Ribbon mics are basically a thin "ribbon" of metal suspended in a magnetic field- sound waves move the ribbon, changing the current flowing through it. [wiki]
Part of Butch Vig's array of microphones for the massive guitar overdubs in Siamese Dream. Mics were placed by cranking Billy's amp up to full gain, boosting the headphone send, then moving the mics around using the guitar echo's phase-shifting hiss as a guide. Ribbon mics were used to obtain a smoother sound with quick, yet mellow, transients.