The circuit consists of three stages, an electret microphone, an audio amplifier and an RF oscillator.
The electret microphone consists of only two parts - a FET transistor and a plastic diaphragm. There is nothing else inside the case. The 3-leaded FET has an input, output and earth lead. The mylar diaphragm has been permanently charged with a static charge and this is one of the secrets of the operation of the microphone.
The FET transistor has a very high input impedance and because it is an active device, we call it a stage.
The gain of this device is extremely high because it is able to amplify the charges from a charged surface to produce an output in the millivolt ra