were used to convert alternating current (AC) into direct current (DC). AC is very easy to generate using a transformer; however, AC moves in a sine-wave pattern, moving first in one direction and then doubling back in the opposite direction. Many electronic devices require a current that moves in only one direction, such as DC. A device was needed that would eliminate the sine-wave motion of AC. Karl Ferdinand Braun, a German physicist, noticed that certain crystalline substances served as rectifiers, allowing current to pass very easily in one direction but inhibiting its motion in the opposite direction. When an AC current was passed through these crystals, it was easily converted into DC. Braun patented his...