Drawing of a Bell Telephone carbon button microphone from about 1912, shown with the structure cut away to reveal the construction. It was used in their telephones into the 1940s. The active element, "(center)" is a cylindrical "button" consisting of two electrodes with carbon granules between them. This was attached at the front to the center of a cylindrical aluminum diaphragm, and the back to a stationary bracket. When someone spoke into the horn mouthpiece "(right)", the sound waves vibrated the diaphragm back and forth, which put varying pressure on the carbon. Current from a battery was applied to the electrodes, and the varying pressure caused the carbon's resistance thus the current to vary.