Ribbon microphone
A microphone in which a corrugated aluminium ribbon a few micrometres thick hangs in a magnetic field and is itself the conductor.
The ribbon is open on both sides, so it responds to the pressure difference between front and back. That gives a natural figure-8 polar pattern: equally sensitive front and rear, deaf at the sides. Harry Olson at RCA designed the RCA 44 in the early 1930s, and it became the microphone of radio and the big bands. The sound is smooth with a slightly recessed top, flattering to voices and to harsh guitar amps alike.
Output is very low, and the ribbon can be stretched by a gust of wind or someone blowing into the grille to test it. Modern designs tolerate phantom power, but a miswired cable into a vintage ribbon can still kill it.