One of the initial diodes, existing even before the vacuum tubes, was a semiconductor known as a cat whisker; this semiconductor was a fine piece of wire in contact with the small piece of mineral galena. This bizarre looking thing had the ability to behave as a rectifier for small radiofrequency (RF) currents. When cat whisker was connected in the circuit like that of the figure given below, the result was a receiver capable of picking up the amplitude modulated (AM) radio signals.
Figure- Schematic diagram of a crystal set radio receiver.
Engineers had to adjust position of the fine wire to find out the best point of contact with galena. A tweezers and magnifying glass were not of any use in this process. A steady hand was required.
The galena called as crystal sometimes, gave rise to the nickname crystal set for the low sensitivity radio. You can build a crystal set today as well, by using a simple RF diode, a coil, a headset, a tuning capacitor, and a long wire antenna. Notice that there is no battery! The audio can be provided by received signal alone.
The diode which is shown in the figure acts to recover audio from radio signal. This is called as detection; the circuit is termed as detector. If the detector is to be made effective, the diode should be of the correct type. It should have low capacitance, so that it works as a rectifier at the radio frequencies, passing the current in one direction but not in the other direction. Some modern RF diodes are microscopic versions of old cat whisker, enclosed in the glass case with axial leads. You have probably seen these in the electronics hobby stores.