We’ve previously shown (here, here, here, and here) other homemade microphones. This idea for a carbon mike appeared in Electronics Illustrated, September 1963. The mike element itself is made of carbon granules. These are obtained by cutting open an old carbon-zinc battery and removing the carbon rod.
You grind them up finely with a file, or by using a metal rod as a rolling pin. The resulting granules are placed in a plastic bottle cap, and wired to a battery and transformer. Almost any type of transformer can be used, either an audio transformer or power transformer. The larger winding is connected to the mike element. The cap is set on top of a paper tube 2-3 inches in diameter, into which you speak.