1930s Universal Model XX Double-Button Carbon Mic Demo Video 

Carbon microphones are big resistors that vary their resistance slightly in response to sound. The Universal model X, an earlier model, had two buttons, each 200 ohms, which seems typical for Universal carbon mics. Each button is, electrically, its own microphone. They called for connecting it with a 1.5-3v battery with negative connected to the casing of the microphoness and ground. Positive is run through a switch into the center tap of the primary side of an audio transformer. The ends of the primary are wired to the buttons of the microphone. The primary has a total impedance of 400 ohms, with 200 to a side to match the microphone. The secondary, with an impedance of 100,000 ohms, is connected to output. Mind you, that secondary impedance was to match the amplifiers in use at that time. You could probably have a unity transformer connected to a modern mixboard because nobody impedance matches microphones anymore. I know it's three years late, but I'm starting down this rabbit hole, myself.