A radio signal is an electromagnetic wave. Scientists have discovered that when an electromagnetic wave passes across a wire in free space, a voltage will be produced. This voltage will be quite small, perhaps a few millionths of a volt for weak stations. But for stronger stations, it may be in the hundredths or even tenths of a volt. At this level, a simple detector will convert the radio frequency signal into an audio frequency signal.
Amplitude modulated (AM) radio frequencies range from 530 Kilohertz to 1700 Kilohertz. A Hertz is one vibration per second, so 1600 Kilohertz is 1,600,000 vibrations per second. The upper range of hearing is about 20,000 Hertz, so the radio waves cannot be heard directly.
Radio transmitters impress the audio frequencies upon the radio frequency of the transmitter called the carrier by a process called modulation. The diode detector separates the carrier from the audio, and allows the voice or music to be heard.
If one connected a diode to an antenna, earphone, and ground, then theoretically one could pick up all signals passing across the antenna simultaneously. This would be confusing and undesirable. To limit the frequencies being received, a tuning system is required. This is accomplished by the combination of the coil and the variable capacitor.
Radio engineers have figured out that a coil, which is an inductor with a certain amount of inductive reactance, in combination with a capacitor, which has a certain amount of capacitive reactance can produce resonance. At resonance, the reactance or tendendacy to reject the flow of electrons is greatly diminished -- by a factor as high as a million times. Thus, a certain value of inductance and capacitance has the ability to pass one radio frequency and reject others.
In our crystal radio, the inductance is set and can't be changed. Inductance varies with the number of turns of wire as well as other factors, but these are unchangeable in the design we are using. However, the capacitor is a variable one. Capacitance is caused by two conductive plates in close proximity and the more plate area the greater the capacitance. As we turn the shaft on the variable capacitor, we are moving the rotor plates in and out of the stator plates, varying the area of the plates that are opposing one another and hence changing the amount of capacitance. By changing the capacitance we are then changing the frequency at which resonance occurs; hence, we are changing the frequency at which the radio will accept signals.
After the diode has demodulated or detected the signals, there is a small audio frequency voltage that is passed into the earphones. The ceramic chip inside the earphone vibrates in step with the changing voltage and thus converts it into sound.