is the simplest possible rectifier. That’s its chief advantage over other, more complicatedrectifier circuits. You’ll learn about the different types of rectifier diodes and circuits inthe next chapter.DetectionOne of the earliest diodes, existing even before vacuum tubes, was a semiconductor.Known as a cat whisker, this semiconductor consisted of a fine piece of wire in contactwith a small piece of the mineral galena. This bizarre-looking thing had the ability to actas a rectifier for small radio-frequency (RF) currents. When the cat whisker was con-nected in a circuit like that of Fig. 20-2, the result was a receiver capable of picking upamplitude-modulated (AM) radio signals.Detection37120-1At A, half-wave rectifier. At B, output of the circuit of A with sine-wave ac input.20-2Schematic diagram of a crystal set radio receiver.A cat whisker was a finicky thing. Engineers had to adjust the position of the finewire to find the best point of contact with the galena. A tweezers and magnifying glasswere invaluable in this process. A steady hand was essential.The galena, sometimes called a “crystal,” gave rise to the nickname crystal set forthis low-sensitivity radio. You can still build a crystal set today, using a simple RF diode,a coil, a tuning capacitor, a headset, and a long-wire antenna. Notice that there’s no bat-tery! The audio is provided by the received signal alone.