The diaphragm is electrically connected to one of the legs of the microphone via the outermost metallic covering.
A JFET (Junction Field Effect Transitor) transistor is used to amplify the electrical current generated. The back plate, fixed in the capsule, is connected with the gate of the transistor, source is connected with the ground of the external circuit and electrical signal appears on the drain.
Sound creates vibrations in surrounding air particles. When a sound is produced near the microphone, vibrating air particles strikes the diaphragm. The resultant air pressure moves diaphragm changing the distance between the plates. The electret material pasted on the back plate, maintains a constant static charge across the capacitor. Relation between the voltage, charge & capacitance is formulated as V=Q/C.
Capacitance of a capacitor depends on the distance between its plates. When the air pressure makes the diaphragm move back and forth, capacitance varies accordingly and thus voltage varies too. This varying voltage produces a variable electric current which is in accordance with the sound vibrations at the diaphragm.