In a ribbon microphone, a thin ribbon -- usually aluminum, duraluminum or nanofilm -- is suspended in a magnetic field. Sound waves move the ribbon, which changes the current flowing through it. Ribbon microphones are bidirectional meaning they pick up sounds from both sides of the mic.
The RCA PB-31 was one of the first ribbon microphones. It was produced in 1931, and changed the audio and broadcasting industries because it set a new standard when it came to clarity. Several other microphone makers made comparable models, including the BBC-Marconi Type A and ST&C Coles 4038.