How carbon microphone works?

Carbon is a resistor, meaning it conducts electricity, but not very well.

carbon microphone consists of two metal plates with a layer of small grains of carbon between. Each metal plate is attached to a wire which connects it to an audio receiver.

A current runs from one plate through the carbon to the other plate. The carbon molecules normally resist it somewhat, lowering the power flow. When a sound wave pushes down on the top plate, however, it squeezes the carbon molecules more tightly between the two plates. This increases their conductivity, creating more electric current. As the plate moves up and down with the sound wave, the current increases and decreases, creating an electric wave in the shape of the sound wave.