A crystal radio receiver is the earliest form of radio invented, and first began to be made after the rectifying property of crystals was discovered in 1874 by the German inventor Karl Braun. Many crystal forms, such as the mineral galena or lead sulfide, which served as the crystal component of early crystal radios, allow electrical current to pass in primarily only one direction. This makes them useful for converting alternating current (AC) radio wave signals to direct current (DC) signals so that audio transmissions can be decoupled from the radio carrier wave and listened to. The principle of rectifying crystals resulted in wide production of crystal radio receivers throughout the world, including their common use in World War I. Since they didn't require batteries to operate and were simple to build, they became widespread until the invention of battery-powered amplifiers and vacuum tubes made more powerful radio receivers possible in the early 1920s.
A crystal set can be built with only four functional parts. These include: the antenna, the tuning coil, the crystal detector, and the earphones. The antenna serves to capture all ambient radio waves present at a designated location and channel their power into the crystal radio as an electrical current. The tuning coil is a copper wire winding that is used to tune out most of the radio waves present so that only the desired one is processed further. This chosen radio wave frequency is channeled into the crystal detector, which separates the carrier radio wave from the audio signal that piggybacks on it as it travels through the air. Since the audio signal is usually quite weak in the absence of a battery-powered amplifier, the earphones are then used for listening to the crystal radio to maximize volume levels.
As of 2011, crystal radios are still made as a subject of school science experiments and by electrical hobbyists, but their low power levels and limited ability to capture far off radio signals have made them obsolete as commercial products. Modern components have also made them easier to build. This includes replacing the cat's whisker receiver with a solid state crystal diode. The original crystal design used a natural crystal to which a wire cat's whisker electrode made contact. The electrode had to be moved around on the crystal until a location was found where the crystal lattice structure would transmit a signal through to the earphones.
Crystal diodes utilize the same principle of rectification as the cat's whisker receiver, but don't require any manual adjustment to transmit a radio signal once it has been tuned by the tuning coil. Several different types of semiconducting crystals have been used to perform this role aside from galena, including silicon carbide, iron pyrite, and zincite-bornite. Any material that has a semiconducting electrical property can also work in place of an actual crystal in a crystal radio. This includes such common objects as copper US pennies that have acquired a tarnished, semiconducting oxidized surface to them with age, but crystals themselves are the easiest material to work with in building a crystal radio, and the most effective.